%%
%% %CopyrightBegin%
%%
%% Copyright Ericsson AB 2004-2024. All Rights Reserved.
%%
%% Licensed under the Apache License, Version 2.0 (the "License");
%% you may not use this file except in compliance with the License.
%% You may obtain a copy of the License at
%%
%%     http://www.apache.org/licenses/LICENSE-2.0
%%
%% Unless required by applicable law or agreed to in writing, software
%% distributed under the License is distributed on an "AS IS" BASIS,
%% WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
%% See the License for the specific language governing permissions and
%% limitations under the License.
%%
%% %CopyrightEnd%
%%
-module(qlc).
-moduledoc({file, "../doc/src/qlc.md"}).

%%% Purpose: Main API module qlc. Functions for evaluation.
%%% Other files:
%%% qlc_pt. Implements the parse transform.

%% External exports 

%% Avoid warning for local function error/1 clashing with autoimported BIF.
-compile({no_auto_import,[error/1]}).
-export([parse_transform/2,
         parse_transform_info/0,
         transform_from_evaluator/2]).

-export([q/1, q/2]).

-export([eval/1, e/1, eval/2, e/2, fold/3, fold/4]).
-export([cursor/1, cursor/2, 
         next_answers/1, next_answers/2, 
         delete_cursor/1]).
-export([append/1, append/2]).

-export([sort/1, sort/2, keysort/2, keysort/3]).

-export([table/2]).

-export([info/1, info/2]).

-export([string_to_handle/1, string_to_handle/2, string_to_handle/3]).

-export([format_error/1]).

%% Exported to qlc_pt.erl only:
-export([template_state/0, aux_name/3, name_suffix/2, vars/1,
         var_ufold/2, var_fold/3, all_selections/1]).

-dialyzer(no_improper_lists).

%% When cache=list lists bigger than ?MAX_LIST_SIZE bytes are put on
%% file. Also used when merge join finds big equivalence classes.
-define(MAX_LIST_SIZE, 512*1024).

-record(qlc_append, % qlc:append/1,2
        {hl
        }).

-record(qlc_table,   % qlc:table/2
        {trav_fun,   % traverse fun
         trav_MS,    % boolean(); true iff traverse fun takes a match spec
         pre_fun,
         post_fun,
         info_fun,
         format_fun,
         lookup_fun,
         parent_fun,
         key_equality, % '==' | '=:=' | undefined (--R12B-5)
         lu_vals,    % undefined | {Position,Values}; values to be looked up
         ms = no_match_spec
                     % match specification; [T || P <- Tab, Fs]
        }).

-record(qlc_sort,    % qlc:sort/1,2 and qlc:keysort/2,3
        {h,
         keypos,     % sort | {keysort, KeyPos}
         unique,
         compressed, % [] | [compressed]
         order,
         fs_opts,    % file_sorter options
         tmpdir_usage = allowed, % allowed | not_allowed 
                                 %  | warning_msg | error_msg | info_msg
         tmpdir
        }).

%% Also in qlc_pt.erl.
-record(qlc_lc,     % qlc:q/1,2
        {lc,
         opt        % #qlc_opt
        }).

-record(qlc_list,   % a prepared list
        {l,
         ms = no_match_spec
        }).

-record(qlc_join,        % a prepared join
        {kind,           % {merge, KeyEquality} |
                         % {lookup, KeyEquality, LookupFun}
         opt,            % #qlc_opt from q/2.
         h1, q1, c1,     % to be traversed by "lookup join"
         h2, q2, c2      % to be looked up by "lookup join"
        }).

%%% A query cursor is a tuple {qlc_cursor, Cursor} where Cursor is a pair
%%% {CursorPid, OwnerPid}.

-record(qlc_cursor, {c}).

-record(qlc_opt,
        {unique = false,      % boolean()
         cache = false,       % boolean() | list (true~ets, false~no)
         max_lookup = -1,     % int() >= 0 | -1 (represents infinity)
         join = any,          % any | nested_loop | merge | lookup
         tmpdir = "",         % global tmpdir
         lookup = any,        % any | boolean()
         max_list = ?MAX_LIST_SIZE,  % int() >= 0
         tmpdir_usage = allowed    % allowed | not_allowed 
                                   %  | warning_msg | error_msg | info_msg
        }).

-record(setup, {parent}).

-define(THROWN_ERROR, {?MODULE, throw_error, _, _}).

-export_type([query_cursor/0, query_handle/0]).

%%% A query handle is a tuple {qlc_handle, Handle} where Handle is one
%%% of #qlc_append, #qlc_table, #qlc_sort, and #qlc_lc.

-record(qlc_handle, {h}).

get_handle(#qlc_handle{h = #qlc_lc{opt = {qlc_opt, U, C, M}}=H}) ->
    %% R11B-0.
    H#qlc_lc{opt = #qlc_opt{unique = U, cache = C, max_lookup = M}};
get_handle(#qlc_handle{h = H}) ->
    H;
get_handle(L) when is_list(L) ->
    L;
get_handle(_) ->
    badarg.

%%%
%%% Exported functions
%%%

-doc "A literal [query list comprehension](`m:qlc#query_list_comprehension`).".
-type(query_list_comprehension() :: term()).
-doc "A [query cursor](`m:qlc#query_cursor`).".
-opaque(query_cursor() :: {qlc_cursor, term()}).
-doc "A [query handle](`m:qlc#query_handle`).".
-opaque(query_handle() :: {qlc_handle, term()}).
-type(query_handle_or_list() :: query_handle() | list()).
-type(answers() :: [answer()]).
-type(answer() :: term()).
-doc """
Parse trees for Erlang expression, see section
[The Abstract Format](`e:erts:absform.md`) in the ERTS User's Guide.
""".
-type(abstract_expr() :: erl_parse:abstract_expr()).
-doc """
Match specification, see section
[Match Specifications in Erlang](`e:erts:match_spec.md`) in the ERTS User's
Guide and `m:ms_transform`.
""".
-type(match_expression() :: ets:match_spec()).
-type(spawn_options() :: default | [proc_lib:spawn_option()]).
-type(sort_options() :: [sort_option()] | sort_option()).
-doc "See `m:file_sorter` for a description of the options.".
-type(sort_option() :: {compressed, boolean()}
                     | {no_files, no_files()}
                     | {order, order()}
                     | {size, pos_integer()}
                     | {tmpdir, tmp_directory()}
                     | {unique, boolean()}).
-type(order() :: ascending | descending | order_fun()).
-type(order_fun() :: fun((term(), term()) -> boolean())).
-type(tmp_directory() :: [] | file:name()).
-doc "An integer > 1.".
-type(no_files() :: pos_integer()). % > 1
-type(key_pos() :: pos_integer() | [pos_integer()]).
-type(max_list_size() :: non_neg_integer()).
-type(cache() :: ets | list | no).
-type(tmp_file_usage() :: allowed | not_allowed | info_msg
                        | warning_msg  | error_msg).

-doc """
Returns a query handle. When evaluating query handle `QH`, all answers to the
first query handle in `QHL` are returned, followed by all answers to the
remaining query handles in `QHL`.
""".
-spec(append(QHL) -> QH when
      QHL :: [query_handle_or_list()],
      QH :: query_handle()).
append(QHs) ->
    Hs = [case get_handle(QH) of
              badarg -> erlang:error(badarg, [QHs]);
              H -> H
          end || QH <- QHs],
    #qlc_handle{h = #qlc_append{hl = Hs}}.

-doc """
Returns a query handle. When evaluating query handle `QH3`, all answers to `QH1`
are returned, followed by all answers to `QH2`.

[`append(QH1, QH2)`](`append/2`) is equivalent to
[`append([QH1, QH2])`](`append/1`).
""".
-spec(append(QH1, QH2) -> QH3 when
      QH1 :: query_handle_or_list(),
      QH2 :: query_handle_or_list(),
      QH3 :: query_handle()).
append(QH1, QH2) ->
    Hs = [case get_handle(QH) of
              badarg -> erlang:error(badarg, [QH1, QH2]);
              H -> H
          end || QH <- [QH1, QH2]],
    #qlc_handle{h = #qlc_append{hl = Hs}}.

-doc(#{equiv => cursor(QH, [])}).
-spec(cursor(QH) -> Cursor when
      QH :: query_handle_or_list(),
      Cursor :: query_cursor()).
cursor(QH) ->
    cursor(QH, []).

-doc """
Creates a query cursor and makes the calling process the owner of the cursor.

The cursor is to be used as argument to [`next_answers/1,2`](`next_answers/1`)
and (eventually) `delete_cursor/1`. Calls `erlang:spawn_opt/2` to spawn and link
to a process that evaluates the query handle. The value of option
`spawn_options` is used as last argument when calling
[`spawn_opt/2`](`spawn_opt/2`). Defaults to `[link]`.

_Example:_

```erlang
1> QH = qlc:q([{X,Y} || X <- [a,b], Y <- [1,2]]),
QC = qlc:cursor(QH),
qlc:next_answers(QC, 1).
[{a,1}]
2> qlc:next_answers(QC, 1).
[{a,2}]
3> qlc:next_answers(QC, all_remaining).
[{b,1},{b,2}]
4> qlc:delete_cursor(QC).
ok
```
""".
-spec(cursor(QH, Options) -> Cursor when
      QH :: query_handle_or_list(),
      Options :: [Option] | Option,
      Option :: {cache_all, cache()} | cache_all
              | {max_list_size, max_list_size()}
              | {spawn_options, spawn_options()}
              | {tmpdir_usage, tmp_file_usage()}
              | {tmpdir, tmp_directory()}
              | {unique_all, boolean()} | unique_all,
      Cursor :: query_cursor()).
cursor(QH, Options) ->
    case {options(Options, [unique_all, cache_all, tmpdir, 
                            spawn_options, max_list_size, 
                            tmpdir_usage]),
          get_handle(QH)} of
        {B1, B2} when B1 =:= badarg; B2 =:= badarg ->
            erlang:error(badarg, [QH, Options]);
        {[GUnique, GCache, TmpDir, SpawnOptions0, MaxList, TmpUsage], H} ->
            SpawnOptions = spawn_options(SpawnOptions0),
            case cursor_process(H, GUnique, GCache, TmpDir, 
                                SpawnOptions, MaxList, TmpUsage) of
                Pid when is_pid(Pid) ->
                    #qlc_cursor{c = {Pid, self()}};
                Error ->
                    Error
            end
    end.

-doc "Deletes a query cursor. Only the owner of the cursor can delete the cursor.".
-spec(delete_cursor(QueryCursor) -> ok when
      QueryCursor :: query_cursor()).
delete_cursor(#qlc_cursor{c = {_, Owner}}=C) when Owner =/= self() ->
    erlang:error(not_cursor_owner, [C]);
delete_cursor(#qlc_cursor{c = {Pid, _}}) ->
    stop_cursor(Pid);
delete_cursor(T) ->
    erlang:error(badarg, [T]).

-doc(#{equiv => eval(QH, [])}).
-spec(e(QH) -> Answers | Error when
      QH :: query_handle_or_list(),
      Answers :: answers(),
      Error :: {error, module(), Reason},
      Reason :: file_sorter:reason()).
e(QH) ->
    eval(QH, []).

-doc(#{equiv => eval(QH, Options)}).
-spec(e(QH, Options) -> Answers | Error when
      QH :: query_handle_or_list(),
      Options :: [Option] | Option,
      Option :: {cache_all, cache()} | cache_all
              | {max_list_size, max_list_size()}
              | {tmpdir_usage, tmp_file_usage()}
              | {tmpdir, tmp_directory()}
              | {unique_all, boolean()} | unique_all,
      Answers :: answers(),
      Error :: {error, module(), Reason},
      Reason :: file_sorter:reason()).
e(QH, Options) ->
    eval(QH, Options).

-doc(#{equiv => eval(QH, [])}).
-spec(eval(QH) -> Answers | Error when
      QH :: query_handle_or_list(),
      Answers :: answers(),
      Error :: {error, module(), Reason},
      Reason :: file_sorter:reason()).
eval(QH) ->
    eval(QH, []).

-doc """
Evaluates a query handle in the calling process and collects all answers in a
list.

_Example:_

```erlang
1> QH = qlc:q([{X,Y} || X <- [a,b], Y <- [1,2]]),
qlc:eval(QH).
[{a,1},{a,2},{b,1},{b,2}]
```
""".
-spec(eval(QH, Options) -> Answers | Error when
      QH :: query_handle_or_list(),
      Answers :: answers(),
      Options :: [Option] | Option,
      Option :: {cache_all, cache()} | cache_all
              | {max_list_size, max_list_size()}
              | {tmpdir_usage, tmp_file_usage()}
              | {tmpdir, tmp_directory()}
              | {unique_all, boolean()} | unique_all,
      Error :: {error, module(), Reason},
      Reason :: file_sorter:reason()).
eval(QH, Options) ->
    case {options(Options, [unique_all, cache_all, tmpdir, max_list_size,
                            tmpdir_usage]),
          get_handle(QH)} of
        {B1, B2} when B1 =:= badarg; B2 =:= badarg ->
            erlang:error(badarg, [QH, Options]);
        {[GUnique, GCache, TmpDir, MaxList, TmpUsage], Handle} ->
            try 
                Prep = prepare_qlc(Handle, [], GUnique, GCache, 
                                   TmpDir, MaxList, TmpUsage),
                case setup_qlc(Prep, #setup{parent = self()}) of
                    {L, Post, _LocalPost} when is_list(L) ->
                        post_funs(Post),
                        L;
                    {Objs, Post, _LocalPost} when is_function(Objs) ->
                        try
                            collect(Objs)
                        after
                            post_funs(Post)
                        end
                end
            catch throw:Term:Stacktrace ->
                case Stacktrace of
                    [?THROWN_ERROR | _] ->
                        Term;
                    _ ->
                        erlang:raise(throw, Term, Stacktrace)
                end
            end
    end.

-doc(#{equiv => fold(Function, Acc0, QH, [])}).
-spec(fold(Function, Acc0, QH) ->
               Acc1 | Error when
      QH :: query_handle_or_list(),
      Function :: fun((answer(), AccIn) -> AccOut),
      Acc0 :: term(),
      Acc1 :: term(),
      AccIn :: term(),
      AccOut :: term(),
      Error :: {error, module(), Reason},
      Reason :: file_sorter:reason()).
fold(Fun, Acc0, QH) ->
    fold(Fun, Acc0, QH, []).

-doc """
Calls `Function` on successive answers to the query handle together with an
extra argument `AccIn`.

The query handle and the function are evaluated in the
calling process. `Function` must return a new accumulator, which is passed to
the next call. `Acc0` is returned if there are no answers to the query handle.

_Example:_

```erlang
1> QH = [1,2,3,4,5,6],
qlc:fold(fun(X, Sum) -> X + Sum end, 0, QH).
21
```
""".
-spec(fold(Function, Acc0, QH, Options) ->
               Acc1 | Error when
      QH :: query_handle_or_list(),
      Function :: fun((answer(), AccIn) -> AccOut),
      Acc0 :: term(),
      Acc1 :: term(),
      AccIn :: term(),
      AccOut :: term(),
      Options :: [Option] | Option,
      Option :: {cache_all, cache()} | cache_all
              | {max_list_size, max_list_size()}
              | {tmpdir_usage, tmp_file_usage()}
              | {tmpdir, tmp_directory()}
              | {unique_all, boolean()} | unique_all,
      Error :: {error, module(), Reason},
      Reason :: file_sorter:reason()).
fold(Fun, Acc0, QH, Options) ->
    case {options(Options, [unique_all, cache_all, tmpdir, max_list_size,
                            tmpdir_usage]), 
          get_handle(QH)} of
        {B1, B2} when B1 =:= badarg; B2 =:= badarg ->
            erlang:error(badarg, [Fun, Acc0, QH, Options]);
        {[GUnique, GCache, TmpDir, MaxList, TmpUsage], Handle} ->
            try
                Prep = prepare_qlc(Handle, not_a_list, GUnique, GCache,
                                   TmpDir, MaxList, TmpUsage),
                case setup_qlc(Prep, #setup{parent = self()}) of
                    {Objs, Post, _LocalPost} when is_function(Objs); 
                                                  is_list(Objs) ->
                        try
                            fold_loop(Fun, Objs, Acc0)
                        after
                            post_funs(Post)
                        end
                end
            catch throw:Term:Stacktrace ->
                case Stacktrace of
                    [?THROWN_ERROR | _] ->
                        Term;
                    _ ->
                        erlang:raise(throw, Term, Stacktrace)
                end
            end
    end.

-doc """
Returns a descriptive string in English of an error tuple returned by some of
the functions of the `qlc` module or the parse transform. This function is
mainly used by the compiler invoking the parse transform.
""".
-spec(format_error(Error) -> Chars when
      Error :: {error, module(), term()},
      Chars :: io_lib:chars()).
format_error(not_a_query_list_comprehension) ->
    io_lib:format("argument is not a query list comprehension", []);
format_error({used_generator_variable, V}) ->
    io_lib:format("generated variable ~w must not be used in list expression",
                  [V]);
format_error(binary_generator) ->
    io_lib:format("cannot handle binary generators", []);
format_error(too_complex_join) ->
    io_lib:format("cannot handle join of three or more generators efficiently",
                  []);
format_error(too_many_joins) ->
    io_lib:format("cannot handle more than one join efficiently", []);
format_error(nomatch_pattern) ->
    io_lib:format("pattern cannot possibly match", []);
format_error(nomatch_filter) ->
    io_lib:format("filter evaluates to 'false'", []);
format_error({Location, Mod, Reason}) when is_integer(Location);
                                           tuple_size(Location) =:= 2 ->
    io_lib:format("~s: ~ts~n",
                  [pos(Location), lists:flatten(Mod:format_error(Reason))]);
%% file_sorter errors
format_error({bad_object, FileName}) ->
    io_lib:format("the temporary file \"~ts\" holding answers is corrupt",
                 [FileName]);
format_error(bad_object) ->
    io_lib:format("the keys could not be extracted from some term", []);
format_error({file_error, FileName, Reason}) ->
    io_lib:format("\"~ts\": ~tp~n",[FileName, file:format_error(Reason)]);
format_error({premature_eof, FileName}) ->
    io_lib:format("\"~ts\": end-of-file was encountered inside some binary term", 
                  [FileName]);
format_error({tmpdir_usage, Why}) ->
    io_lib:format("temporary file was needed for ~w~n", [Why]);
format_error({error, Module, Reason}) ->
    Module:format_error(Reason);
format_error(E) ->
    io_lib:format("~tp~n", [E]).

pos({Line,Col}) ->
    io_lib:format("~w:~w", [Line,Col]);
pos(Line) ->
    io_lib:format("~w", [Line]).

-doc(#{equiv => info(QH, [])}).
-spec(info(QH) -> Info when
      QH :: query_handle_or_list(),
      Info :: abstract_expr() | string()).
info(QH) ->
    info(QH, []).

-doc """
Returns information about a query handle. The information describes the
simplifications and optimizations that are the results of preparing the query
for evaluation. This function is probably mainly useful during debugging.

The information has the form of an Erlang expression where QLCs most likely
occur. Depending on the format functions of mentioned QLC tables, it is not
certain that the information is absolutely accurate.

Options:

- The default is to return a sequence of QLCs in a block, but if option
  `{flat, false}` is specified, one single QLC is returned.
- The default is to return a string, but if option `{format, abstract_code}` is
  specified, abstract code is returned instead. In the abstract code, port
  identifiers, references, and pids are represented by strings.
- The default is to return all elements in lists, but if option
  `{n_elements, NElements}` is specified, only a limited number of elements are
  returned.
- The default is to show all parts of objects and match specifications, but if
  option `{depth, Depth}` is specified, parts of terms below a certain depth are
  replaced by `'...'`.

_Examples:_

In the following example two simple QLCs are inserted only to hold option
`{unique, true}`:

```erlang
1> QH = qlc:q([{X,Y} || X <- [x,y], Y <- [a,b]]),
io:format("~s~n", [qlc:info(QH, unique_all)]).
begin
    V1 =
        qlc:q([
               SQV ||
                   SQV <- [x, y]
              ],
              [{unique, true}]),
    V2 =
        qlc:q([
               SQV ||
                   SQV <- [a, b]
              ],
              [{unique, true}]),
    qlc:q([
           {X,Y} ||
               X <- V1,
               Y <- V2
          ],
          [{unique, true}])
end
```

In the following example QLC `V2` has been inserted to show the joined
generators and the join method chosen. A convention is used for lookup join: the
first generator (`G2`) is the one traversed, the second (`G1`) is the table
where constants are looked up.

```erlang
1> E1 = ets:new(e1, []),
E2 = ets:new(e2, []),
true = ets:insert(E1, [{1,a},{2,b}]),
true = ets:insert(E2, [{a,1},{b,2}]),
Q = qlc:q([{X,Z,W} ||
{X, Z} <- ets:table(E1),
{W, Y} <- ets:table(E2),
X =:= Y]),
io:format("~s~n", [qlc:info(Q)]).
begin
    V1 =
        qlc:q([
               P0 ||
                   P0 = {W, Y} <-
                       ets:table(#Ref<0.3098908599.2283929601.256549>)
              ]),
    V2 =
        qlc:q([
               [G1 | G2] ||
                   G2 <- V1,
                   G1 <-
                       ets:table(#Ref<0.3098908599.2283929601.256548>),
                   element(2, G1) =:= element(1, G2)
              ],
              [{join, lookup}]),
    qlc:q([
           {X, Z, W} ||
               [{X, Z} | {W, Y}] <- V2
          ])
end
```
""".
-spec(info(QH, Options) -> Info when
      QH :: query_handle_or_list(),
      Options :: [Option] | Option,
      Option :: EvalOption | ReturnOption,
      EvalOption :: {cache_all, cache()} | cache_all
                  | {max_list_size, max_list_size()}
                  | {tmpdir_usage, tmp_file_usage()}
                  | {tmpdir, tmp_directory()}
                  | {unique_all, boolean()} | unique_all,
      ReturnOption :: {depth, Depth}
                    | {flat, boolean()}
                    | {format, Format}
                    | {n_elements, NElements},
      Depth :: infinity | non_neg_integer(),
      Format :: abstract_code | string,
      NElements :: infinity | pos_integer(),
      Info :: abstract_expr() | string()).
info(QH, Options) ->
    case {options(Options, [unique_all, cache_all, flat, format, n_elements, 
                            depth, tmpdir, max_list_size, tmpdir_usage]),
          get_handle(QH)} of
        {B1, B2} when B1 =:= badarg; B2 =:= badarg ->
            erlang:error(badarg, [QH, Options]);
        {[GUnique, GCache, Flat, Format, NElements, 
          Depth, TmpDir, MaxList, TmpUsage],
         H} ->
            try
                Prep = prepare_qlc(H, [], GUnique, GCache, 
                                   TmpDir, MaxList, TmpUsage),
                Info = le_info(Prep, {NElements,Depth}),
                AbstractCode = abstract(Info, Flat, NElements, Depth),
                case Format of
                    abstract_code ->
                        abstract_code(AbstractCode);
                    string ->
                        Hook = fun({special, _Anno, String}, _I, _P, _F) ->
                                       String
                               end,
                        lists:flatten(erl_pp:expr(AbstractCode, 0, Hook));
                    debug -> % Not documented. Intended for testing only.
                        Info
                end
            catch throw:Term:Stacktrace ->
                case Stacktrace of
                    [?THROWN_ERROR | _] ->
                        Term;
                    _ ->
                        erlang:raise(throw, Term, Stacktrace)
                end
            end
    end.

-doc(#{equiv => keysort(KeyPos, QH1, [])}).
-spec(keysort(KeyPos, QH1) -> QH2 when
      KeyPos :: key_pos(),
      QH1 :: query_handle_or_list(),
      QH2 :: query_handle()).
keysort(KeyPos, QH) ->
    keysort(KeyPos, QH, []).

-doc """
Returns a query handle. When evaluating query handle `QH2`, the answers to query
handle `QH1` are sorted by `file_sorter:keysort/4` according to the options.

The sorter uses temporary files only if `QH1` does not evaluate to a list and
the size of the binary representation of the answers exceeds `Size` bytes, where
`Size` is the value of option `size`.
""".
-spec(keysort(KeyPos, QH1, SortOptions) -> QH2 when
      KeyPos :: key_pos(),
      SortOptions :: sort_options(),
      QH1 :: query_handle_or_list(),
      QH2 :: query_handle()).
keysort(KeyPos, QH, Options) ->
    case {is_keypos(KeyPos), 
          options(Options, [tmpdir, order, unique, compressed, 
                            size, no_files]),
          get_handle(QH)} of
        {true, [TmpDir, Order, Unique,Compressed | _], H} when H =/= badarg ->
            #qlc_handle{h = #qlc_sort{h = H, keypos = {keysort,KeyPos}, 
                                      unique = Unique, 
                                      compressed = Compressed,
                                      order = Order, 
                                      fs_opts = listify(Options), 
                                      tmpdir = TmpDir}};
        _ ->
            erlang:error(badarg, [KeyPos, QH, Options])
    end.

-define(DEFAULT_NUM_OF_ANSWERS, 10).

-doc(#{equiv => next_answers(C, ?DEFAULT_NUM_OF_ANSWERS)}).
-spec(next_answers(QueryCursor) ->
            Answers | Error when
      QueryCursor :: query_cursor(),
      Answers :: answers(),
      Error :: {error, module(), Reason},
      Reason :: file_sorter:reason()).
next_answers(C) ->
    next_answers(C, ?DEFAULT_NUM_OF_ANSWERS).

-doc """
Returns some or all of the remaining answers to a query cursor. Only the owner
of `QueryCursor` can retrieve answers.

Argument `NumberOfAnswers` determines the maximum number of answers
returned. If less than the requested number of answers is
returned, subsequent calls to `next_answers` return `[]`.
""".
-spec(next_answers(QueryCursor, NumberOfAnswers) ->
            Answers | Error when
      QueryCursor :: query_cursor(),
      Answers :: answers(),
      NumberOfAnswers :: all_remaining | pos_integer(),
      Error :: {error, module(), Reason},
      Reason :: file_sorter:reason()).
next_answers(#qlc_cursor{c = {_, Owner}}=C, 
             NumOfAnswers) when Owner =/= self() ->
    erlang:error(not_cursor_owner, [C, NumOfAnswers]);
next_answers(#qlc_cursor{c = {Pid, _}}=C, NumOfAnswers) ->
    N = case NumOfAnswers of
            all_remaining -> -1;
            _ when is_integer(NumOfAnswers), NumOfAnswers > 0 -> NumOfAnswers;
            _ -> erlang:error(badarg, [C, NumOfAnswers])
        end,
    next_loop(Pid, [], N);
next_answers(T1, T2) ->
    erlang:error(badarg, [T1, T2]).

-doc false.
-spec(parse_transform(Forms, Options) -> Forms2 when
      Forms :: [erl_parse:abstract_form()],
      Forms2 :: [erl_parse:abstract_form()],
      Options :: [Option],
      Option :: type_checker | compile:option()).

parse_transform(Forms, Options) ->
    qlc_pt:parse_transform(Forms, Options).

-doc false.
-spec parse_transform_info() -> #{'error_location' => 'column'}.

parse_transform_info() ->
    #{error_location => column}.

%% The funcspecs qlc:q/1 and qlc:q/2 are known by erl_eval.erl and
%% erl_lint.erl.
-doc(#{equiv => q(QLC, [])}).
-spec(q(QLC) -> QH when
      QLC :: query_list_comprehension(),
      QH :: query_handle()).
q(QLC_lc) ->
    q(QLC_lc, []).

-doc """
Returns a query handle for a QLC. The QLC must be the first argument to this
function, otherwise it is evaluated as an ordinary list comprehension. It is
also necessary to add the following line to the source code:

```erlang
-include_lib("stdlib/include/qlc.hrl").
```

This causes a parse transform to substitute a fun for the QLC. The (compiled)
fun is called when the query handle is evaluated.

When calling `qlc:q/1,2` from the Erlang shell, the parse transform is
automatically called. When this occurs, the fun substituted for the QLC is not
compiled but is evaluated by `m:erl_eval`. This is also true when expressions
are evaluated by `file:eval/1,2` or in the debugger.

To be explicit, this does not work:

```erlang
...
A = [X || {X} <- [{1},{2}]],
QH = qlc:q(A),
...
```

Variable `A` is bound to the evaluated value of the list comprehension
(`[1,2]`). The compiler complains with an error message ("argument is not a
query list comprehension"); the shell process stops with a `badarg` reason.

Options:

- Option `{cache, ets}` can be used to cache the answers to a QLC. The answers
  are stored in one ETS table for each cached QLC. When a cached QLC is
  evaluated again, answers are fetched from the table without any further
  computations. Therefore, when all answers to a cached QLC have been found, the
  ETS tables used for caching answers to the qualifiers of the QLC can be
  emptied. Option `cache` is equivalent to `{cache, ets}`.
- Option `{cache, list}` can be used to cache the answers to a QLC like
  `{cache, ets}`. The difference is that the answers are kept in a list (on the
  process heap). If the answers would occupy more than a certain amount of RAM
  memory, a temporary file is used for storing the answers. Option
  `max_list_size` sets the limit in bytes and the temporary file is put on the
  directory set by option `tmpdir`.

  Option `cache` has no effect if it is known that the QLC is to be evaluated at
  most once. This is always true for the top-most QLC and also for the list
  expression of the first generator in a list of qualifiers. Notice that in the
  presence of side effects in filters or callback functions, the answers to QLCs
  can be affected by option `cache`.

- Option `{unique, true}` can be used to remove duplicate answers to a QLC. The
  unique answers are stored in one ETS table for each QLC. The table is emptied
  every time it is known that there are no more answers to the QLC. Option
  `unique` is equivalent to `{unique, true}`. If option `unique` is combined
  with option `{cache, ets}`, two ETS tables are used, but the full answers are
  stored in one table only. If option `unique` is combined with option
  `{cache, list}`, the answers are sorted twice using `keysort/3`; once to
  remove duplicates and once to restore the order.

Options `cache` and `unique` apply not only to the QLC itself but also to the
results of looking up constants, running match specifications, and joining
handles.

_Example:_

In the following example the cached results of the merge join are traversed for
each value of `A`. Notice that without option `cache` the join would have been
carried out three times, once for each value of `A`.

```erlang
1> Q = qlc:q([{A,X,Z,W} ||
A <- [a,b,c],
{X,Z} <- [{a,1},{b,4},{c,6}],
{W,Y} <- [{2,a},{3,b},{4,c}],
X =:= Y],
{cache, list}),
io:format("~s~n", [qlc:info(Q)]).
begin
    V1 =
        qlc:q([
               P0 ||
                   P0 = {X, Z} <-
                       qlc:keysort(1, [{a, 1}, {b, 4}, {c, 6}], [])
              ]),
    V2 =
        qlc:q([
               P0 ||
                   P0 = {W, Y} <-
                       qlc:keysort(2, [{2, a}, {3, b}, {4, c}], [])
              ]),
    V3 =
        qlc:q([
               [G1 | G2] ||
                   G1 <- V1,
                   G2 <- V2,
                   element(1, G1) == element(2, G2)
              ],
              [{join, merge}, {cache, list}]),
    qlc:q([
           {A, X, Z, W} ||
               A <- [a, b, c],
               [{X, Z} | {W, Y}] <- V3,
               X =:= Y
          ])
end
```

[`sort/1,2`](`sort/1`) and [`keysort/2,3`](`keysort/2`) can also be used for
caching answers and for removing duplicates. When sorting answers are cached in
a list, possibly stored on a temporary file, and no ETS tables are used.

Sometimes (see `table/2`) traversal of tables can be done by looking up key
values, which is assumed to be fast. Under certain (rare) circumstances there
can be too many key values to look up. [](){: #max_lookup } Option
`{max_lookup, MaxLookup}` can then be used to limit the number of lookups: if
more than `MaxLookup` lookups would be required, no lookups are done but the
table is traversed instead. Defaults to `infinity`, which means that there is no
limit on the number of keys to look up.

_Example:_

In the following example, using the `gb_table` module from section
[Implementing a QLC Table](`m:qlc#implementing_a_qlc_table`), there are six keys
to look up: `{1,a}`, `{1,b}`, `{1,c}`, `{2,a}`, `{2,b}`, and `{2,c}`. The reason
is that the two elements of key `{X, Y}` are compared separately.

```erlang
1> T = gb_trees:empty(),
QH = qlc:q([X || {{X,Y},_} <- gb_table:table(T),
((X == 1) or (X == 2)) andalso
((Y == a) or (Y == b) or (Y == c))]),
io:format("~s~n", [qlc:info(QH)]).
ets:match_spec_run(
       lists:flatmap(fun(K) ->
                            case
                                gb_trees:lookup(K,
                                                gb_trees:from_orddict([]))
                            of
                                {value, V} ->
                                    [{K, V}];
                                none ->
                                    []
                            end
                     end,
                     [{1, a},
                      {1, b},
                      {1, c},
                      {2, a},
                      {2, b},
                      {2, c}]),
       ets:match_spec_compile([{{{'$1', '$2'}, '_'},
                                [],
                                ['$1']}]))
```

Options:

- Option `{lookup, true}` can be used to ensure that the `qlc` module looks up
  constants in some QLC table. If there are more than one QLC table among the
  list expressions of the generators, constants must be looked up in at least
  one of the tables. The evaluation of the query fails if there are no constants
  to look up. This option is useful when it would be unacceptable to traverse
  all objects in some table. Setting option `lookup` to `false` ensures that no
  constants are looked up (`{max_lookup, 0}` has the same effect). Defaults to
  `any`, which means that constants are looked up whenever possible.
- Option `{join, Join}` can be used to ensure that a certain join method is
  used:

  - `{join, lookup}` invokes the lookup join method.
  - `{join, merge}` invokes the merge join method.
  - `{join, nested_loop}` invokes the method of matching every pair of objects
    from two handles. This method is mostly very slow.

  The evaluation of the query fails if the `qlc` module cannot carry out the
  chosen join method. Defaults to `any`, which means that some fast join method
  is used if possible.
""".
-spec(q(QLC, Options) -> QH when
      QH :: query_handle(),
      Options :: [Option] | Option,
      Option :: {max_lookup, MaxLookup}
              | {cache, cache()} | cache
              | {join, Join}
              | {lookup, Lookup}
              | {unique, boolean()} | unique,
      MaxLookup :: non_neg_integer() | infinity,
      Join :: any | lookup | merge | nested_loop,
      Lookup :: boolean() | any,
      QLC :: query_list_comprehension()).
q(#qlc_lc{}=QLC_lc, Options) ->
    case options(Options, [unique, cache, max_lookup, join, lookup]) of
        [Unique, Cache, Max, Join, Lookup] ->
            Opt = #qlc_opt{unique = Unique, cache = Cache, 
                           max_lookup = Max, join = Join, lookup = Lookup},
            #qlc_handle{h = QLC_lc#qlc_lc{opt = Opt}};
        _ ->
            erlang:error(badarg, [QLC_lc, Options])
    end;
q(T1, T2) ->
    erlang:error(badarg, [T1, T2]).

-doc(#{equiv => sort(QH, [])}).
-spec(sort(QH1) -> QH2 when
      QH1 :: query_handle_or_list(),
      QH2 :: query_handle()).
sort(QH) ->
    sort(QH, []).

-doc """
Returns a query handle. When evaluating query handle `QH2`, the answers to query
handle `QH1` are sorted by `file_sorter:sort/3` according to the options.

The sorter uses temporary files only if `QH1` does not evaluate to a list and
the size of the binary representation of the answers exceeds `Size` bytes, where
`Size` is the value of option `size`.
""".
-spec(sort(QH1, SortOptions) -> QH2 when
      SortOptions :: sort_options(),
      QH1 :: query_handle_or_list(),
      QH2 :: query_handle()).
sort(QH, Options) ->
    case {options(Options, [tmpdir, order, unique, compressed, 
                            size, no_files]), get_handle(QH)} of
        {B1, B2} when B1 =:= badarg; B2 =:= badarg ->
            erlang:error(badarg, [QH, Options]);
        {[TD, Order, Unique, Compressed | _], H} ->
            #qlc_handle{h = #qlc_sort{h = H, keypos = sort, unique = Unique, 
                                      compressed = Compressed, order = Order,
                                      fs_opts = listify(Options), 
                                      tmpdir = TD}}
    end.

%% Note that the generated code is evaluated by (the slow) erl_eval.
-doc(#{equiv => string_to_handle(QueryString, [])}).
-spec(string_to_handle(QueryString) -> QH | Error when
      QueryString :: string(),
      QH :: query_handle(),
      Error :: {error, module(), Reason},
      Reason :: erl_parse:error_info() | erl_scan:error_info()).
string_to_handle(Str) ->
    string_to_handle(Str, []).

-doc(#{equiv => string_to_handle(QueryString, Options, erl_eval:new_bindings())}).
-spec(string_to_handle(QueryString, Options) -> QH | Error when
      QueryString :: string(),
      Options :: [Option] | Option,
      Option :: {max_lookup, MaxLookup}
              | {cache, cache()} | cache
              | {join, Join}
              | {lookup, Lookup}
              | {unique, boolean()} | unique,
      MaxLookup :: non_neg_integer() | infinity,
      Join :: any | lookup | merge | nested_loop,
      Lookup :: boolean() | any,
      QH :: query_handle(),
      Error :: {error, module(), Reason},
      Reason :: erl_parse:error_info() | erl_scan:error_info()).
string_to_handle(Str, Options) ->
    string_to_handle(Str, Options, erl_eval:new_bindings()).

-doc """
A string version of [`q/1,2`](`q/1`). When the query handle is evaluated, the
fun created by the parse transform is interpreted by `m:erl_eval`. The query
string is to be one single QLC terminated by a period.

_Example:_

```erlang
1> L = [1,2,3],
Bs = erl_eval:add_binding('L', L, erl_eval:new_bindings()),
QH = qlc:string_to_handle("[X+1 || X <- L].", [], Bs),
qlc:eval(QH).
[2,3,4]
```

This function is probably mainly useful when called from outside of Erlang, for
example from a driver written in C.

> #### Note {: .info }
>
> Query handles created this way may have worse performance than when created
> directly via [`q/1,2`](`q/1`).
""".
-spec(string_to_handle(QueryString, Options, Bindings) -> QH | Error when
      QueryString :: string(),
      Options :: [Option] | Option,
      Option :: {max_lookup, MaxLookup}
              | {cache, cache()} | cache
              | {join, Join}
              | {lookup, Lookup}
              | {unique, boolean()} | unique,
      MaxLookup :: non_neg_integer() | infinity,
      Join :: any | lookup | merge | nested_loop,
      Lookup :: boolean() | any,
      Bindings :: erl_eval:binding_struct(),
      QH :: query_handle(),
      Error :: {error, module(), Reason},
      Reason :: erl_parse:error_info() | erl_scan:error_info()).
string_to_handle(Str, Options, Bindings) when is_list(Str) ->
    case options(Options, [unique, cache, max_lookup, join, lookup]) of
        badarg ->
            erlang:error(badarg, [Str, Options, Bindings]);
        [Unique, Cache, MaxLookup, Join, Lookup] ->
            case erl_scan:string(Str, 1, [text]) of
                {ok, Tokens, _} ->
                    ScanRes =
                        case erl_eval:extended_parse_exprs(Tokens) of
                            {ok, [Expr0]} ->
                                {ok, Expr0};
                            {ok, _ExprList} ->
                                erlang:error(badarg,
                                             [Str, Options, Bindings]);
                            E ->
                                E
                        end,
                    case ScanRes of
                        {ok, Expr} ->
                            case qlc_pt:transform_expression(Expr, Bindings) of
                                {ok, {call, _, _QlcQ,  Handle}} ->
                                    {value, QLC_lc, _} = 
                                        erl_eval:exprs(Handle, Bindings),
                                    O = #qlc_opt{unique = Unique, 
                                                 cache = Cache,
                                                 max_lookup = MaxLookup, 
                                                 join = Join,
                                                 lookup = Lookup},
                                    #qlc_handle{h = QLC_lc#qlc_lc{opt = O}};
                                {not_ok, [{error, Error} | _]} ->
                                    error(Error)
                            end;
                        {error, ErrorInfo} ->
                            error(ErrorInfo)
                    end;
                {error, ErrorInfo, _EndLocation} ->
                    error(ErrorInfo)
            end
    end;
string_to_handle(T1, T2, T3) ->    
    erlang:error(badarg, [T1, T2, T3]).

-doc """
Returns a query handle for a QLC table. In Erlang/OTP there is support for ETS,
Dets, and Mnesia tables, but many other data structures can be turned into QLC
tables. This is accomplished by letting function(s) in the module implementing
the data structure create a query handle by calling `qlc:table/2`.

The different ways to traverse the table and properties of the table are handled
by callback functions provided as options to `qlc:table/2`.

- Callback function `TraverseFun` is used for traversing the table. It is to
  return a list of objects terminated by either `[]` or a nullary fun to be used
  for traversing the not yet traversed objects of the table. Any other return
  value is immediately returned as value of the query evaluation. Unary
  `TraverseFun`s are to accept a match specification as argument. The match
  specification is created by the parse transform by analyzing the pattern of
  the generator calling `qlc:table/2` and filters using variables introduced in
  the pattern. If the parse transform cannot find a match specification
  equivalent to the pattern and filters, `TraverseFun` is called with a match
  specification returning every object.

  - Modules that can use match specifications for optimized traversal of tables
    are to call `qlc:table/2` with an unary `TraverseFun`. An example is
    `ets:table/2`.
  - Other modules can provide a nullary `TraverseFun`. An example is
    [`gb_table:table/1`](`m:qlc#gb_table`) in section
    [Implementing a QLC Table](`m:qlc#implementing_a_qlc_table`).

- Unary callback function `PreFun` is called once before the table is read for
  the first time. If the call fails, the query evaluation fails.

  Argument `PreArgs` is a list of tagged values. There are two tags,
  `parent_value` and `stop_fun`, used by Mnesia for managing transactions.

  - The value of `parent_value` is the value returned by `ParentFun`, or
    `undefined` if there is no `ParentFun`. `ParentFun` is called once just
    before the call of `PreFun` in the context of the process calling
    [`eval/1,2`](`eval/1`), [`fold/3,4`](`fold/3`), or
    [`cursor/1,2`](`cursor/1`).
  - The value of `stop_fun` is a nullary fun that deletes the cursor if called
    from the parent, or `undefined` if there is no cursor.

- Nullary callback function `PostFun` is called once after the table was last
  read. The return value, which is caught, is ignored. If `PreFun` has been
  called for a table, `PostFun` is guaranteed to be called for that table, even
  if the evaluation of the query fails for some reason.

  The pre (post) functions for different tables are evaluated in unspecified
  order.

  Other table access than reading, such as calling `InfoFun`, is assumed to be
  OK at any time.

- [](){: #lookup_fun } Binary callback function `LookupFun` is used for looking
  up objects in the table. The first argument `Position` is the key position or
  an indexed position and the second argument `Keys` is a sorted list of unique
  values. The return value is to be a list of all objects (tuples), such that
  the element at `Position` is a member of `Keys`. Any other return value is
  immediately returned as value of the query evaluation. `LookupFun` is called
  instead of traversing the table if the parse transform at compile time can
  determine that the filters match and compare the element at `Position` in such
  a way that only `Keys` need to be looked up to find all potential answers.

  The key position is obtained by calling `InfoFun(keypos)` and the indexed
  positions by calling `InfoFun(indices)`. If the key position can be used for
  lookup, it is always chosen, otherwise the indexed position requiring the
  least number of lookups is chosen. If there is a tie between two indexed
  positions, the one occurring first in the list returned by `InfoFun` is
  chosen. Positions requiring more than [max_lookup](`m:qlc#max_lookup`) lookups
  are ignored.

- Unary callback function `InfoFun` is to return information about the table.
  `undefined` is to be returned if the value of some tag is unknown:

  - **`indices`** - Returns a list of indexed positions, a list of positive
    integers.

  - **`is_unique_objects`** - Returns `true` if the objects returned by
    `TraverseFun` are unique.

  - **`keypos`** - Returns the position of the table key, a positive integer.

  - **`is_sorted_key`** - Returns `true` if the objects returned by
    `TraverseFun` are sorted on the key.

  - **`num_of_objects`** - Returns the number of objects in the table, a
    non-negative integer.

- Unary callback function `FormatFun` is used by [`info/1,2`](`info/1`) for
  displaying the call that created the query handle of the table. Defaults to
  `undefined`, which means that `info/1,2` displays a call to `'$MOD':'$FUN'/0`.
  It is up to `FormatFun` to present the selected objects of the table in a
  suitable way. However, if a character list is chosen for presentation, it must
  be an Erlang expression that can be scanned and parsed (a trailing dot is
  added by `info/1,2` though).

  `FormatFun` is called with an argument that describes the selected objects
  based on optimizations done as a result of analyzing the filters of the QLC
  where the call to `qlc:table/2` occurs. The argument can have the following
  values:

  - **`{lookup, Position, Keys, NElements, DepthFun}`.** - `LookupFun` is used
    for looking up objects in the table.

  - **`{match_spec, MatchExpression}`** - No way of finding all possible answers
    by looking up keys was found, but the filters could be transformed into a
    match specification. All answers are found by calling
    `TraverseFun(MatchExpression)`.

  - **`{all, NElements, DepthFun}`** - No optimization was found. A match
    specification matching all objects is used if `TraverseFun` is unary.

    `NElements` is the value of the `info/1,2` option `n_elements`.

    `DepthFun` is a function that can be used for limiting the size of terms;
    calling `DepthFun(Term)` substitutes `'...'` for parts of `Term` below the
    depth specified by the `info/1,2` option `depth`.

    If calling `FormatFun` with an argument including `NElements` and `DepthFun`
    fails, `FormatFun` is called once again with an argument excluding
    `NElements` and `DepthFun` (`{lookup, Position, Keys}` or `all`).

- [](){: #key_equality } The value of option `key_equality` is to be `'=:='` if
  the table considers two keys equal if they match, and to be `'=='` if two keys
  are equal if they compare equal. Defaults to `'=:='`.

For the various options recognized by `table/1,2` in respective module, see
[`ets`](`ets:table/1`), [`dets`](`dets:table/1`), and
[`mnesia`](`mnesia:table/1`).
""".
-spec table(TraverseFun, Options) -> QH when
      TraverseFun :: TraverseFun0 | TraverseFun1,
      TraverseFun0 :: fun(() -> TraverseResult),
      TraverseFun1 :: fun((match_expression()) -> TraverseResult),
      TraverseResult :: Objects | term(),
      Objects :: [] | [term() | ObjectList],
      ObjectList :: TraverseFun0 | Objects,
      Options :: [Option] | Option,
      Option :: {format_fun, FormatFun}
              | {info_fun, InfoFun}
              | {lookup_fun, LookupFun}
              | {parent_fun, ParentFun}
              | {post_fun, PostFun}
              | {pre_fun, PreFun}
              | {key_equality, KeyComparison},
      FormatFun :: undefined  | fun((SelectedObjects) -> FormatedTable),
      SelectedObjects :: all
                       | {all, NElements, DepthFun}
                       | {match_spec, match_expression()}
                       | {lookup, Position, Keys}
                       | {lookup, Position, Keys, NElements, DepthFun},
      NElements :: infinity | pos_integer(),
      DepthFun :: fun((term()) -> term()),
      FormatedTable :: {Mod, Fun, Args}
                     | abstract_expr()
                     | string(),
      InfoFun :: undefined  | fun((InfoTag) -> InfoValue),
      InfoTag :: indices | is_unique_objects | keypos | num_of_objects,
      InfoValue :: undefined  | term(),
      LookupFun :: undefined  | fun((Position, Keys) -> LookupResult),
      LookupResult :: [term()] | term(),
      ParentFun :: undefined  | fun(() -> ParentFunValue),
      PostFun :: undefined  | fun(() -> term()),
      PreFun :: undefined  | fun((PreArgs) -> term()),
      PreArgs :: [PreArg],
      PreArg :: {parent_value, ParentFunValue}  | {stop_fun, StopFun},
      ParentFunValue :: undefined  | term(),
      StopFun :: undefined  | fun(() -> term()),
      KeyComparison :: '=:=' | '==',
      Position :: pos_integer(),
      Keys :: [term()],
      Mod :: atom(),
      Fun :: atom(),
      Args :: [term()],
      QH :: query_handle().
table(TraverseFun, Options) when is_function(TraverseFun) ->
    case {is_function(TraverseFun, 0), 
          IsFun1 = is_function(TraverseFun, 1)} of
        {false, false} ->
            erlang:error(badarg, [TraverseFun, Options]);
        _ ->
            case options(Options, [pre_fun, post_fun, info_fun, format_fun, 
                                   lookup_fun, parent_fun, key_equality]) of
                [PreFun, PostFun, InfoFun, FormatFun, LookupFun, ParentFun,
                 KeyEquality] ->
                    T = #qlc_table{trav_fun = TraverseFun, pre_fun = PreFun,
                                   post_fun = PostFun, info_fun = InfoFun, 
                                   parent_fun = ParentFun,
                                   trav_MS = IsFun1,
                                   format_fun = FormatFun, 
                                   lookup_fun = LookupFun,
                                   key_equality = KeyEquality},
                    #qlc_handle{h = T};
                badarg ->
                    erlang:error(badarg, [TraverseFun, Options])
            end
    end;
table(T1, T2) ->
    erlang:error(badarg, [T1, T2]).

-doc false.
-spec(transform_from_evaluator(LC, Bs) -> Return when
      LC :: abstract_expr(),
      Bs :: erl_eval:binding_struct(),
      Return :: {ok, abstract_expr()}
              | {not_ok, {error, module(), Reason :: term()}}).

transform_from_evaluator(LC, Bs0) ->
    qlc_pt:transform_from_evaluator(LC, Bs0).

-define(TEMPLATE_STATE, 1).

-doc false.
template_state() ->
    ?TEMPLATE_STATE.

-doc false.
aux_name(Name, N, AllNames) ->
    {VN, _} = aux_name1(Name, N, AllNames),
    VN.

-doc false.
name_suffix(A, Suff) ->
    list_to_atom(lists:concat([A, Suff])).

-doc false.
vars(E) ->
    var_ufold(fun({var,_A,V}) -> V end, E).

-doc false.
var_ufold(F, E) ->
    ordsets:from_list(var_fold(F, [], E)).

-doc false.
all_selections([]) ->
    [[]];
all_selections([{I,Cs} | ICs]) ->
    [[{I,C} | L] || C <- Cs, L <- all_selections(ICs)].

%%%
%%% Local functions
%%%

aux_name1(Name, N, AllNames) ->
    SN = name_suffix(Name, N),
    case gb_sets:is_member(SN, AllNames) of
        true -> aux_name1(Name, N + 1, AllNames);
        false -> {SN, N}
    end.

-doc false.
var_fold(F, A, {var,_,V}=Var) when V =/= '_' ->
    [F(Var) | A];
var_fold(F, A, T) when is_tuple(T) ->
    var_fold(F, A, tuple_to_list(T));
var_fold(F, A, [E | Es]) ->
    var_fold(F, var_fold(F, A, E), Es);
var_fold(_F, A, _T) ->
    A.

options(Options, Keys) when is_list(Options) ->
    options(Options, Keys, []);
options(Option, Keys) ->
    options([Option], Keys, []).

options(Options0, [Key | Keys], L) when is_list(Options0) ->
    Options = case lists:member(Key, Options0) of
                  true -> 
                      [atom_option(Key) | lists:delete(Key, Options0)];
                  false ->
                      Options0
              end,
    V = case lists:keyfind(Key, 1, Options) of
            {format_fun, U=undefined} ->
                {ok, U};
            {info_fun, U=undefined} ->
                {ok, U};
            {lookup_fun, U=undefined} ->
                {ok, U};
            {parent_fun, U=undefined} ->
                {ok, U};
            {post_fun, U=undefined} ->
                {ok, U};
            {pre_fun, U=undefined} ->
                {ok, U};
            {info_fun, Fun} when is_function(Fun, 1) ->
                {ok, Fun};
            {pre_fun, Fun} when is_function(Fun, 1) ->
                {ok, Fun};
            {post_fun, Fun} when is_function(Fun, 0) ->
                {ok, Fun};
            {lookup_fun, Fun} when is_function(Fun, 2) ->
                {ok, Fun};
            {max_lookup, Max} when is_integer(Max), Max >= 0 ->
                {ok, Max};
            {max_lookup, infinity} ->
                {ok, -1};
            {format_fun, Fun} when is_function(Fun, 1) ->
                {ok, Fun};
            {parent_fun, Fun} when is_function(Fun, 0) ->
                {ok, Fun};
            {key_equality, KE='=='} ->
                {ok, KE};
            {key_equality, KE='=:='} ->
                {ok, KE};
            {join, J=any} ->
                {ok, J};
            {join, J=nested_loop} ->
                {ok, J};
            {join, J=merge} ->
                {ok, J};
            {join, J=lookup} ->
                {ok, J};
            {lookup, LookUp} when is_boolean(LookUp); LookUp =:= any ->
                {ok, LookUp};
            {max_list_size, Max} when is_integer(Max), Max >= 0 ->
                {ok, Max};
            {tmpdir_usage, TmpUsage} when TmpUsage =:= allowed;
                                          TmpUsage =:= not_allowed;
                                          TmpUsage =:= info_msg;
                                          TmpUsage =:= warning_msg;
                                          TmpUsage =:= error_msg ->
                {ok, TmpUsage};
            {unique, Unique} when is_boolean(Unique) ->
                {ok, Unique};
            {cache, Cache} when is_boolean(Cache); Cache =:= list ->
                {ok, Cache};
            {cache, ets} ->
                {ok, true};
            {cache, no} ->
                {ok, false};
            {unique_all, UniqueAll} when is_boolean(UniqueAll) ->
                {ok, UniqueAll};
            {cache_all, CacheAll} when is_boolean(CacheAll);
                                       CacheAll =:= list ->
                {ok, CacheAll};
            {cache_all, ets} ->
                {ok, true};
            {cache_all, no} ->
                {ok, false};
            {spawn_options, default} ->
                {ok, default};
            {spawn_options, SpawnOptions} ->
                case is_proper_list(SpawnOptions) of
                    true -> 
                        {ok, SpawnOptions};
                    false ->
                        badarg
                end;
            {flat, Flat} when is_boolean(Flat) ->
                {ok, Flat};
            {format, Format} when Format =:= string;
                                  Format =:= abstract_code;
                                  Format =:= debug ->
                {ok, Format};
            {n_elements, NElements} when NElements =:= infinity;
                                         is_integer(NElements),
                                         NElements > 0 ->
                {ok, NElements};
            {depth, Depth} when Depth =:= infinity;
                                is_integer(Depth), Depth >= 0 ->
                {ok, Depth};
            {order, Order} when is_function(Order, 2);
                                (Order =:= ascending);
                                (Order =:= descending) ->
                {ok, Order};
            {compressed, Comp} when Comp ->
                {ok, [compressed]};
            {compressed, Comp} when not Comp ->
                {ok, []};
            {tmpdir, T} ->
                {ok, T};
            {size, Size} when is_integer(Size), Size > 0 ->
                {ok, Size};
            {no_files, NoFiles} when is_integer(NoFiles), NoFiles > 1 ->
                {ok, NoFiles};
            {Key, _} ->
                badarg;
            false ->
                Default = default_option(Key),
                {ok, Default}
        end,
    case V of
        badarg ->
            badarg;
        {ok, Value} ->
            NewOptions = lists:keydelete(Key, 1, Options),
            options(NewOptions, Keys, [Value | L])
    end;
options([], [], L) ->
    lists:reverse(L);
options(_Options, _, _L) ->
    badarg.

default_option(pre_fun) -> undefined;
default_option(post_fun) -> undefined;
default_option(info_fun) -> undefined;
default_option(format_fun) -> undefined;
default_option(lookup_fun) -> undefined;
default_option(max_lookup) -> -1;
default_option(join) -> any;
default_option(lookup) -> any;
default_option(parent_fun) -> undefined;
default_option(key_equality) -> '=:=';
default_option(spawn_options) -> default;
default_option(flat) -> true;
default_option(format) -> string;
default_option(n_elements) -> infinity;
default_option(depth) -> infinity;
default_option(max_list_size) -> ?MAX_LIST_SIZE;
default_option(tmpdir_usage) -> allowed;
default_option(cache) -> false;
default_option(cache_all) -> false;
default_option(unique) -> false;
default_option(unique_all) -> false;
default_option(order) -> ascending; % default values from file_sorter.erl
default_option(compressed) -> [];
default_option(tmpdir) -> "";
default_option(size) -> 524288;
default_option(no_files) -> 16.

atom_option(cache) -> {cache, true};
atom_option(unique) -> {unique, true};
atom_option(cache_all) -> {cache_all, true};
atom_option(unique_all) -> {unique_all, true};
atom_option(lookup) -> {lookup, true};
atom_option(flat) -> {flat, true};
atom_option(Key) -> Key.

is_proper_list([_ | L]) ->
    is_proper_list(L);
is_proper_list(L) ->
    L =:= [].

spawn_options(default) ->
    [link];
spawn_options(SpawnOptions) ->
    lists:delete(monitor, 
                 case lists:member(link, SpawnOptions) of
                     true -> 
                         SpawnOptions;
                     false ->
                         [link | SpawnOptions]
                 end).

is_keypos(Keypos) when is_integer(Keypos), Keypos > 0 ->
    true;
is_keypos([]) ->
    false;
is_keypos(L) ->
    is_keyposs(L).

is_keyposs([Kp | Kps]) when is_integer(Kp), Kp > 0 ->
    is_keyposs(Kps);
is_keyposs(Kps) ->
    Kps =:= [].

listify(L) when is_list(L) ->
    L;
listify(T) ->
    [T].

%% Optimizations to be carried out.
-record(optz,
        {unique = false,    % boolean()
         cache = false,     % boolean() | list
         join_option = any, % constraint set by the 'join' option
         fast_join = no,    % no | #qlc_join. 'no' means nested loop.
         opt                % #qlc_opt
        }).

%% Prepared #qlc_lc.
-record(qlc,
        {lcf,       % fun() -> Val
         codef,
         qdata,     % with evaluated list expressions
         init_value,
         optz       % #optz
        }).

%% Prepared simple #qlc_lc.
-record(simple_qlc,
        {p,         % atom(), pattern variable
         le,
         anno :: erl_anno:anno(),
         init_value,
         optz       % #optz
         }).

-record(prepared,
        {qh,     % #qlc_append | #qlc_table | #qlc | #simple_qlc | 
                 % #qlc_sort | list()
         sorted = no,  % yes | no | ascending | descending
         sort_info = [], % 
         sort_info2 = [], % 'sort_info' updated with pattern info; qh is LE
         lu_skip_quals = [], % qualifiers to skip due to lookup
         join = {[],[]},  % {Lookup, Merge}
         n_objs = undefined,   % for join (not used yet)
         is_unique_objects = false, % boolean()
         is_cached = false          % boolean() (true means 'ets' or 'list')
        }).

%%% Cursor process functions.

cursor_process(H, GUnique, GCache, TmpDir, SpawnOptions, MaxList, TmpUsage) ->
    Parent = self(),
    Setup = #setup{parent = Parent},
    CF = fun() -> 
                 %% Unless exit/2 is trapped no cleanup can be done.
                 %% The user is assumed not to set the flag to false.
                 process_flag(trap_exit, true),
                 MonRef = erlang:monitor(process, Parent),
                 {Objs, Post, _LocalPost} = 
                     try 
                         Prep = prepare_qlc(H, not_a_list, GUnique, GCache, 
                                            TmpDir, MaxList, TmpUsage),
                         setup_qlc(Prep, Setup)
                     catch Class:Reason:Stacktrace ->
                           Parent ! {self(),
                                     {caught, Class, Reason, Stacktrace}},
                           exit(normal)
                     end,
                 Parent ! {self(), ok},
                 wait_for_request(Parent, MonRef, Post), 
                 reply(Parent, MonRef, Post, Objs)
         end,
    Pid = spawn_opt(CF, SpawnOptions),
    parent_fun(Pid, Parent).

%% Expect calls from tables calling the parent_fun and finally an 'ok'.
parent_fun(Pid, Parent) ->
    receive 
        {Pid, ok} -> Pid;
        {TPid, {parent_fun, Fun}} ->
            V = try 
                    {value, Fun()}
                catch Class:Reason:Stacktrace ->
                    {parent_fun_caught, Class, Reason, Stacktrace}
            end,
            TPid ! {Parent, V},
            parent_fun(Pid, Parent);
        {Pid, {caught, throw, Error, [?THROWN_ERROR | _]}} ->
            Error;
        {Pid, {caught, Class, Reason, Stacktrace}} ->
            erlang:raise(Class, Reason, Stacktrace)
    end.

reply(Parent, MonRef, Post, []) ->
    no_more(Parent, MonRef, Post);
reply(Parent, MonRef, Post, [Answer | Cont]) ->
    Parent ! {self(), {answer, Answer}},    
    wait_for_request(Parent, MonRef, Post),
    reply(Parent, MonRef, Post, Cont);
reply(Parent, MonRef, Post, Cont) ->
    Reply = try 
                if 
                    is_function(Cont) ->
                        Cont();
                    true ->
                        throw_error(Cont)
                end
            catch 
                Class:Reason:Stacktrace ->
                   post_funs(Post),
                   Message = {caught, Class, Reason, Stacktrace},
                   Parent ! {self(), Message},
                   exit(normal)
            end,
    reply(Parent, MonRef, Post, Reply).

no_more(Parent, MonRef, Post) ->
    Parent ! {self(), no_more},
    wait_for_request(Parent, MonRef, Post),
    no_more(Parent, MonRef, Post).

wait_for_request(Parent, MonRef, Post) ->
    receive 
        {Parent, stop} ->
            post_funs(Post),
            exit(normal);
        {Parent, more} ->
            ok;
        {'EXIT', Parent, _Reason} ->
            post_funs(Post),
            exit(normal);
        {'DOWN', MonRef, process, Parent, _Info} ->
            post_funs(Post),
            exit(normal);
        {'EXIT', Pid, _Reason} when Pid =:= self() ->
            %% Trapped signal. The cursor ignores it...
            wait_for_request(Parent, MonRef, Post);
        Other ->
            error_logger:error_msg(
              "The qlc cursor ~w received an unexpected message:\n~tp\n",
              [self(), Other]),
            wait_for_request(Parent, MonRef, Post)
    end.

%%% End of cursor process functions.

abstract_code({special, Anno, String}) ->
    {string, Anno, String};
abstract_code(Tuple) when is_tuple(Tuple) ->
    list_to_tuple(abstract_code(tuple_to_list(Tuple)));
abstract_code([H | T]) ->
    [abstract_code(H) | abstract_code(T)];
abstract_code(Term) ->
    Term.

%% Also in qlc_pt.erl.
-define(Q, q).
-define(QLC_Q(A1, A2, A3, A4, LC, Os),
        {call,A1,{remote,A2,{atom,A3,?MODULE},{atom,A4,?Q}},[LC | Os]}).

abstract(Info, false=_Flat, NElements, Depth) ->
    abstract(Info, NElements, Depth);
abstract(Info, true=_Flat, NElements, Depth) ->
    Abstract = abstract(Info, NElements, Depth),
    Vars = abstract_vars(Abstract),
    {_, Body0, Expr} = flatten_abstr(Abstract, 1, Vars, []),
    case Body0 of
        [] -> 
            Expr;
        [{match,_,Expr,Q}] ->
            Q;
        [{match,_,Expr,Q} | Body] ->
            {block, anno0(), lists:reverse(Body, [Q])};
        _ ->
            {block, anno0(), lists:reverse(Body0, [Expr])}
    end.

abstract(Info, NElements, Depth) ->
    abstract1(Info, NElements, Depth, anno1()).

abstract1({qlc, E0, Qs0, Opt}, NElements, Depth, A) ->
    Qs = lists:map(fun({generate, P, LE}) ->
                           {generate, A, binary_to_term(P),
                            abstract1(LE, NElements, Depth, A)};
                      (F) -> 
                           binary_to_term(F)
                   end, Qs0),
    E = binary_to_term(E0),
    Os = case Opt of
             [] -> [];
             _ -> [abstract_term(Opt, 1)]
         end,
    ?QLC_Q(A, A, A, A, {lc,A,E,Qs}, Os);
abstract1({table, {M, F, As0}}, _NElements, _Depth, Anno)
                          when is_atom(M), is_atom(F), is_list(As0) ->
    As = [abstract_term(A, 1) || A <- As0],
    {call, Anno, {remote, Anno, {atom, Anno, M}, {atom, Anno, F}}, As};
abstract1({table, TableDesc}, _NElements, _Depth, _A) ->
    case io_lib:deep_char_list(TableDesc) of
        true ->
            {ok, Tokens, _} =
                erl_scan:string(lists:flatten(TableDesc++"."), 1, [text]),
            {ok, [Expr]} = erl_eval:extended_parse_exprs(Tokens),
            special(Expr);
        false -> % abstract expression
            TableDesc
    end;
abstract1({append, Infos}, NElements, Depth, A) ->
    As = lists:foldr(fun(Info, As0) -> 
                             {cons,A,abstract1(Info, NElements, Depth, A),
                              As0}
                     end, {nil, A}, Infos),
    {call, A, {remote, A, {atom, A, ?MODULE}, {atom, A, append}}, [As]};
abstract1({sort, Info, SortOptions}, NElements, Depth, A) ->
    {call, A, {remote, A, {atom, A, ?MODULE}, {atom, A, sort}},
     [abstract1(Info, NElements, Depth, A), abstract_term(SortOptions, 1)]};
abstract1({keysort, Info, Kp, SortOptions}, NElements, Depth, A) ->
    {call, A, {remote, A, {atom, A, ?MODULE}, {atom, A, keysort}},
     [abstract_term(Kp, 1), abstract1(Info, NElements, Depth, A),
      abstract_term(SortOptions, 1)]};
abstract1({list,L,MS}, NElements, Depth, A) ->
    {call, A, {remote, A, {atom, A, ets}, {atom, A, match_spec_run}},
     [abstract1(L, NElements, Depth, A),
      {call, A, {remote, A, {atom, A, ets}, {atom, A, match_spec_compile}},
       [abstract_term(depth(MS, Depth), 1)]}]};
abstract1({list, L}, NElements, Depth, _A) when NElements =:= infinity;
                                                NElements >= length(L) ->
    abstract_term(depth(L, Depth), 1);
abstract1({list, L}, NElements, Depth, _A) ->
    abstract_term(depth(lists:sublist(L, NElements), Depth) ++ '...', 1).

special({value, _, Thing}) ->
    abstract_term(Thing);
special(Tuple) when is_tuple(Tuple) ->
    list_to_tuple(special(tuple_to_list(Tuple)));
special([E|Es]) ->
    [special(E)|special(Es)];
special(Expr) ->
    Expr.

depth(List, infinity) ->
    List;
depth(List, Depth) ->
    [depth1(E, Depth) || E <- List].

depth_fun(infinity = _Depth) ->
    fun(E) -> E end;
depth_fun(Depth) ->
    fun(E) -> depth1(E, Depth) end.

depth1([]=L, _D) ->
    L;
depth1(_Term, 0) ->
    '...';
depth1(Tuple, D) when is_tuple(Tuple) ->
    depth_tuple(Tuple, tuple_size(Tuple), 1, D - 1, []);
depth1(List, D) when is_list(List) ->
    if 
        D =:= 1 ->
            ['...'];
        true -> 
            depth_list(List, D - 1)
    end;
depth1(Binary, D) when byte_size(Binary) > D - 1 ->
    D1 = D - 1,
    <<Bin:D1/bytes,_/bytes>> = Binary,
    <<Bin/bytes,"...">>;
depth1(T, _Depth) ->
    T.

depth_list([]=L, _D) ->
    L;
depth_list(_L, 0) ->
    '...';
depth_list([E | Es], D) ->
    [depth1(E, D) | depth_list(Es, D - 1)].

depth_tuple(_Tuple, Sz, I, _D, L) when I > Sz ->
    list_to_tuple(lists:reverse(L));
depth_tuple(_L, _Sz, _I, 0, L) ->
    list_to_tuple(lists:reverse(L, ['...']));
depth_tuple(Tuple, Sz, I, D, L) ->
    E = depth1(element(I, Tuple), D),
    depth_tuple(Tuple, Sz, I + 1, D - 1, [E | L]).

abstract_term(Term) ->
    abstract_term(Term, 0).

abstract_term(Term, Line) ->
    abstr_term(Term, anno(Line)).

abstr_term(Tuple, Anno) when is_tuple(Tuple) ->
    {tuple,Anno,[abstr_term(E, Anno) || E <- tuple_to_list(Tuple)]};
abstr_term([_ | _]=L, Anno) ->
    case io_lib:char_list(L) of
        true ->
            erl_parse:abstract(L, erl_anno:line(Anno));
        false ->
            abstr_list(L, Anno)
    end;
abstr_term(Fun, Anno) when is_function(Fun) ->
    case erl_eval:fun_data(Fun) of
        {fun_data, _Bs, Cs} ->
            {'fun', Anno, {clauses, Cs}};
        {named_fun_data, _Bs, Name, Cs} ->
            {named_fun, Anno, Name, Cs};
        false ->
            {name, Name} = erlang:fun_info(Fun, name),
            {arity, Arity} = erlang:fun_info(Fun, arity),
            case erlang:fun_info(Fun, type) of
                {type, external} ->
                    {module, Module} = erlang:fun_info(Fun, module),
                    {'fun', Anno, {function,
				   {atom,Anno,Module},
				   {atom,Anno,Name},
				   {integer,Anno,Arity}}};
                {type, local} ->
                    {'fun', Anno, {function,Name,Arity}}
            end
    end;
abstr_term(PPR, Anno) when is_pid(PPR); is_port(PPR); is_reference(PPR) ->
    {special, Anno, lists:flatten(io_lib:write(PPR))};
abstr_term(Map, Anno) when is_map(Map) ->
    {map,Anno,
     [{map_field_assoc,Anno,abstr_term(K, Anno),abstr_term(V, Anno)} ||
         K := V <- Map]};
abstr_term(Simple, Anno) ->
    erl_parse:abstract(Simple, erl_anno:line(Anno)).

abstr_list([H | T], Anno) ->
    {cons, Anno, abstr_term(H, Anno), abstr_list(T, Anno)};
abstr_list(T, Anno) ->
    abstr_term(T, Anno).

%% Since generator pattern variables cannot be used in list
%% expressions, it is OK to flatten out QLCs using temporary
%% variables.
flatten_abstr(?QLC_Q(A1, A2, A3, A4, LC0, Os), VN0, Vars, Body0) ->
    {lc,Anno,E,Qs0} = LC0,
    F = fun({generate,AnnoG,P,LE0}, {VN1,Body1}) ->
                {VN2,Body2,LE} = flatten_abstr(LE0, VN1, Vars, Body1),
                {{generate,AnnoG,P,LE}, {VN2,Body2}};
           (Fil, VN_Body) ->
                {Fil, VN_Body}
        end,
    {Qs, {VN3,Body}} = lists:mapfoldl(F, {VN0,Body0}, Qs0),
    LC = {lc,Anno,E,Qs},
    {V, VN} = aux_name1('V', VN3, Vars),
    Var = {var, A1, V},
    QLC = ?QLC_Q(A1, A2, A3, A4, LC, Os),
    {VN + 1, [{match, A1, Var, QLC} | Body], Var};
flatten_abstr(T0, VN0, Vars, Body0) when is_tuple(T0) ->
    {VN, Body, L} = flatten_abstr(tuple_to_list(T0), VN0, Vars, Body0),
    {VN, Body, list_to_tuple(L)};
flatten_abstr([E0 | Es0], VN0, Vars, Body0) ->
    {VN1, Body1, E} = flatten_abstr(E0, VN0, Vars, Body0),
    {VN, Body, Es} = flatten_abstr(Es0, VN1, Vars, Body1),
    {VN, Body, [E | Es]};
flatten_abstr(E, VN, _Vars, Body) ->
    {VN, Body, E}.

abstract_vars(Abstract) ->
    gb_sets:from_list(ordsets:to_list(vars(Abstract))).

collect([]=L) ->
    L;
collect([Answer | Cont]) ->
    [Answer | collect(Cont)];
collect(Cont) ->
    case Cont() of
        Answers when is_list(Answers) ->
            collect(Answers);
        Term ->
            throw_error(Term)
    end.

fold_loop(Fun, [Obj | Cont], Acc) ->
    fold_loop(Fun, Cont, Fun(Obj, Acc));
fold_loop(_Fun, [], Acc) ->
    Acc;
fold_loop(Fun, Cont, Acc) ->
    case Cont() of
        Objects when is_list(Objects) ->
            fold_loop(Fun, Objects, Acc);
        Term -> 
            Term
    end.

next_loop(Pid, L, N) when N =/= 0 ->
    case monitor_request(Pid, more) of
        no_more ->
            lists:reverse(L);
        {answer, Answer} ->
            next_loop(Pid, [Answer | L], N - 1);
        {caught, throw, Error, [?THROWN_ERROR | _]} ->
            Error;
        {caught, Class, Reason, Stacktrace} ->
            {current_stacktrace, CurrentStacktrace} =
                erlang:process_info(self(), current_stacktrace),
            erlang:raise(Class, Reason, Stacktrace ++ CurrentStacktrace);
        error ->
            erlang:error({qlc_cursor_pid_no_longer_exists, Pid})
    end;
next_loop(_Pid, L, _N) ->
    lists:reverse(L).

stop_cursor(Pid) ->
    erlang:monitor(process, Pid),
    unlink(Pid),
    receive
        {'EXIT',Pid,_Reason} -> % Simply ignore the error.
            receive 
                {'DOWN',_,process,Pid,_} -> ok
            end
    after 0 -> 
            Pid ! {self(),stop},
            receive
                {'DOWN',_,process,Pid,_} -> ok
            end
    end.

monitor_request(Pid, Req) ->
    Ref = erlang:monitor(process, Pid),
    Pid ! {self(), Req},
    receive 
        {'DOWN', Ref, process, Pid, _Info} ->
            receive
                {'EXIT', Pid, _Reason} -> ok
            after 1 -> ok end,
            error;
        {'EXIT', Pid, _Reason} ->
            receive 
                {'DOWN', _, process, Pid, _} -> error
            end;
        {Pid, Reply} ->
            erlang:demonitor(Ref, [flush]),
            Reply
    end.

%% Marker for skipped filter or unused generator.
-define(SKIP, (-1)).

%% Qual = {gen, LE} | fil
-define(qual_data(QNum, GoToIndex, State, Qual), 
        {QNum, GoToIndex, State, Qual}).

-record(join, % generated by qlc_pt
        {op, q1, q2, wh1, wh2, cs_fun}). % op is unused

%% le_info/1 returns an intermediate information format only used for
%% testing purposes. Changes will happen without notice.
%%
%% QueryDesc = {qlc, TemplateDesc, [QualDesc], [QueryOpt]} 
%%           | {table, TableDesc}
%%           | {append, [QueryDesc]}
%%           | {sort, QueryDesc, [sort_option()]}
%%           | {keysort, key_pos(), QueryDesc, [sort_option()]}
%%           | {list, list()}
%%           | {list, QueryDesc, match_expression()}
%% TableDesc = {Mod, Fun, Args}
%%           | erl_parse:abstract_expr()
%%           | string()
%% Mod = module()
%% Fun = atom()
%% Args = [term()]
%% QualDesc = FilterDesc
%%          | {generate, PatternDesc, QueryDesc}
%% QueryOpt = {cache, boolean()} | cache
%%          | {unique, boolean()} | unique
%% FilterDesc = PatternDesc = TemplateDesc = binary()

le_info(#prepared{qh = #simple_qlc{le = LE, p = P, anno = Anno, optz = Optz}},
        InfOpt) ->
    QVar = term_to_binary({var, Anno, P}),
    {qlc, QVar, [{generate, QVar, le_info(LE, InfOpt)}], opt_info(Optz)};
le_info(#prepared{qh = #qlc{codef = CodeF, qdata = Qdata, optz = Optz}},
        InfOpt) ->
    Code = CodeF(),
    TemplateState = template_state(),
    E = element(TemplateState, Code),
    QualInfo0 = qual_info(Qdata, Code, InfOpt),
    QualInfo1 = case Optz#optz.fast_join of
                    #qlc_join{} = Join ->
                        join_info(Join, QualInfo0, Qdata, Code);
                    no ->
                        QualInfo0
                end,
    QualInfo = [I || I <- QualInfo1, I =/= skip],
    {qlc, E, QualInfo, opt_info(Optz)};
le_info(#prepared{qh = #qlc_table{format_fun = FormatFun, trav_MS = TravMS, 
                                  ms = MS, lu_vals = LuVals}}, InfOpt) ->
    {NElements, Depth} = InfOpt,
    %% The 'depth' option applies to match specifications as well.
    %% This is for limiting imported variables (parameters).
    DepthFun = depth_fun(Depth),
    case LuVals of
        _ when FormatFun =:= undefined ->
            {table, {'$MOD', '$FUN', []}};
        {Pos, Vals} ->
            Formated = try FormatFun({lookup, Pos, Vals, NElements, DepthFun})
                       catch _:_ -> FormatFun({lookup, Pos, Vals})
                       end,
            if
                MS =:= no_match_spec ->
                    {table, Formated};
                true ->
                    {list, {table, Formated}, depth(MS, Depth)}
            end;
        _ when TravMS, is_list(MS) ->
            {table, FormatFun({match_spec, depth(MS, Depth)})};
        _ when MS =:= no_match_spec ->
            try {table, FormatFun({all, NElements, DepthFun})}
            catch _:_ -> {table, FormatFun(all)}
            end
    end;
le_info(#prepared{qh = #qlc_append{hl = HL}}, InfOpt) ->
    {append, [le_info(H, InfOpt) || H <- HL]};
le_info(#prepared{qh = #qlc_sort{h = H, keypos = sort, 
                                 fs_opts = SortOptions0, tmpdir = TmpDir}},
        InfOpt) ->
    SortOptions = sort_options_global_tmp(SortOptions0, TmpDir),
    {sort, le_info(H, InfOpt), SortOptions};
le_info(#prepared{qh = #qlc_sort{h = H, keypos = {keysort, Kp}, 
                                 fs_opts = SortOptions0, tmpdir = TmpDir}},
        InfOpt) ->
    SortOptions = sort_options_global_tmp(SortOptions0, TmpDir),
    {keysort, le_info(H, InfOpt), Kp, SortOptions};
le_info(#prepared{qh = #qlc_list{l = L, ms = no_match_spec}}, _InfOpt) ->
    {list, L};
le_info(#prepared{qh = #qlc_list{l = L, ms = MS}},_InfOpt) when is_list(L) ->
    {list, {list, L}, MS};
le_info(#prepared{qh = #qlc_list{l = L, ms = MS}}, InfOpt) ->
    {list, le_info(L, InfOpt), MS}.

qual_info([?qual_data(_QNum, _GoI, ?SKIP, fil) | Qdata], Code, InfOpt) ->
    %% see skip_lookup_filters()
    [skip | qual_info(Qdata, Code, InfOpt)];
qual_info([?qual_data(QNum, _GoI, _SI, fil) | Qdata], Code, InfOpt) ->
    [element(QNum + 1, Code) | qual_info(Qdata, Code, InfOpt)];
qual_info([?qual_data(_QNum, _GoI, _SI, {gen,#join{}}) | Qdata], 
          Code, InfOpt) ->
    [skip | qual_info(Qdata, Code, InfOpt)];
qual_info([?qual_data(QNum, _GoI, _SI, {gen,LE}) | Qdata], Code, InfOpt) ->
    [{generate,element(QNum + 1, Code),le_info(LE, InfOpt)} | 
     qual_info(Qdata, Code, InfOpt)];
qual_info([], _Code, _InfOpt) ->
    [].

join_info(Join, QInfo, Qdata, Code) ->
    #qlc_join{kind = Kind, q1 = QNum1a, c1 = C1, q2 = QNum2a, c2 = C2, 
              opt = Opt} = Join,
    {?qual_data(JQNum,_,_,_), Rev, QNum1, QNum2, _WH1, _WH2, CsFun} = 
        find_join_data(Qdata, QNum1a, QNum2a),
    {Cs1_0, Cs2_0, Compat} = CsFun(),
    [Cs1, Cs2] = case Compat of
                     [] -> % --R12B-5
                         [[{C,[{V,'=:='} || V <- Vs]} || {C,Vs} <- CVs] || 
                             CVs <- [Cs1_0, Cs2_0]];
                     _ -> % 'v1', R13A --
                         %% Only compared constants (==).
                         [Cs1_0, Cs2_0]
                 end,
    Anno = anno0(),
    G1_0 = {var,Anno,'G1'}, G2_0 = {var,Anno,'G2'},
    JP = element(JQNum + 1, Code),
    %% Create code for wh1 and wh2 in #join{}:
    {{I1,G1}, {I2,G2}, QInfoL} = 
        case Kind of
            {merge, _} -> 
                {JG1,QInfo1} = join_merge_info(QNum1, QInfo, Code, G1_0, Cs1),
                {JG2,QInfo2} = join_merge_info(QNum2, QInfo, Code, G2_0, Cs2),
                {JG1, JG2, QInfo1 ++ QInfo2};
            _ when Rev ->
                {JG2,QInfo2} = join_merge_info(QNum2, QInfo, Code, G2_0, Cs2),
                {J1, QInfo1} = join_lookup_info(QNum1, QInfo, G1_0),
                {{J1,G1_0}, JG2, QInfo2 ++ [QInfo1]};
            _ ->
                {JG1,QInfo1} = join_merge_info(QNum1, QInfo, Code, G1_0, Cs1),
                {J2, QInfo2} = join_lookup_info(QNum2, QInfo, G2_0),
                {JG1, {J2,G2_0}, QInfo1 ++ [QInfo2]}
        end,
    {JOptVal, JOp} = kind2op(Kind),
    JOpt = [{join, JOptVal}] ++ opt_info(join_unique_cache(Opt)),
    JFil = term_to_binary({op,Anno,JOp,
                           {call,Anno,{atom,Anno,element},
                            [{integer,Anno,C1},G1]},
                           {call,Anno,{atom,Anno,element},
                            [{integer,Anno,C2},G2]}}),
    P = term_to_binary({cons, Anno, G1, G2}),
    JInfo = {generate, JP, {qlc, P, QInfoL ++ [JFil], JOpt}},
    {Before, [I1 | After]} = lists:split(QNum1 - 1, QInfo),
    Before ++ [JInfo] ++ lists:delete(I2, After).

kind2op({merge, _KE}) -> {merge, '=='};
kind2op({lookup, KE, _LU_fun}) -> {lookup, KE}.

%% qlc:q(P0 || P0 = Pattern <- H1, ConstFilters), 
%% where "P0" is a fresh variable and ConstFilters are filters that
%% test constant values of pattern columns.
join_merge_info(QNum, QInfo, Code, G, ExtraConstants) ->
    {generate, _, LEInfo}=I = lists:nth(QNum, QInfo),
    P = binary_to_term(element(QNum + 1, Code)),
    case {P, ExtraConstants} of
        {{var, _, _}, []} ->
            %% No need to introduce a QLC.
            TP = term_to_binary(G),
            I2 = {generate, TP, LEInfo},
            {{I,G}, [I2]};
        _ -> 
            {EPV, M} = 
                case P of
                    {var, _, _} -> 
                        %% No need to introduce a pattern variable.
                        {P, P};
                    _ -> 
                        {PV, _} = aux_name1('P', 0, abstract_vars(P)),
                        Anno = anno0(),
                        V = {var, Anno, PV},
                        {V, {match, Anno, V, P}}
                 end,
            DQP = term_to_binary(EPV),
            LEI = {generate, term_to_binary(M), LEInfo},
            TP = term_to_binary(G),
            CFs = [begin
                       A = anno0(),
                       Call = {call,A,{atom,A,element},[{integer,A,Col},EPV]},
                       F = list2op([{op,A,Op,abstract_term(Con),Call}
                                      || {Con,Op} <- ConstOps], 'or', A),
                       term_to_binary(F)
                   end ||
                      {Col,ConstOps} <- ExtraConstants],
            {{I,G}, [{generate, TP, {qlc, DQP, [LEI | CFs], []}}]}
    end.

list2op([E], _Op, _Anno) ->
    E;
list2op([E | Es], Op, Anno) ->
    {op,Anno,Op,E,list2op(Es, Op, Anno)}.

join_lookup_info(QNum, QInfo, G) ->
    {generate, _, LEInfo}=I = lists:nth(QNum, QInfo),
    TP = term_to_binary(G),
    {I, {generate, TP, LEInfo}}.

opt_info(#optz{unique = Unique, cache = Cache0, join_option = JoinOption}) ->
    %% No 'nested_loop' options are added here, even if there are
    %% nested loops to carry out, unless a 'nested_loop' was given as
    %% option. The reason is that the qlc module does not know about
    %% all instances of nested loops.
    Cache = if
                Cache0 -> ets;
                true -> Cache0
            end,
    [{T,V} || {T,V} <- [{cache,Cache},{unique,Unique}],
              V =/= default_option(T)] ++
    [{T,V} || {T,V} <- [{join,JoinOption}], V =:= nested_loop].

prepare_qlc(H, InitialValue, GUnique, GCache, TmpDir, MaxList, TmpUsage) ->
    GOpt = #qlc_opt{unique = GUnique, cache = GCache, 
                    tmpdir = TmpDir, max_list = MaxList, 
                    tmpdir_usage = TmpUsage},
    case opt_le(prep_le(H, GOpt), 1) of
        #prepared{qh = #qlc{} = QLC}=Prep ->
            Prep#prepared{qh = QLC#qlc{init_value = InitialValue}};
        #prepared{qh = #simple_qlc{}=SimpleQLC}=Prep ->
            Prep#prepared{qh = SimpleQLC#simple_qlc{init_value = InitialValue}};
        Prep ->
            Prep
    end.

%%% The options given to append, q and table (unique and cache) as well
%%% as the type of expression (list, table, append, qlc...) are
%%% analyzed by prep_le. The results are is_unique_objects and
%%% is_cached. It is checked that the evaluation (in the Erlang sense)
%%% of list expressions yields qlc handles.

prep_le(#qlc_lc{lc = LC_fun, opt = #qlc_opt{} = Opt0}=H, GOpt) ->
    #qlc_opt{unique = GUnique, cache = GCache, 
             tmpdir = TmpDir, max_list = MaxList, 
             tmpdir_usage = TmpUsage} = GOpt,
    Unique = Opt0#qlc_opt.unique or GUnique,
    Cache = if
                not GCache -> Opt0#qlc_opt.cache;
                true -> GCache
            end,
    Opt = Opt0#qlc_opt{unique = Unique, cache = Cache, 
                       tmpdir = TmpDir, max_list = MaxList, 
                       tmpdir_usage = TmpUsage},
    prep_qlc_lc(LC_fun(), Opt, GOpt, H);
prep_le(#qlc_table{info_fun = IF}=T, GOpt) ->
    {SortInfo, Sorted} = table_sort_info(T),
    IsUnique = grd(IF, is_unique_objects),
    Prep = #prepared{qh = T, sort_info = SortInfo, sorted = Sorted,
                     is_unique_objects = IsUnique},
    Opt = if 
              IsUnique or not GOpt#qlc_opt.unique,
              T#qlc_table.ms =:= no_match_spec -> 
                  GOpt#qlc_opt{cache = false};
              true ->
                  GOpt
          end,
    may_create_simple(Opt, Prep);
prep_le(#qlc_append{hl = HL}, GOpt) ->
    case lists:flatmap(fun(#prepared{qh = #qlc_list{l = []}}) -> [];
                          (#prepared{qh = #qlc_append{hl = HL1}}) -> HL1;
                          (H) -> [H] end, 
                       [prep_le(H, GOpt) || H <- HL]) of
        []=Nil -> 
            short_list(Nil);
        [Prep] -> 
            Prep;
        PrepL -> 
            Cache = lists:all(fun(#prepared{is_cached = IsC}) -> IsC =/= false
                              end, PrepL),
            %% The handles in hl are replaced by prepared handles:
            Prep = #prepared{qh = #qlc_append{hl = PrepL}, is_cached = Cache},
            may_create_simple(GOpt, Prep)
    end;
prep_le(#qlc_sort{h = H0}=Q0, GOpt) ->
    %% The handle h is replaced by a prepared handle:
    Q = Q0#qlc_sort{h = prep_le(H0, GOpt)},
    prep_sort(Q, GOpt);
prep_le([_, _ | _]=L, GOpt) ->
    Prep = #prepared{qh = #qlc_list{l = L}, is_cached = true},
    Opt = if 
              not GOpt#qlc_opt.unique -> 
                  GOpt#qlc_opt{cache = false}; 
             true -> GOpt 
          end,
    may_create_simple(Opt, Prep);
prep_le(L, _GOpt) when is_list(L) ->
    short_list(L);
prep_le(T, _GOpt) ->
    erlang:error({unsupported_qlc_handle, #qlc_handle{h = T}}).

eval_le(LE_fun, GOpt) ->
    case LE_fun() of
        {error, ?MODULE, _} = Error ->
            throw_error(Error);
        R -> 
            case get_handle(R) of
                badarg ->
                    erlang:error(badarg, [R]);
                H -> 
                    prep_le(H, GOpt)
            end
    end.

prep_qlc_lc({simple_v1, PVar, LE_fun, L}, Opt, GOpt, _H) ->
    check_lookup_option(Opt, false),
    prep_simple_qlc(PVar, anno(L), eval_le(LE_fun, GOpt), Opt);
prep_qlc_lc({qlc_v1, QFun, CodeF, Qdata0, QOpt}, Opt, GOpt, _H) ->
    F = fun(?qual_data(_QNum, _GoI, _SI, fil)=QualData, ModGens) -> 
                {QualData, ModGens};
           (?qual_data(_QNum, _GoI, _SI, {gen, #join{}})=QualData, ModGens) ->
                {QualData, ModGens};
           (?qual_data(QNum, GoI, SI, {gen, LE_fun}), ModGens0) -> 
                Prep1 = eval_le(LE_fun, GOpt),
                {Prep, ModGens} = 
                    prep_generator(QNum, Prep1, QOpt, Opt, ModGens0),
                {?qual_data(QNum, GoI, SI, {gen, Prep}), ModGens}
        end,
    {Qdata, ModGens} = lists:mapfoldl(F, [], Qdata0),
    SomeLookUp = lists:keymember(true, 2, ModGens),
    check_lookup_option(Opt, SomeLookUp),
    case ModGens of
        [{_QNum, _LookUp, all, OnePrep}] ->
            check_join_option(Opt),
            OnePrep;
        _ -> 
            Prep0 = prep_qlc(QFun, CodeF, Qdata, QOpt, Opt),
            LU_SkipQuals = 
                lists:flatmap(fun({QNum,_LookUp,Fs,_Prep}) -> [{QNum,Fs}]
                              end, ModGens),        
            Prep1 = Prep0#prepared{lu_skip_quals = LU_SkipQuals},
            prep_join(Prep1, QOpt, Opt)
    end;
prep_qlc_lc(_, _Opt, _GOpt, H) ->
    erlang:error({unsupported_qlc_handle, #qlc_handle{h = H}}).

prep_generator(QNum, Prep0, QOpt, Opt, ModGens) ->
    PosFun = fun(KeyEquality) -> pos_fun(KeyEquality, QOpt, QNum) end,
    MSFs = case match_specs(QOpt, QNum) of
               undefined ->
                   {no_match_spec, []};
               {_, _}=MSFs0 ->
                   MSFs0
         end,
    #prepared{qh = LE} = Prep0,
    case prep_gen(LE, Prep0, PosFun, MSFs, Opt) of
        {replace, Fs, LookUp, Prep} ->
            {Prep, [{QNum,LookUp,Fs,Prep} | ModGens]};
        {skip, SkipFils, LookUp, Prep} ->
            {Prep, [{QNum,LookUp,SkipFils,Prep} | ModGens]};
        {no, _Fs, _LookUp, Prep} ->
            {Prep, ModGens}
    end.

pos_fun(undefined, QOpt, QNum) ->
    {'=:=', constants(QOpt, QNum)}; %% --R12B-5
pos_fun('=:=', QOpt, QNum) ->
    {'=:=', constants(QOpt, QNum)};
pos_fun('==', QOpt, QNum) ->    
    try {'==', equal_constants(QOpt, QNum)} % R13A--
    catch _:_ -> {'=:=', constants(QOpt, QNum)}
    end.

prep_gen(#qlc_table{lu_vals = LuV0, ms = MS0, trav_MS = TravMS,
                    info_fun = IF, lookup_fun = LU_fun,
                    key_equality = KeyEquality}=LE0,
         Prep0, PosFun0, {MS, Fs}, Opt) ->
    PosFun = PosFun0(KeyEquality),
    {LuV, {STag,SkipFils}} = find_const_positions(IF, LU_fun, PosFun, Opt),
    LU = LuV =/= false,
    if
        LuV0 =/= undefined; MS0 =/= no_match_spec ->
            {no, [], false, Prep0};
        MS =/= no_match_spec, LU ->
            MS1 = if 
                      Fs =:= SkipFils; STag =:= Fs ->
                          %% The guard of the match specification 
                          %% is covered by the lookup.
                          case MS of
                              [{'$1',_Guard,['$1']}] -> % no transformation
                                  no_match_spec;
                              [{Head,_Guard,Body}] ->
                                  [{Head,[],Body}] % true guard
                          end;
                      true ->
                          MS
                  end,
            Prep = Prep0#prepared{qh = LE0#qlc_table{lu_vals = LuV,ms = MS1}},
            {replace, Fs, LU, Prep};
        LU ->
            Prep = Prep0#prepared{qh = LE0#qlc_table{lu_vals = LuV}},
            {skip, SkipFils, LU, Prep};
        TravMS, MS =/= no_match_spec ->
            Prep = Prep0#prepared{qh = LE0#qlc_table{ms = MS},
                                  is_unique_objects = false},
            {replace, Fs, false, may_create_simple(Opt, Prep)};
        true -> 
            {no, [], false, Prep0}
    end;
prep_gen(#qlc_list{l = []}, Prep0, _PosFun, {_MS, Fs}, _Opt) ->
    %% unique and cached
    {replace, Fs, false, Prep0};
prep_gen(#qlc_list{ms = no_match_spec}=LE0, Prep0, _PosFun, {MS, Fs}, Opt) 
                                  when MS =/= no_match_spec ->
    Prep = Prep0#prepared{qh = LE0#qlc_list{ms = MS}, 
                          is_cached = false},
    {replace, Fs, false, may_create_simple(Opt, Prep)};
prep_gen(#qlc_list{}, Prep0, _PosFun, {MS, Fs}, Opt) 
                                  when MS =/= no_match_spec ->
    ListMS = #qlc_list{l = Prep0, ms = MS},
    LE = #prepared{qh = ListMS, is_cached = false},
    {replace, Fs, false, may_create_simple(Opt, LE)};
prep_gen(_LE0, Prep0, _PosFun, _MSFs, _Opt) ->
    {no, [], false, Prep0}.

-define(SIMPLE_QVAR, 'SQV').

may_create_simple(#qlc_opt{unique = Unique, cache = Cache} = Opt, 
                  #prepared{is_cached = IsCached, 
                            is_unique_objects = IsUnique} = Prep) ->
    if 
        Unique and not IsUnique; 
        (Cache =/= false) and not IsCached ->
            prep_simple_qlc(?SIMPLE_QVAR, anno(1), Prep, Opt);
        true ->
            Prep
    end.

prep_simple_qlc(PVar, Anno, LE, Opt) ->
    check_join_option(Opt),
    #prepared{is_cached = IsCached, 
              sort_info = SortInfo, sorted = Sorted,
              is_unique_objects = IsUnique} = LE,
    #qlc_opt{unique = Unique, cache = Cache} = Opt,
    Cachez = if
                 Unique -> Cache;
                 not IsCached -> Cache;
                 true -> false
             end,
    Optz = #optz{unique = Unique and not IsUnique, 
                 cache = Cachez, opt = Opt},
    QLC = #simple_qlc{p = PVar, le = LE, anno = Anno,
                      init_value = not_a_list, optz = Optz},
    %% LE#prepared.join is not copied
    #prepared{qh = QLC, is_unique_objects = IsUnique or Unique, 
              sort_info = SortInfo, sorted = Sorted,
              is_cached = IsCached or (Cachez =/= false)}.

prep_sort(#qlc_sort{h = #prepared{sorted = yes}=Prep}, _GOpt) ->
    Prep;
prep_sort(#qlc_sort{h = #prepared{is_unique_objects = IsUniqueObjs}}=Q, 
          GOpt) ->
    S1 = sort_unique(IsUniqueObjs, Q),
    S2 = sort_tmpdir(S1, GOpt),
    S = S2#qlc_sort{tmpdir_usage = GOpt#qlc_opt.tmpdir_usage},
    {SortInfo, Sorted} = sort_sort_info(S),
    #prepared{qh = S, is_cached = true, sort_info = SortInfo,
              sorted = Sorted,
              is_unique_objects = S#qlc_sort.unique or IsUniqueObjs}.

prep_qlc(QFun, CodeF, Qdata0, QOpt, Opt) ->
    #qlc_opt{unique = Unique, cache = Cache, join = Join} = Opt,
    Optz = #optz{unique = Unique, cache = Cache, 
                 join_option = Join, opt = Opt},
    {Qdata, SortInfo} = qlc_sort_info(Qdata0, QOpt),
    QLC = #qlc{lcf = QFun, codef = CodeF, qdata = Qdata, 
               init_value = not_a_list, optz = Optz},
    #prepared{qh = QLC, sort_info = SortInfo,
              is_unique_objects = Unique, 
              is_cached = Cache =/= false}.

%% 'sorted', 'sorted_info', and 'sorted_info2' are used to avoid
%% sorting on a key when there is no need to sort on the key. 'sorted'
%% is set by qlc:sort() only; its purpose is to assure that if columns
%% 1 to i are constant, then column i+1 is key-sorted (always true if
%% the tuples are sorted). Note: the implementation is (too?) simple.
%% For instance, each column is annotated with 'ascending' or
%% 'descending' (not yet). More exact would be, as examples, 'always
%% ascending' and 'ascending if all preceding columns are constant'.
%%
%% The 'size' of the template is not used (size_of_qualifier(QOpt, 0)).

qlc_sort_info(Qdata0, QOpt) ->
    F = fun(?qual_data(_QNum, _GoI, _SI, fil)=Qd, Info) -> 
                {Qd, Info};
           (?qual_data(_QNum, _GoI, _SI, {gen, #join{}})=Qd, Info) ->
                {Qd, Info};
           (?qual_data(QNum, GoI, SI, {gen, PrepLE0}), Info) -> 
                PrepLE = sort_info(PrepLE0, QNum, QOpt),
                Qd = ?qual_data(QNum, GoI, SI, {gen, PrepLE}),
                I = [{{Column,Order}, [{traverse,QNum,C}]} ||
                        {{C,Order},What} <- PrepLE#prepared.sort_info2,
                        What =:= [], % Something else later...
                        Column <- equal_template_columns(QOpt, {QNum,C})],
                {Qd, [I | Info]}
        end,
    {Qdata, SortInfoL} = lists:mapfoldl(F, [], Qdata0),
    SortInfo0 = [{{Pos,Ord}, [template]} || 
                    Pos <- constant_columns(QOpt, 0), 
                    Ord <- orders(yes)]
           ++ lists:append(SortInfoL),
    SortInfo = family_union(SortInfo0),
    {Qdata, SortInfo}.

sort_info(#prepared{sort_info = SI, sorted = S} = Prep, QNum, QOpt) ->
    SI1 = [{{C,Ord},[]} || 
              S =/= no, 
              is_integer(Sz = size_of_qualifier(QOpt, QNum)), 
              Sz > 0, % the size of the pattern
              (NConstCols = size_of_constant_prefix(QOpt, QNum)) < Sz, 
              C <- [NConstCols+1],
              Ord <- orders(S)]
       ++ [{{Pos,Ord},[]} || Pos <- constant_columns(QOpt, QNum),
                             Ord <- orders(yes)]
       ++ [{PosOrd,[]} || {PosOrd,_} <- SI],
    SI2 = lists:usort(SI1),
    Prep#prepared{sort_info2 = SI2}. 

%orders(descending=O) ->
%    [O];
orders(ascending=O) ->
    [O];
orders(yes) ->
    [ascending
%   ,descending
    ].

sort_unique(true, #qlc_sort{fs_opts = SortOptions, keypos = sort}=Sort) ->
    Sort#qlc_sort{unique = false, 
                  fs_opts = 
                      lists:keydelete(unique, 1, 
                                      lists:delete(unique, SortOptions))};
sort_unique(_, Sort) ->
    Sort.

sort_tmpdir(S, #qlc_opt{tmpdir = ""}) ->
    S;
sort_tmpdir(S, Opt) ->
    S#qlc_sort{tmpdir = Opt#qlc_opt.tmpdir}.

short_list(L) ->
    %% length(L) < 2: all elements are known be equal
    #prepared{qh = #qlc_list{l = L}, sorted = yes, is_unique_objects = true, 
              is_cached = true}.
    
find_const_positions(IF, LU_fun, {KeyEquality, PosFun}, 
                     #qlc_opt{max_lookup = Max, lookup = Lookup}) 
           when is_function(LU_fun), is_function(PosFun), is_function(IF),
                Lookup =/= false ->
    case call(IF, keypos, undefined, [])  of
        undefined ->
            Indices = call(IF, indices, undefined, []),
            find_const_position_idx(Indices, KeyEquality, PosFun, Max, []);
        KeyPos ->
            case pos_vals(KeyPos, KeyEquality, PosFun(KeyPos), Max) of
                false ->
                    find_const_position_idx(IF(indices), KeyEquality,
                                            PosFun, Max, []);
                PosValuesSkip ->
                    PosValuesSkip
            end
    end;
find_const_positions(_IF, _LU_fun, _KE_PosFun, _Opt0) ->
    {false, {some,[]}}.

find_const_position_idx([I | Is], KeyEquality, PosFun, Max, L0) ->
    case pos_vals(I, KeyEquality, PosFun(I), Max) of
        false ->
            find_const_position_idx(Is, KeyEquality, PosFun, Max, L0);
        {{_Pos, Values}, _SkipFils}=PosValuesFils ->
            L = [{length(Values), PosValuesFils} | L0],
            find_const_position_idx(Is, KeyEquality, PosFun, Max, L)
    end;
find_const_position_idx(_, _KeyEquality, _PosFun, _Max, []) ->
    {false, {some,[]}};
find_const_position_idx(_, _KeyEquality, _PosFun, _Max, L) ->
    [{_,PVF} | _] = lists:sort(L),
    PVF.

pos_vals(Pos, '==', {usort_needed, Values, SkipFils}, Max) ->
    pos_vals_max(Pos, lists:usort(Values), SkipFils, Max);
pos_vals(Pos, '=:=', {usort_needed, Values, SkipFils}, Max) ->
    pos_vals_max(Pos, lists:sort(nub(Values)), SkipFils, Max);
pos_vals(Pos, _KeyEquality, {values, Values, SkipFils}, Max) ->
    pos_vals_max(Pos, Values, SkipFils, Max);
pos_vals(_Pos, _KeyEquality, _T, _Max) ->
    false.

nub([]) ->
    [];
nub([E | L]) ->
    case lists:member(E, Es=nub(L)) of
        true ->
            Es;
        false ->
            [E | Es]
    end.

%% length(Values) >= 1
pos_vals_max(Pos, Values, Skip, Max) when Max =:= -1; Max >= length(Values) ->
    {{Pos, Values}, Skip};
pos_vals_max(_Pos, _Value, _Skip, _Max) ->
    false.

prep_join(Prep, QOpt, Opt) ->
    case join_opt(QOpt) of
        undefined ->
            check_join_option(Opt),
            Prep;
        EqualMatch ->
            {Ix, M} = case EqualMatch of
                          {NEqual, NMatch} ->
                              pref_join(NEqual, NMatch, Prep, QOpt, Opt);
                          EM ->
                              pref_join(EM, EM, Prep, QOpt, Opt)
                      end,
            SI = family_union(Prep#prepared.sort_info ++ M),
            Prep#prepared{join = {Ix, M}, sort_info = SI}
    end.

%% The parse transform ensures that only two tables are involved.
pref_join(Equal, Match, Prep, QOpt, #qlc_opt{join = JoinOpt}) ->
    JQs = [{KeyEquality, QCs} ||
              {KeyEquality, QCsL} <- [{'==',Equal}, {'=:=',Match}],
              QCs <- QCsL],
    IxL = [pref_lookup_join(KE, QCs, Prep, QOpt) || 
              JoinOpt =:= any orelse JoinOpt =:= lookup,
              {KE, QCs} <- JQs],
    ML = [pref_merge_join(KE, QCs, Prep, QOpt) ||
             JoinOpt =:= any orelse JoinOpt =:= merge,
             {KE, QCs} <- JQs],
    {lists:usort(lists:append(IxL)), lists:usort(lists:append(ML))}.

pref_lookup_join(KeyEquality, {[{Q1,C1},{Q2,C2}],Skip}, Prep, QOpt) 
                                       when is_integer(C1), is_integer(C2) ->
    #prepared{qh = #qlc{qdata = QData}} = Prep,
    Is1 = lookup_qual_data(QData, Q1, KeyEquality),
    Lu2 = [pref_lookup_join2(Q2, C2, Q1, C1, Skip, QOpt, KeyEquality) ||
              IC1 <- Is1, IC1 =:= C1],
    Is2 = lookup_qual_data(QData, Q2, KeyEquality),
    Lu1 = [pref_lookup_join2(Q1, C1, Q2, C2, Skip, QOpt, KeyEquality) ||
              IC2 <- Is2, IC2 =:= C2],
    family(Lu1 ++ Lu2);
pref_lookup_join(KE, [{_,Cs1},{_,Cs2}]=L, Prep, QOpt) when is_list(Cs1), 
                                                           is_list(Cs2) ->
    %% --R12B-5
    lists:append([pref_lookup_join(KE, QC,Prep,QOpt) || 
                     QC <- selections_no_skip(L)]).

lookup_qual_data(QData, QNum, KeyEquality) ->
    case lists:keysearch(QNum, 1, QData) of
        {value, ?qual_data(QNum, _, _, {gen, PrepLE})} ->
            join_indices(PrepLE, KeyEquality)
    end.

%% If the table has a match specification (ms =/= no_match_spec) that
%% has _not_ been derived from a filter but from a query handle then
%% the lookup join cannot be done. This particular case has not been
%% excluded here but is taken care of in opt_join().
join_indices(#prepared{qh = #qlc_table{info_fun = IF, 
                                       lookup_fun = LU_fun,
                                       key_equality = KeyEquality,
                                       lu_vals = undefined}},
             KE) when is_function(LU_fun),
                      KE =:= KeyEquality orelse
                      KE =:= '=:=' andalso 
                            KeyEquality =:= undefined -> % --R12B-5
    KpL = case call(IF, keypos, undefined, []) of
              undefined -> [];
              Kp -> [Kp]
          end,
    case call(IF, indices, undefined, []) of
        undefined -> KpL;
        Is0 -> lists:usort(KpL ++ Is0)
    end;
join_indices(_Prep, _KeyEquality) ->
    [].

pref_lookup_join2(Q1, C1, Q2, C2, Skip, QOpt, KeyEquality) ->
    TemplCols = compared_template_columns(QOpt, {Q1,C1}, KeyEquality),
    {{Q1,C1,Q2,C2},{lookup_join,TemplCols,KeyEquality,Skip}}.

pref_merge_join(KE, {[{Q1,C1},{Q2,C2}],Skip}, Prep, QOpt) 
                                       when is_integer(C1), is_integer(C2) ->
    #prepared{qh = #qlc{qdata = QData}} = Prep,
    Sort1 = merge_qual_data(QData, Q1),
    Sort2 = merge_qual_data(QData, Q2),
    Merge = pref_merge(KE, Q1, C1, Q2, C2, Skip, Sort1, Sort2, QOpt),
    family_union(Merge);
pref_merge_join(KE, [{_,Cs1},{_,Cs2}]=L, Prep, QOpt) when is_list(Cs1), 
                                                      is_list(Cs2) ->
    %% --R12B-5
    lists:append([pref_merge_join(KE, QC, Prep, QOpt) || 
                     QC <- selections_no_skip(L)]).

selections_no_skip(L) ->
    [{C,{some,[]}} || C <- all_selections(L)].

merge_qual_data(QData, QNum) ->
    case lists:keysearch(QNum, 1, QData) of
        {value, ?qual_data(QNum, _, _, {gen, PrepLE})} ->
            #prepared{sort_info2 = SortInfo} = PrepLE,
            SortInfo
    end.

pref_merge(KE, Q1, C1, Q2, C2, Skip, Sort1, Sort2, QOpt) ->
    Col1 = {Q1,C1},
    Col2 = {Q2,C2},
    DoSort = [QC || {{_QNum,Col}=QC,SortL} <- [{Col1,Sort1}, {Col2,Sort2}],
                    lists:keymember({Col, ascending}, 1, SortL) =:= false],
    J = [{{Q1,C1,Q2,C2}, {merge_join,DoSort,KE,Skip}}],
    %% true = (QOpt(template))(Col1, '==') =:= (QOpt(template))(Col2, '==')
    [{{Column, ascending}, J} || 
        Column <- equal_template_columns(QOpt, Col1)] ++ [{other, J}].

table_sort_info(#qlc_table{info_fun = IF}) ->
    case call(IF, is_sorted_key, undefined, []) of
        undefined -> 
            {[], no};
        false -> 
            {[], no};
        true -> 
            case call(IF, keypos, undefined, []) of
                undefined -> % strange
                    {[], no};
                KeyPos ->
                    {[{{KeyPos,ascending},[]}], no}
            end
    end.

sort_sort_info(#qlc_sort{keypos = sort, order = Ord0}) ->
    {[], sort_order(Ord0)};
sort_sort_info(#qlc_sort{keypos = {keysort,Kp0}, order = Ord0}) ->
    Kp = case Kp0 of
             [Pos | _] -> Pos;
             _ -> Kp0
         end,
    {[{{Kp,sort_order(Ord0)},[]}], no}.

sort_order(F) when is_function(F) ->
    no;
sort_order(Order) ->
    Order.

check_join_option(#qlc_opt{join = any}) ->
    ok;
check_join_option(#qlc_opt{join = Join}) ->
    erlang:error(no_join_to_carry_out, [{join,Join}]).

check_lookup_option(#qlc_opt{lookup = true}, false) ->
    erlang:error(no_lookup_to_carry_out, [{lookup,true}]);
check_lookup_option(_QOpt, _LuV) ->
    ok.

compared_template_columns(QOpt, QNumColumn, KeyEquality) ->
    (QOpt(template))(QNumColumn, KeyEquality).

equal_template_columns(QOpt, QNumColumn) ->
    (QOpt(template))(QNumColumn, '==').

%eq_template_columns(QOpt, QNumColumn) ->
%    (QOpt(template))(QNumColumn, '=:=').

size_of_constant_prefix(QOpt, QNum) ->
    (QOpt(n_leading_constant_columns))(QNum).

constants(QOpt, QNum) ->
    (QOpt(constants))(QNum).

equal_constants(QOpt, QNum) ->
    (QOpt(equal_constants))(QNum).

join_opt(QOpt) ->
    QOpt(join).

match_specs(QOpt, QNum) ->
    (QOpt(match_specs))(QNum).

constant_columns(QOpt, QNum) ->
    (QOpt(constant_columns))(QNum).

size_of_qualifier(QOpt, QNum) ->
    (QOpt(size))(QNum).

%% Two optimizations are carried out:
%% 1. The first generator is never cached if the QLC itself is cached.
%% Since the answers do not need to be cached, the top-most QLC is
%% never cached either. Simple QLCs not holding any options are
%% removed. Simple QLCs are coalesced when possible.
%% 2. Merge join and lookup join is done if possible.

opt_le(#prepared{qh = #simple_qlc{le = LE0, optz = Optz0}=QLC}=Prep0, 
       GenNum) ->
    case LE0 of
        #prepared{qh = #simple_qlc{p = LE_Pvar, le = LE2, optz = Optz2}} ->
            %% Coalesce two simple QLCs.
            Cachez = case Optz2#optz.cache of
                         false -> Optz0#optz.cache;
                         Cache2 -> Cache2
                     end,
            Optz = Optz0#optz{cache = Cachez,
                              unique = Optz0#optz.unique or Optz2#optz.unique},
            PVar = if 
                       LE_Pvar =:= ?SIMPLE_QVAR -> QLC#simple_qlc.p;
                       true -> LE_Pvar
                   end,
            Prep = Prep0#prepared{qh = QLC#simple_qlc{p = PVar, le = LE2,
                                                      optz = Optz}},
            opt_le(Prep, GenNum);
        _ ->
            Optz1 = no_cache_of_first_generator(Optz0, GenNum),
            case {opt_le(LE0, 1), Optz1} of
                {LE, #optz{unique = false, cache = false}} ->
                    LE;
                {LE, _} ->
                    Prep0#prepared{qh = QLC#simple_qlc{le = LE, optz = Optz1}}
            end
    end;
opt_le(#prepared{qh = #qlc{}, lu_skip_quals = LU_SkipQuals0}=Prep0, GenNum) ->
    #prepared{qh = #qlc{qdata = Qdata0, optz = Optz0}=QLC} = Prep0,
    #optz{join_option = JoinOption, opt = Opt} = Optz0,
    JoinOption = Optz0#optz.join_option,
    {LU_QNum, Join, JoinSkipFs, DoSort} = 
        opt_join(Prep0#prepared.join, JoinOption, Qdata0, Opt, LU_SkipQuals0),
    {LU_Skip, LU_SkipQuals} = 
        lists:partition(fun({QNum,_Fs}) -> QNum =:= LU_QNum end,
                        LU_SkipQuals0),
    LU_SkipFs = lists:flatmap(fun({_QNum,Fs}) -> Fs end, LU_SkipQuals),
    %% If LU_QNum has a match spec it must be applied _after_ the
    %% lookup join (the filter must not be skipped!). 
    Qdata1 = if 
                 LU_Skip =:= [] -> Qdata0;
                 true -> activate_join_lookup_filter(LU_QNum, Qdata0)
             end,
    Qdata2 = skip_lookup_filters(Qdata1, LU_SkipFs ++ JoinSkipFs),
    F = fun(?qual_data(QNum, GoI, SI, {gen, #prepared{}=PrepLE}), GenNum1) ->
                NewPrepLE = maybe_sort(PrepLE, QNum, DoSort, Opt),
                {?qual_data(QNum, GoI, SI, {gen, opt_le(NewPrepLE, GenNum1)}),
                 GenNum1 + 1};
           (Qd, GenNum1) ->
                {Qd, GenNum1}
        end,
    {Qdata, _} = lists:mapfoldl(F, 1, Qdata2),
    Optz1 = no_cache_of_first_generator(Optz0, GenNum),
    Optz = Optz1#optz{fast_join = Join},
    Prep0#prepared{qh = QLC#qlc{qdata = Qdata, optz = Optz}};
opt_le(#prepared{qh = #qlc_append{hl = HL}}=Prep, GenNum) ->
    Hs = [opt_le(H, GenNum) || H <- HL],
    Prep#prepared{qh = #qlc_append{hl = Hs}};
opt_le(#prepared{qh = #qlc_sort{h = H}=Sort}=Prep, GenNum) ->
    Prep#prepared{qh = Sort#qlc_sort{h = opt_le(H, GenNum)}};
opt_le(Prep, _GenNum) ->
    Prep.

no_cache_of_first_generator(Optz, GenNum) when GenNum > 1 ->
    Optz;
no_cache_of_first_generator(Optz, 1) ->
    Optz#optz{cache = false}.

maybe_sort(LE, QNum, DoSort, Opt) ->
    case lists:keyfind(QNum, 1, DoSort) of
        {QNum, Col} ->
            #qlc_opt{tmpdir = TmpDir, tmpdir_usage = TmpUsage} = Opt,
            SortOpts = [{tmpdir,Dir} || Dir <- [TmpDir], Dir =/= ""],
            Sort = #qlc_sort{h = LE, keypos = {keysort, Col}, unique = false,
                             compressed = [], order = ascending,
                             fs_opts = SortOpts, tmpdir_usage = TmpUsage,
                             tmpdir = TmpDir},
            #prepared{qh = Sort, sorted = no, join = no};
        false ->
            LE
    end.

skip_lookup_filters(Qdata, []) ->
    Qdata;
skip_lookup_filters(Qdata0, LU_SkipFs) ->
    [case lists:member(QNum, LU_SkipFs) of
         true ->
             ?qual_data(QNum, GoI, ?SKIP, fil);
         false ->
             Qd
     end || ?qual_data(QNum, GoI, _, _)=Qd <- Qdata0].

%% If the qualifier used for lookup by the join (QNum) has a match
%% specification it must be applied _after_ the lookup join (the
%% filter must not be skipped!).
activate_join_lookup_filter(QNum, Qdata) ->
    {_,GoI2,SI2,{gen,Prep2}} = lists:keyfind(QNum, 1, Qdata),
    Table2 = Prep2#prepared.qh,
    NPrep2 = Prep2#prepared{qh = Table2#qlc_table{ms = no_match_spec}},
    %% Table2#qlc_table.ms has been reset; the filter will be run.
    lists:keyreplace(QNum, 1, Qdata, ?qual_data(QNum,GoI2,SI2,{gen,NPrep2})).

opt_join(Join, JoinOption, Qdata, Opt, LU_SkipQuals) ->
    %% prep_qlc_lc() assures that no unwanted join is carried out
    {Ix0, M0} = Join,
    Ix1 = opt_join_lu(Ix0, Qdata, LU_SkipQuals),
    Ix = lists:reverse(lists:keysort(2, Ix1)), % prefer to skip
    case Ix of 
        [{{Q1,C1,Q2,C2},Skip,KE,LU_fun} | _] ->
            J = #qlc_join{kind = {lookup, KE, LU_fun}, q1 = Q1,
                          c1 = C1, q2 = Q2, c2 = C2, opt = Opt},
            {Q2, J, Skip, []};
        [] ->
            M = opt_join_merge(M0),
            case M of
                [{{Q1,C1,Q2,C2},{merge_join,DoSort,KE,Skip}}|_] ->
                    J = #qlc_join{kind = {merge, KE}, opt = Opt,
                                  q1 = Q1, c1 = C1, q2 = Q2, c2 = C2},
                    {not_a_qnum, J, Skip, DoSort};
                [] when JoinOption =:= nested_loop ->
                    {not_a_qnum, no, [], []};
                _ when JoinOption =/= any ->
                    erlang:error(cannot_carry_out_join, [JoinOption]);
                _ ->
                    {not_a_qnum, no, [], []}
            end
    end.

opt_join_lu([{{_Q1,_C1,Q2,_C2}=J,[{lookup_join,_KEols,JKE,Skip0} | _]} | LJ], 
            Qdata, LU_SkipQuals) ->
    {Q2,_,_,{gen,Prep2}} = lists:keyfind(Q2, 1, Qdata),
    #qlc_table{ms = MS, key_equality = KE, 
               lookup_fun = LU_fun} = Prep2#prepared.qh,
    %% If there is no filter to skip (the match spec was derived 
    %% from a query handle) then the lookup join cannot be done.
    case 
        MS =/= no_match_spec andalso 
        lists:keymember(Q2, 1, LU_SkipQuals) =:= false
    of 
        true ->
            opt_join_lu(LJ, Qdata, LU_SkipQuals);
        false ->
            %% The join is preferred before evaluating the match spec
            %% (if there is one).
            Skip = skip_if_possible(JKE, KE, Skip0),
            [{J,Skip,KE,LU_fun} | opt_join_lu(LJ, Qdata, LU_SkipQuals)]
    end;
opt_join_lu([], _Qdata, _LU_SkipQuals) ->
    [].

opt_join_merge(M) ->
    %% Prefer not to sort arguments. Prefer to skip join filter.
    L = [{-length(DoSort),length(Skip),
         {QCs,{merge_join,DoSort,KE,Skip}}} ||
            {_KpOrder_or_other,MJ} <- M,
            {QCs,{merge_join,DoSort,KE,Skip0}} <- MJ,
            Skip <- [skip_if_possible(KE, '==', Skip0)]],
    lists:reverse([J || {_,_,J} <- lists:sort(L)]).

%% Cannot skip the join filter the join operator is '=:=' and the join
%% is performed using '=='. Note: the tag 'some'/'all' is not used.
skip_if_possible('=:=', '==', _) ->
    [];
skip_if_possible(_, _, {_SkipTag, Skip}) ->
    Skip.

%% -> {Objects, Post, LocalPost} | throw()
%% Post is a list of funs (closures) to run afterwards.
%% LocalPost should be run when all objects have been found (optimization).
%% LocalPost will always be a subset of Post.
%% List expressions are evaluated, resulting in lists of objects kept in
%% RAM or on disk.
%% An error term is thrown as soon as cleanup according Post has been
%% done. (This is opposed to errors during evaluation; such errors are
%% returned as terms.)
setup_qlc(Prep, Setup) ->
    Post0 = [],
    setup_le(Prep, Post0, Setup).

setup_le(#prepared{qh = #simple_qlc{le = LE, optz = Optz}}, Post0, Setup) ->
    {Objs, Post, LocalPost} = setup_le(LE, Post0, Setup),
    unique_cache(Objs, Post, LocalPost, Optz);
setup_le(#prepared{qh = #qlc{lcf = QFun, qdata = Qdata, init_value = V, 
                             optz = Optz}}, Post0, Setup) ->
    {GoTo, FirstState, Post, LocalPost} = 
        setup_quals(Qdata, Post0, Setup, Optz),
    Objs = fun() -> QFun(FirstState, V, GoTo) end,
    unique_cache(Objs, Post, LocalPost, Optz);
setup_le(#prepared{qh = #qlc_table{post_fun = PostFun}=Table}, Post, Setup) ->
    H = table_handle(Table, Post, Setup),
    %% The pre fun has been called from table_handle():
    {H, [PostFun | Post], []};
setup_le(#prepared{qh = #qlc_append{hl = PrepL}}, Post0, Setup) ->
    F = fun(Prep, {Post1, LPost1}) -> 
                {Objs, Post2, LPost2} = setup_le(Prep, Post1, Setup),
                {Objs, {Post2, LPost1++LPost2}}
        end,
    {ObjsL, {Post, LocalPost}} = lists:mapfoldl(F, {Post0,[]}, PrepL),
    {fun() -> append_loop(ObjsL, 0) end, Post, LocalPost};
setup_le(#prepared{qh = #qlc_sort{h = Prep, keypos = Kp, 
                                  unique = Unique, compressed = Compressed,
                                  order = Order, fs_opts = SortOptions0, 
                                  tmpdir_usage = TmpUsage,tmpdir = TmpDir}},
         Post0, Setup) ->
    SortOptions = sort_options_global_tmp(SortOptions0, TmpDir),
    LF = fun(Objs) -> 
                 sort_list(Objs, Order, Unique, Kp, SortOptions, Post0)
         end,
    case setup_le(Prep, Post0, Setup) of
        {L, Post, LocalPost} when is_list(L) ->
            {LF(L), Post, LocalPost};
        {Objs, Post, LocalPost} ->
            FF = fun(Objs1) ->
                         file_sort_handle(Objs1, Kp, SortOptions, TmpDir, 
                                          Compressed, Post, LocalPost)
                 end,
            sort_handle(Objs, LF, FF, SortOptions, Post, LocalPost, 
                        {TmpUsage, sorting})
    end;
setup_le(#prepared{qh = #qlc_list{l = L, ms = MS}}, Post, _Setup) 
              when (no_match_spec =:= MS); L =:= [] ->
    {L, Post, []};
setup_le(#prepared{qh = #qlc_list{l = L, ms = MS}}, Post, _Setup) 
                                   when is_list(L) ->
    {ets:match_spec_run(L, ets:match_spec_compile(MS)), Post, []};
setup_le(#prepared{qh = #qlc_list{l = H0, ms = MS}}, Post0, Setup) ->
    {Objs0, Post, LocalPost} = setup_le(H0, Post0, Setup),
    Objs = ets:match_spec_run(Objs0, ets:match_spec_compile(MS)),
    {Objs, Post, LocalPost}.

%% The goto table (a tuple) is created at runtime. It is accessed by
%% the generated code in order to find next clause to execute. For
%% generators there is also a fun; calling the fun runs the list
%% expression of the generator. There are two elements for a filter:
%% the first one is the state to go when the filter is false; the
%% other the state when the filter is true. There are three elements
%% for a generator G: the first one is the state of the generator
%% before G (or the stop state if there is no generator); the second
%% one is the state of the qualifier following the generator (or the
%% template if there is no next generator); the third one is the list
%% expression fun. 
%% There are also join generators which are "activated" when it is 
%% possbible to do a join.

setup_quals(Qdata, Post0, Setup, Optz) ->
    {GoTo0, Post1, LocalPost0} = 
        setup_quals(0, Qdata, [], Post0, [], Setup),
    GoTo1 = lists:keysort(1, GoTo0),
    FirstState0 = next_state(Qdata),
    {GoTo2, FirstState, Post, LocalPost1} =
        case Optz#optz.fast_join of
            #qlc_join{kind = {merge,_KE}, c1 = C1, c2 = C2, opt = Opt} = MJ ->
                MF = fun(_Rev, {H1, WH1}, {H2, WH2}) -> 
                             fun() -> 
                                  merge_join(WH1(H1), C1, WH2(H2), C2, Opt) 
                             end
                     end,
                setup_join(MJ, Qdata, GoTo1, FirstState0, MF, Post1);
            #qlc_join{kind = {lookup,_KE,LuF}, c1 = C1, c2 = C2} = LJ ->
                LF = fun(Rev, {H1, WH1}, {H2, WH2}) ->
                             {H, W} = if 
                                          Rev -> {H2, WH2};
                                          true -> {H1, WH1} 
                                      end,
                             fun() -> 
                                     lookup_join(W(H), C1, LuF, C2, Rev)
                             end
                     end,
                setup_join(LJ, Qdata, GoTo1, FirstState0, LF, Post1);
            no ->
                {flat_goto(GoTo1), FirstState0, Post1, []}
        end,
    GoTo = list_to_tuple(GoTo2),
    {GoTo, FirstState, Post, LocalPost0 ++ LocalPost1}.

setup_quals(GenLoopS, [?qual_data(_QNum,GoI,?SKIP,fil) | Qdata], 
            Gs, P, LP, Setup) ->
    %% ?SKIP causes runtime error. See also skip_lookup_filters().
    setup_quals(GenLoopS, Qdata, [{GoI,[?SKIP,?SKIP]} | Gs], P, LP, Setup);
setup_quals(GenLoopS, [?qual_data(_QNum,GoI,_SI,fil) | Qdata], 
            Gs, P, LP, Setup) ->
    setup_quals(GenLoopS, Qdata, [{GoI,[GenLoopS,next_state(Qdata)]} | Gs], 
                P, LP, Setup);
setup_quals(GenLoopS, [?qual_data(_QNum,GoI,_SI, {gen,#join{}}) | Qdata],
            Gs, P, LP, Setup) ->
    setup_quals(GenLoopS, Qdata, [{GoI,[?SKIP,?SKIP,?SKIP]} | Gs],P,LP,Setup);
setup_quals(GenLoopS, [?qual_data(_QNum,GoI,SI,{gen,LE}) | Qdata], 
            Gs, P, LP, Setup) ->
    {V, NP, LP1} = setup_le(LE, P, Setup),
    setup_quals(SI + 1, Qdata, [{GoI, [GenLoopS,next_state(Qdata),V]} | Gs], 
                NP, LP ++ LP1, Setup);
setup_quals(GenLoopS, [], Gs, P, LP, _Setup) ->
    {[{1,[GenLoopS]} | Gs], P, LP}.

%% Finds the qualifier in Qdata that performs the join between Q1 and
%% Q2, and sets it up using the handles already set up for Q1 and Q2.
%% Removes Q1 and Q2 from GoTo0 and updates the join qualifier in GoTo0.
%% Note: the parse transform has given each generator three slots
%% in the GoTo table. The position of these slots within the GoTo table
%% is fixed (at runtime).
%% (Assumes there is only one join-generator in Qdata.)
setup_join(J, Qdata, GoTo0, FirstState0, JoinFun, Post0) ->
    #qlc_join{q1 = QNum1a, q2 = QNum2a, opt = Opt} = J,
    {?qual_data(_QN,JGoI,JSI,_), Rev, QNum1, QNum2, WH1, WH2, _CsFun} = 
        find_join_data(Qdata, QNum1a, QNum2a),
    [{GoI1,SI1}] = [{GoI,SI} ||
                       ?qual_data(QNum,GoI,SI,_) <- Qdata, QNum =:= QNum1],
    [{GoI2,SI2}] = [{GoI,SI} ||
                       ?qual_data(QNum,GoI,SI,_) <- Qdata, QNum =:= QNum2],

    [H1] = [H || {GoI,[_Back,_Forth,H]} <- GoTo0, GoI =:= GoI1],
    [{BackH2,H2}] = 
           [{Back,H} || {GoI,[Back,_Forth,H]} <- GoTo0, GoI =:= GoI2],
    H0 = JoinFun(Rev, {H1,WH1}, {H2,WH2}),
    %% The qlc expression options apply to the introduced qlc expr as well.
    {H, Post, LocalPost} = 
         unique_cache(H0, Post0, [], join_unique_cache(Opt)),
    [JBack] = [Back || {GoI,[Back,_,_]} <- GoTo0, GoI =:= GoI1],
    JForth = next_after(Qdata, SI1, QNum2),
    GoTo1 = lists:map(fun({GoI,_}) when GoI =:= JGoI -> 
                              {JGoI, [JBack, JForth, H]};
                         ({GoI,_}) when GoI =:= GoI1; GoI =:= GoI2 ->
                              {GoI, [?SKIP,?SKIP,?SKIP]}; % not necessary
                         (Go) ->
                              Go
                      end, GoTo0),
    GoTo = lists:map(fun(S) when S =:= SI1 -> 
                             JSI;
                        (S) when S =:= SI2 -> 
                             next_after(Qdata, S, QNum2);
                        (S) when S =:= SI1+1 -> 
                             JSI+1;
                        (S) when S =:= SI2+1, SI1 + 1 =:= BackH2  -> 
                             JSI+1;
                        (S) when S =:= SI2+1 -> 
                             BackH2;
                        (S) -> S
                     end, flat_goto(GoTo1)),
    FirstState = if
                     SI1 =:= FirstState0 -> JSI;
                     true -> FirstState0
                 end,
    {GoTo, FirstState, Post, LocalPost}.

join_unique_cache(#qlc_opt{cache = Cache, unique = Unique}=Opt) ->
    #optz{cache = Cache, unique = Unique, opt = Opt}.

flat_goto(GoTo) ->
    lists:flatmap(fun({_,L}) -> L end, GoTo).

next_after([?qual_data(_, _, S, _) | Qdata], S, QNum2) ->
    case Qdata of
        [?qual_data(QNum2, _, _, _) | Qdata1] ->
            next_state(Qdata1);
        _ -> 
            next_state(Qdata)
    end;
next_after([_ | Qdata], S, QNum2) ->
    next_after(Qdata, S, QNum2).

next_state([?qual_data(_,_,_,{gen,#join{}}) | Qdata]) ->
    next_state(Qdata);
next_state([?qual_data(_,_,?SKIP,fil) | Qdata]) ->
    %% see skip_lookup_filters()
    next_state(Qdata);
next_state([?qual_data(_,_,S,_) | _]) ->
    S;
next_state([]) ->
    template_state().

find_join_data(Qdata, QNum1, QNum2) ->
    [QRev] = [{Q,Rev,QN1,QN2,H1,H2,CsF} || 
                 ?qual_data(_QN,_GoI,_SI,
                            {gen,#join{q1 = QN1,q2 = QN2, 
                                       wh1 = H1, wh2 = H2,
                                       cs_fun = CsF}})= Q <- Qdata,
                 if 
                     QN1 =:= QNum1, QN2 =:= QNum2 ->
                         not (Rev = false);
                     QN1 =:= QNum2, QN2 =:= QNum1 ->
                         Rev = true;
                     true -> 
                         Rev = false
                 end],
    QRev.

table_handle(#qlc_table{trav_fun = TraverseFun, trav_MS = TravMS, 
                        pre_fun = PreFun, lookup_fun = LuF, 
                        parent_fun = ParentFun, lu_vals = LuVals, ms = MS}, 
             Post, Setup) ->
    #setup{parent = Parent} = Setup,
    ParentValue = 
        if 
            ParentFun =:= undefined ->
                undefined;
            Parent =:= self() ->
                try
                    ParentFun() 
                catch Class:Reason:Stacktrace ->
                    post_funs(Post),
                    erlang:raise(Class, Reason, Stacktrace)
                end;
            true ->
                case monitor_request(Parent, {parent_fun, ParentFun}) of
                    error -> % parent has died
                        post_funs(Post),
                        exit(normal);
                    {value, Value} ->
                        Value;
                    {parent_fun_caught, Class, Reason, Stacktrace} ->
                        %% No use augmenting Stacktrace here.
                        post_funs(Post),
                        erlang:raise(Class, Reason, Stacktrace)
                end
        end,
    StopFun = 
        if 
            Parent =:= self() ->
                undefined;
            true ->
                Cursor = #qlc_cursor{c = {self(), Parent}},
                fun() -> delete_cursor(Cursor) end
        end,
    PreFunArgs = [{parent_value, ParentValue}, {stop_fun, StopFun}],
    _ = call(PreFun, PreFunArgs, ok, Post),
    case LuVals of
        {Pos, Vals} when MS =:= no_match_spec ->
            LuF(Pos, Vals);
        {Pos, Vals} ->
            case LuF(Pos, Vals) of
                [] -> 
                    [];
                Objs when is_list(Objs) -> 
                    ets:match_spec_run(Objs, 
                                       ets:match_spec_compile(MS));
                Error ->
                    post_funs(Post),
                    throw_error(Error)
            end;
        _ when not TravMS ->
            MS = no_match_spec, % assertion
            TraverseFun;
        _ when MS =:= no_match_spec ->
            fun() -> TraverseFun([{'$1',[],['$1']}]) end;
        _ ->
            fun() -> TraverseFun(MS) end
    end.

-define(CHUNK_SIZE, 64*1024).

open_file(FileName, Extra, Post) ->
    case file:open(FileName, [read, raw, binary | Extra]) of
        {ok, Fd} ->
            {fun() -> 
                 case file:position(Fd, bof) of
                     {ok, 0} -> 
                         TF = fun([], _) -> 
                                      [];
                                 (Ts, C) when is_list(Ts) -> 
                                      lists:reverse(Ts, C) 
                              end,
                         file_loop_read(<<>>, ?CHUNK_SIZE, {Fd,FileName}, TF);
                     Error -> 
                         file_error(FileName, Error)
                 end
             end, Fd};
        Error ->
            post_funs(Post),
            throw_file_error(FileName, Error)
    end.

file_loop(Bin0, Fd_FName, Ts0, TF) ->
    case 
        try file_loop2(Bin0, Ts0) 
        catch _:_ ->        
            {_Fd, FileName} = Fd_FName,
            error({bad_object, FileName})
        end
    of
        {terms, <<Size:4/unit:8, B/bytes>>=Bin, []} ->
            file_loop_read(Bin, Size - byte_size(B) + 4, Fd_FName, TF);
        {terms, <<Size:4/unit:8, _/bytes>>=Bin, Ts} ->
            C = fun() -> file_loop_read(Bin, Size+4, Fd_FName, TF) end,
            TF(Ts, C);
        {terms, B, Ts} ->
            C = fun() -> file_loop_read(B, ?CHUNK_SIZE, Fd_FName, TF) end,
            TF(Ts, C);
        Error ->
            Error
    end.

file_loop2(<<Size:4/unit:8, B:Size/bytes, Bin/bytes>>, Ts) ->
    file_loop2(Bin, [binary_to_term(B) | Ts]);
file_loop2(Bin, Ts) ->
    {terms, Bin, Ts}.

%% After power failures (and only then) files with corrupted Size
%% fields have been observed in a disk_log file. If file:read/2 is
%% asked to read a huge amount of data the emulator may crash. Nothing
%% has been done here to prevent such crashes (by inspecting
%% BytesToRead in some way) since temporary files will never be read
%% after a power failure.
file_loop_read(B, MinBytesToRead, {Fd, FileName}=Fd_FName, TF) ->
    BytesToRead = erlang:max(?CHUNK_SIZE, MinBytesToRead),
    case file:read(Fd, BytesToRead) of
        {ok, Bin} when byte_size(B) =:= 0 ->
            file_loop(Bin, Fd_FName, [], TF);
        {ok, Bin} ->
            case B of 
                <<Size:4/unit:8, Tl/bytes>> 
                                when byte_size(Bin) + byte_size(Tl) >= Size ->
                    {B1, B2} = split_binary(Bin, Size - byte_size(Tl)),
                    Foo = fun([T], Fun) -> [T | Fun] end,
                    %% TF should be applied exactly once.
                    case 
                        file_loop(list_to_binary([B, B1]), Fd_FName, [], Foo) 
                    of
                        [T | Fun] -> 
                            true = is_function(Fun),
                            file_loop(B2, Fd_FName, [T], TF);
                        Error ->
                            Error
                    end;
                _ ->
                    file_loop(list_to_binary([B, Bin]), Fd_FName, [], TF)
            end;
        eof when byte_size(B) =:= 0 ->
            TF([], foo);
        eof ->
            error({bad_object, FileName});
        Error ->
            file_error(FileName, Error)
    end.

sort_cursor_input(H, NoObjects) ->
    fun(close) ->
            ok;
       (read) ->
            sort_cursor_input_read(H, NoObjects)
    end.
                    
sort_cursor_list_output(TmpDir, Z, Unique) ->
    fun(close) ->
            {terms, []};
       ({value, NoObjects}) ->
            fun(BTerms) when Unique; length(BTerms) =:= NoObjects ->
                    fun(close) ->
                            {terms, BTerms};
                       (BTerms1) ->
                            sort_cursor_file(BTerms ++ BTerms1, TmpDir, Z)
                    end;
               (BTerms) ->
                    sort_cursor_file(BTerms, TmpDir, Z)
            end
    end.

sort_cursor_file(BTerms, TmpDir, Z) ->
    FName = tmp_filename(TmpDir),
    case file:open(FName, [write, raw, binary | Z]) of
        {ok, Fd} ->
            WFun = write_terms(FName, Fd),
            WFun(BTerms);
        Error ->
            throw_file_error(FName, Error)
    end.

sort_options_global_tmp(S, "") ->
    S;
sort_options_global_tmp(S, TmpDir) ->
    [{tmpdir,TmpDir} | lists:keydelete(tmpdir, 1, S)].

tmp_filename(TmpDirOpt) ->
    U = "_",
    Node = node(),
    Pid = os:getpid(),
    Unique = erlang:unique_integer(),
    F = lists:concat([?MODULE,U,Node,U,Pid,U,Unique]),
    TmpDir = case TmpDirOpt of
                 "" ->
                     {ok, CurDir} = file:get_cwd(),
                     CurDir;
                 TDir ->
                     TDir
             end,
    filename:join(filename:absname(TmpDir), F).

write_terms(FileName, Fd) ->
    fun(close) ->
            _ = file:close(Fd),
            {file, FileName};
       (BTerms) ->
            case file:write(Fd, size_bin(BTerms, [])) of
                ok ->
                    write_terms(FileName, Fd);
                Error ->
                    _ = file:close(Fd),
                    throw_file_error(FileName, Error)
            end
    end.

size_bin([], L) ->
    L;
size_bin([BinTerm | BinTerms], L) ->
    size_bin(BinTerms, [L, <<(byte_size(BinTerm)):4/unit:8>> | BinTerm]).

sort_cursor_input_read([], NoObjects) ->
    {end_of_input, NoObjects};
sort_cursor_input_read([Object | Cont], NoObjects) ->
    {[term_to_binary(Object)], sort_cursor_input(Cont, NoObjects + 1)};
sort_cursor_input_read(F, NoObjects) ->
    case F() of
        Objects when is_list(Objects) ->
            sort_cursor_input_read(Objects, NoObjects);
        Term ->
            throw_error(Term)
    end.

unique_cache(L, Post, LocalPost, Optz) when is_list(L) ->
    case Optz#optz.unique of
        true -> 
            {unique_sort_list(L), Post, LocalPost};
        false ->
            %% If Optz#optz.cache then an ETS table could be used. 
            {L, Post, LocalPost}
    end;
unique_cache(H, Post, LocalPost, #optz{unique = false, cache = false}) ->
    {H, Post, LocalPost};
unique_cache(H, Post, LocalPost, #optz{unique = true, cache = false}) ->
    E = ets:new(qlc, [set, private]),
    {fun() -> no_dups(H, E) end, [del_table(E) | Post], LocalPost};
unique_cache(H, Post, LocalPost, #optz{unique = false, cache = true}) ->
    E = ets:new(qlc, [set, private]),
    {L, P} = unique_cache_post(E),
    {fun() -> cache(H, E, LocalPost) end, [P | Post], [L]};
unique_cache(H, Post, LocalPost, #optz{unique = true, cache = true}) ->
    UT = ets:new(qlc, [bag, private]),
    MT = ets:new(qlc, [set, private]),
    {L1, P1} = unique_cache_post(UT),
    {L2, P2} = unique_cache_post(MT),
    {fun() -> ucache(H, UT, MT, LocalPost) end, [P1, P2 | Post], [L1, L2]};
unique_cache(H, Post, LocalPost, #optz{unique = false, cache = list}=Optz) ->
    Ref = make_ref(),
    F = del_lcache(Ref),
    #qlc_opt{tmpdir = TmpDir, max_list = MaxList, tmpdir_usage = TmpUsage} =
        Optz#optz.opt,
    {fun() -> lcache(H, Ref, LocalPost, TmpDir, MaxList, TmpUsage) end, 
     [F | Post], [F]};
unique_cache(H, Post0, LocalPost0, #optz{unique = true, cache = list}=Optz) ->
    #qlc_opt{tmpdir = TmpDir, max_list = MaxList, tmpdir_usage = TmpUsage} =
        Optz#optz.opt,
    Size = if
               MaxList >= 1 bsl 31 -> (1 bsl 31) - 1;
               MaxList =:= 0 -> 1;
               true ->  MaxList
           end,
    SortOptions = [{size, Size}, {tmpdir, TmpDir}],
    USortOptions = [{unique, true} | SortOptions],
    TmpUsageM = {TmpUsage, caching},
    LF1 = fun(Objs) -> lists:ukeysort(1, Objs) end,
    FF1 = fun(Objs) ->
                  file_sort_handle(Objs, {keysort, 1}, USortOptions, 
                                   TmpDir, [], Post0, LocalPost0)
          end,
    {UH, Post1, LocalPost1} = sort_handle(tag_objects(H, 1), LF1, FF1, 
                                          USortOptions, Post0, LocalPost0, 
                                          TmpUsageM),
    LF2 = fun(Objs) -> lists:keysort(2, Objs) end,
    FF2 = fun(Objs) ->
                  file_sort_handle(Objs, {keysort, 2}, SortOptions, TmpDir,
                                   [], Post1, LocalPost1)
          end,
    {SH, Post, LocalPost} = 
        sort_handle(UH, LF2, FF2, SortOptions, Post1, LocalPost1, TmpUsageM),
    if
        is_list(SH) ->
            %% Remove the tag once and for all.
            {untag_objects2(SH), Post, LocalPost};
        true ->
            %% Every traversal untags the objects...
            {fun() -> untag_objects(SH) end, Post, LocalPost}
    end.

unique_cache_post(E) ->
    {empty_table(E), del_table(E)}.

unique_sort_list(L) ->
    E = ets:new(qlc, [set, private]),
    unique_list(L, E).

unique_list([], E) ->
    true = ets:delete(E),
    [];
unique_list([Object | Objects], E) ->
    case ets:member(E, Object) of
        false ->
            true = ets:insert(E, {Object}),
            [Object | unique_list(Objects, E)];
        true ->
            unique_list(Objects, E)
    end.

sort_list(L, CFun, true, sort, _SortOptions, _Post) when is_function(CFun) ->
    lists:usort(CFun, L);
sort_list(L, CFun, false, sort, _SortOptions, _Post) when is_function(CFun) ->
    lists:sort(CFun, L);
sort_list(L, ascending, true, sort, _SortOptions, _Post) ->
    lists:usort(L);
sort_list(L, descending, true, sort, _SortOptions, _Post) ->
    lists:reverse(lists:usort(L));
sort_list(L, ascending, false, sort, _SortOptions, _Post) ->
    lists:sort(L);
sort_list(L, descending, false, sort, _SortOptions, _Post) ->
    lists:reverse(lists:sort(L));
sort_list(L, Order, Unique, {keysort, Kp}, _SortOptions, _Post) 
           when is_integer(Kp), is_atom(Order) ->
    case {Order, Unique} of
        {ascending, true} ->
            lists:ukeysort(Kp, L);
        {ascending, false} ->
            lists:keysort(Kp, L);
        {descending, true} ->
            lists:reverse(lists:ukeysort(Kp, L));
        {descending, false} ->
            lists:reverse(lists:keysort(Kp, L))
    end;
sort_list(L, _Order, _Unique, Sort, SortOptions, Post) ->
    In = fun(_) -> {L, fun(_) -> end_of_input end} end,
    Out = sort_list_output([]),
    TSortOptions = [{format,term} | SortOptions],
    do_sort(In, Out, Sort, TSortOptions, Post).

sort_list_output(L) ->
    fun(close) ->
            lists:append(lists:reverse(L));
       (Terms) when is_list(Terms) ->
            sort_list_output([Terms | L])
    end.

%% Don't use the file_sorter unless it is known that objects will be
%% put on a temporary file (optimization).
sort_handle(H, ListFun, FileFun, SortOptions, Post, LocalPost, TmpUsageM) ->
    Size = case lists:keyfind(size, 1, SortOptions) of
               {size, Size0} -> Size0;
               false -> default_option(size)
           end,
    sort_cache(H, [], Size, {ListFun, FileFun, Post, LocalPost, TmpUsageM}).

sort_cache([], CL, _Sz, {LF, _FF, Post, LocalPost, _TmpUsageM}) ->
    {LF(lists:reverse(CL)), Post, LocalPost};
sort_cache(Objs, CL, Sz, C) when Sz < 0 ->
    sort_cache2(Objs, CL, false, C);
sort_cache([Object | Cont], CL, Sz0, C) ->
    Sz = decr_list_size(Sz0, Object),
    sort_cache(Cont, [Object | CL], Sz, C);
sort_cache(F, CL, Sz, C) ->
    case F() of
        Objects when is_list(Objects) ->
            sort_cache(Objects, CL, Sz, C);
        Term -> 
            {_LF, _FF, Post, _LocalPost, _TmpUsageM} = C,
            post_funs(Post),
            throw_error(Term)
    end.

sort_cache2([], CL, _X, {LF, _FF, Post, LocalPost, _TmpUsageM}) ->
    {LF(lists:reverse(CL)), Post, LocalPost};
sort_cache2([Object | Cont], CL, _, C) ->
    sort_cache2(Cont, [Object | CL], true, C);
sort_cache2(F, CL, false, C) ->
    %% Find one extra object to be sure that temporary file(s) will be
    %% used when calling the file_sorter. This works even if
    %% duplicates are removed.
    case F() of
        Objects when is_list(Objects) ->
            sort_cache2(Objects, CL, true, C);
        Term -> 
            {_LF, _FF, Post, _LocalPost, _TmpUsageM} = C,
            post_funs(Post),
            throw_error(Term)
    end;
sort_cache2(_Cont, _CL, true, {_LF,_FF,Post,_LocalPost, {not_allowed,M}}) ->
    post_funs(Post),
    throw_reason({tmpdir_usage, M});
sort_cache2(Cont, CL, true, {_LF, FF, _Post, _LocalPost, {TmpUsage, M}}) ->
    maybe_error_logger(TmpUsage, M),
    FF(lists:reverse(CL, Cont)).

file_sort_handle(H, Kp, SortOptions, TmpDir, Compressed, Post, LocalPost) ->
    In = sort_cursor_input(H, 0),
    Unique = lists:member(unique, SortOptions) 
             orelse
             lists:keymember(unique, 1, SortOptions),
    Out = sort_cursor_list_output(TmpDir, Compressed, Unique),
    Reply = do_sort(In, Out, Kp, SortOptions, Post),
    case Reply of
        {file, FileName} ->
            {F, Fd} = open_file(FileName, Compressed, Post),
            P = fun() -> _ = file:close(Fd),
                         _ = file:delete(FileName)
                end,
            {F, [P | Post], LocalPost};
        {terms, BTerms} ->
            try 
                {[binary_to_term(B) || B <- BTerms], Post, LocalPost}
            catch Class:Reason:Stacktrace ->
                post_funs(Post),
                erlang:raise(Class, Reason, Stacktrace)
            end
    end.

do_sort(In, Out, Sort, SortOptions, Post) ->
    try
        case do_sort(In, Out, Sort, SortOptions) of
            {error, Reason} -> throw_reason(Reason);
            Reply -> Reply
        end
    catch Class:Term:Stacktrace ->
        post_funs(Post),
        erlang:raise(Class, Term, Stacktrace)
    end.

do_sort(In, Out, sort, SortOptions) ->
    file_sorter:sort(In, Out, SortOptions);
do_sort(In, Out, {keysort, KeyPos}, SortOptions) ->
    file_sorter:keysort(KeyPos, In, Out, SortOptions).

del_table(Ets) ->
    fun() -> true = ets:delete(Ets) end.

empty_table(Ets) ->
    fun() -> true = ets:delete_all_objects(Ets) end.

append_loop([[_ | _]=L], _N) ->
    L;
append_loop([F], _N) ->
    F();
append_loop([L | Hs], N) ->
    append_loop(L, N, Hs).

append_loop([], N, Hs) ->
    append_loop(Hs, N);
append_loop([Object | Cont], N, Hs) ->
    [Object | append_loop(Cont, N + 1, Hs)];
append_loop(F, 0, Hs) ->
    case F() of
        [] ->
            append_loop(Hs, 0);
        [Object | Cont] ->
            [Object | append_loop(Cont, 1, Hs)];
        Term ->
            Term
    end;
append_loop(F, _N, Hs) -> % when _N > 0
    fun() -> append_loop(F, 0, Hs) end.

no_dups([]=Cont, UTab) ->
    true = ets:delete_all_objects(UTab),
    Cont;
no_dups([Object | Cont], UTab) ->
    case ets:member(UTab, Object)  of
        false ->
            true = ets:insert(UTab, {Object}),
            %% A fun is created here, even if Cont is a list; objects
            %% will not be copied to the ETS table unless requested.
            [Object | fun() -> no_dups(Cont, UTab) end];
        true ->
            no_dups(Cont, UTab)
    end;
no_dups(F, UTab) ->
    case F() of
        Objects when is_list(Objects) ->
            no_dups(Objects, UTab);
        Term ->
            Term
    end.

%% When all objects have been returned from a cached QLC, the
%% generators of the expression will never be called again, and so the
%% tables used by the generators (LocalPost) can be emptied.

cache(H, MTab, LocalPost) ->
    case ets:member(MTab, 0) of
        false ->
            true = ets:insert(MTab, {0}),
            cache(H, MTab, 1, LocalPost);
        true ->
            cache_recall(MTab, 1)
    end.

cache([]=Cont, _MTab, _SeqNo, LocalPost) ->
    local_post(LocalPost),
    Cont;
cache([Object | Cont], MTab, SeqNo, LocalPost) ->
    true = ets:insert(MTab, {SeqNo, Object}),
    %% A fun is created here, even if Cont is a list; objects
    %% will not be copied to the ETS table unless requested.
    [Object | fun() -> cache(Cont, MTab, SeqNo + 1, LocalPost) end];
cache(F, MTab, SeqNo, LocalPost) ->
    case F() of
        Objects when is_list(Objects) ->
            cache(Objects, MTab, SeqNo, LocalPost);
        Term -> 
            Term
    end.

cache_recall(MTab, SeqNo) ->
    case ets:lookup(MTab, SeqNo) of
        []=Cont ->
            Cont;
        [{SeqNo, Object}] ->
            [Object | fun() -> cache_recall(MTab, SeqNo + 1) end]
    end.

ucache(H, UTab, MTab, LocalPost) ->
    case ets:member(MTab, 0) of
        false ->
            true = ets:insert(MTab, {0}),
            ucache(H, UTab, MTab, 1, LocalPost);
        true ->
            ucache_recall(UTab, MTab, 1)
    end.

ucache([]=Cont, _UTab, _MTab, _SeqNo, LocalPost) ->
    local_post(LocalPost),
    Cont;
ucache([Object | Cont], UTab, MTab, SeqNo, LocalPost) ->
    %% Always using 28 bits hash value...
    Hash = erlang:phash2(Object),
    case ets:lookup(UTab, Hash) of
        [] ->
            ucache3(Object, Cont, Hash, UTab, MTab, SeqNo, LocalPost);
        HashSeqObjects ->
            case lists:keymember(Object, 3, HashSeqObjects) of
                true -> 
                    ucache(Cont, UTab, MTab, SeqNo, LocalPost);
                false -> 
                    ucache3(Object, Cont, Hash, UTab, MTab, SeqNo, LocalPost)
            end
    end;
ucache(F, UTab, MTab, SeqNo, LocalPost) ->
    case F() of
        Objects when is_list(Objects) ->
            ucache(Objects, UTab, MTab, SeqNo, LocalPost);
        Term ->
            Term
    end.

ucache3(Object, Cont, Hash, UTab, MTab, SeqNo, LocalPost) ->
    true = ets:insert(UTab, {Hash, SeqNo, Object}),
    true = ets:insert(MTab, {SeqNo, Hash}),
    %% A fun is created here, even if Cont is a list; objects
    %% will not be copied to the ETS table unless requested.
    [Object | fun() -> ucache(Cont, UTab, MTab, SeqNo+1, LocalPost) end].

ucache_recall(UTab, MTab, SeqNo) ->
    case ets:lookup(MTab, SeqNo) of
        []=Cont ->
            Cont;
        [{SeqNo, Hash}] ->
            Object = case ets:lookup(UTab, Hash) of
                         [{Hash, SeqNo, Object0}] -> Object0;
                         HashSeqObjects -> 
                             {Hash, SeqNo, Object0} =
                                 lists:keyfind(SeqNo, 2, HashSeqObjects),
                             Object0
                     end,
            [Object | fun() -> ucache_recall(UTab, MTab, SeqNo + 1) end]
    end.

-define(LCACHE_FILE(Ref), {Ref, '$_qlc_cache_tmpfiles_'}).

lcache(H, Ref, LocalPost, TmpDir, MaxList, TmpUsage) ->
    Key = ?LCACHE_FILE(Ref),
    case get(Key) of
        undefined ->
            lcache1(H, {Key, LocalPost, TmpDir, MaxList, TmpUsage}, 
                    MaxList, []);
        {file, _Fd, _TmpFile, F} ->
            F();
        L when is_list(L) ->
            L
    end.
    
lcache1([]=Cont, {Key, LocalPost, _TmpDir, _MaxList, _TmpUsage}, _Sz, Acc) ->
    local_post(LocalPost),
    case get(Key) of
        undefined -> 
            put(Key, lists:reverse(Acc)),
            Cont;
        {file, Fd, TmpFile, _F} ->
            case lcache_write(Fd, TmpFile, Acc) of
                ok ->
                    Cont;
                Error ->
                    Error
            end
    end;
lcache1(H, State, Sz, Acc) when Sz < 0 ->
    {Key, LocalPost, TmpDir, MaxList, TmpUsage} = State,
    GetFile = 
        case get(Key) of
            {file, Fd0, TmpFile, _F} ->
                {TmpFile, Fd0};
            undefined when TmpUsage =:= not_allowed ->
                error({tmpdir_usage, caching});
            undefined ->
                maybe_error_logger(TmpUsage, caching),
                FName = tmp_filename(TmpDir),
                {F, Fd0} = open_file(FName, [write], LocalPost),
                put(Key, {file, Fd0, FName, F}),
                {FName, Fd0}
        end,
    case GetFile of
        {FileName, Fd} ->
            case lcache_write(Fd, FileName, Acc) of
                ok -> 
                    lcache1(H, State, MaxList, []);
                Error ->
                    Error
            end;
        Error ->
            Error
    end;
lcache1([Object | Cont], State, Sz0, Acc) ->
    Sz = decr_list_size(Sz0, Object),
    [Object | lcache2(Cont, State, Sz, [Object | Acc])];
lcache1(F, State, Sz, Acc) ->
    case F() of
        Objects when is_list(Objects) ->
            lcache1(Objects, State, Sz, Acc);
        Term ->
            Term
    end.

lcache2([Object | Cont], State, Sz0, Acc) when Sz0 >= 0 ->
    Sz = decr_list_size(Sz0, Object),
    [Object | lcache2(Cont, State, Sz, [Object | Acc])];
lcache2(Cont, State, Sz, Acc) ->
    fun() -> lcache1(Cont, State, Sz, Acc) end.

lcache_write(Fd, FileName, L) ->
    write_binary_terms(t2b(L, []), Fd, FileName).

t2b([], Bs) ->
    Bs;
t2b([T | Ts], Bs) ->
    t2b(Ts, [term_to_binary(T) | Bs]).

del_lcache(Ref) ->
    fun() -> 
            Key = ?LCACHE_FILE(Ref),
            case get(Key) of
                undefined ->
                    ok;
                {file, Fd, TmpFile, _F} ->
                    _ = file:close(Fd),
                    _ = file:delete(TmpFile),
                    erase(Key);
                _L -> 
                    erase(Key)
            end
    end.

tag_objects([Object | Cont], T) ->
    [{Object, T} | tag_objects2(Cont, T + 1)];
tag_objects([]=Cont, _T) ->
    Cont;
tag_objects(F, T) ->
    case F() of
        Objects when is_list(Objects) ->
            tag_objects(Objects, T);
        Term ->
            Term
    end.

tag_objects2([Object | Cont], T) ->
    [{Object, T} | tag_objects2(Cont, T + 1)];
tag_objects2(Objects, T) ->
    fun() -> tag_objects(Objects, T) end.

untag_objects([]=Objs) ->
    Objs;
untag_objects([{Object, _N} | Cont]) ->
    [Object | untag_objects2(Cont)];
untag_objects(F) ->
    case F() of
        Objects when is_list(Objects) ->
            untag_objects(Objects);
        Term -> % Cannot happen
            Term
    end.

untag_objects2([{Object, _N} | Cont]) ->
    [Object | untag_objects2(Cont)];
untag_objects2([]=Cont) ->
    Cont;
untag_objects2(Objects) ->
    fun() -> untag_objects(Objects) end.

%%% Merge join.
%%% Temporary files are used when many objects have the same key.

-define(JWRAP(E1, E2), [E1 | E2]).

-record(m, {id, tmpdir, max_list, tmp_usage}).

merge_join([]=Cont, _C1, _T2, _C2, _Opt) ->
    Cont;
merge_join([E1 | L1], C1, L2, C2, Opt) ->
    #qlc_opt{tmpdir = TmpDir, max_list = MaxList, 
             tmpdir_usage = TmpUsage} = Opt,
    M = #m{id = merge_join_id(), tmpdir = TmpDir, max_list = MaxList, 
           tmp_usage = TmpUsage},
    merge_join2(E1, element(C1, E1), L1, C1, L2, C2, M);
merge_join(F1, C1, L2, C2, Opt) ->
    case F1() of
        L1 when is_list(L1) ->
            merge_join(L1, C1, L2, C2, Opt);
        T1 ->
            T1
    end.

merge_join1(_E2, _K2, []=Cont, _C1, _L2, _C2, M) ->
    end_merge_join(Cont, M);
merge_join1(E2, K2, [E1 | L1], C1, L2, C2, M) ->
    K1 = element(C1, E1),
    if 
        K1 == K2 ->
            same_keys2(E1, K1, L1, C1, L2, C2, E2, M);
        K1 > K2 ->
            merge_join2(E1, K1, L1, C1, L2, C2, M);
        true -> % K1 < K2
            merge_join1(E2, K2, L1, C1, L2, C2, M)
    end;
merge_join1(E2, K2, F1, C1, L2, C2, M) ->
    case F1() of
        L1 when is_list(L1) ->
            merge_join1(E2, K2, L1, C1, L2, C2, M);
        T1 ->
            T1
    end.

merge_join2(_E1, _K1, _L1, _C1, []=Cont, _C2, M) ->
    end_merge_join(Cont, M);
merge_join2(E1, K1, L1, C1, [E2 | L2], C2, M) ->
    K2 = element(C2, E2),
    if
        K1 == K2 ->
            same_keys2(E1, K1, L1, C1, L2, C2, E2, M);
        K1 > K2 ->
            merge_join2(E1, K1, L1, C1, L2, C2, M);
        true -> % K1 < K2
            merge_join1(E2, K2, L1, C1, L2, C2, M)
    end;
merge_join2(E1, K1, L1, C1, F2, C2, M) ->
    case F2() of
        L2 when is_list(L2) ->
            merge_join2(E1, K1, L1, C1, L2, C2, M);
        T2 ->
            T2
    end.

%% element(C2, E2_0) == K1
same_keys2(E1, K1, L1, C1, [], _C2, E2_0, M) ->
    Cont = fun(_L1b) -> end_merge_join([], M) end,
    loop_same_keys(E1, K1, L1, C1, [E2_0], Cont, M);
same_keys2(E1, K1, L1, C1, [E2 | L2]=L2_0, C2, E2_0, M) ->
    K2 = element(C2, E2),
    if
        K1 == K2 ->
            same_keys1(E1, K1, L1, C1, E2, C2, E2_0, L2, M);
        K1 < K2 ->
            [?JWRAP(E1, E2_0) |
             fun() -> same_loop1(L1, K1, C1, E2_0, L2_0, C2, M) end]
    end;
same_keys2(E1, K1, L1, C1, F2, C2, E2_0, M) ->
    case F2() of
        L2 when is_list(L2) ->
            same_keys2(E1, K1, L1, C1, L2, C2, E2_0, M);
        T2 ->
            Cont = fun(_L1b) -> T2 end,
            loop_same_keys(E1, K1, L1, C1, [E2_0], Cont, M)
    end.

same_loop1([], _K1_0, _C1, _E2_0, _L2, _C2, M) ->
    end_merge_join([], M);
same_loop1([E1 | L1], K1_0, C1, E2_0, L2, C2, M) ->
    K1 = element(C1, E1),
    if
        K1 == K1_0 ->
            [?JWRAP(E1, E2_0) | 
             fun() -> same_loop1(L1, K1_0, C1, E2_0, L2, C2, M) end];
        K1_0 < K1 ->
            merge_join2(E1, K1, L1, C1, L2, C2, M)
    end;
same_loop1(F1, K1_0, C1, E2_0, L2, C2, M) ->
    case F1() of
        L1 when is_list(L1) ->
            same_loop1(L1, K1_0, C1, E2_0, L2, C2, M);
        T1 ->
            T1
    end.

%% element(C2, E2_0) == K1, element(C2, E2) == K1_0
same_keys1(E1_0, K1_0, []=L1, C1, E2, C2, E2_0, L2, M) ->
    [?JWRAP(E1_0, E2_0), ?JWRAP(E1_0, E2) |
     fun() -> same_keys(K1_0, E1_0, L1, C1, L2, C2, M) end];
same_keys1(E1_0, K1_0, [E1 | _]=L1, C1, E2, C2, E2_0, L2, M) ->
    K1 = element(C1, E1),
    if 
        K1_0 == K1 ->
            E2s = [E2, E2_0],
            Sz0 = decr_list_size(M#m.max_list, E2s),
            same_keys_cache(E1_0, K1_0, L1, C1, L2, C2, E2s, Sz0, M);
        K1_0 < K1  ->
            [?JWRAP(E1_0, E2_0), ?JWRAP(E1_0, E2) |
             fun() -> same_keys(K1_0, E1_0, L1, C1, L2, C2, M) end]
    end;
same_keys1(E1_0, K1_0, F1, C1, E2, C2, E2_0, L2, M) ->
    case F1() of
        L1 when is_list(L1) ->
            same_keys1(E1_0, K1_0, L1, C1, E2, C2, E2_0, L2, M);
        T1 ->
            Cont = fun() -> T1 end,
            loop_same(E1_0, [E2, E2_0], Cont)
    end.

%% There is no such element E in L1 such that element(C1, E) == K1.
same_keys(_K1, _E1, _L1, _C1, []=Cont, _C2, M) ->
    end_merge_join(Cont, M);
same_keys(K1, E1, L1, C1, [E2 | L2], C2, M) ->
    K2 = element(C2, E2),
    if
        K1 == K2 ->
            [?JWRAP(E1, E2) | 
             fun() -> same_keys(K1, E1, L1, C1, L2, C2, M) end];
        K1 < K2 ->
            merge_join1(E2, K2, L1, C1, L2, C2, M)
    end;
same_keys(K1, E1, L1, C1, F2, C2, M) ->
    case F2() of
        L2 when is_list(L2) ->
            same_keys(K1, E1, L1, C1, L2, C2, M);
        T2 ->
            T2
    end.

%% There are at least two elements in [E1 | L1] that are to be combined
%% with the elements in E2s (length(E2s) > 1). This loop covers the case
%% when all elements in E2 with key K1 can be kept in RAM.
same_keys_cache(E1, K1, L1, C1, [], _C2, E2s, _Sz, M) ->
    Cont = fun(_L1b) -> end_merge_join([], M) end,
    loop_same_keys(E1, K1, L1, C1, E2s, Cont, M);
same_keys_cache(E1, K1, L1, C1, L2, C2, E2s, Sz0, M) when Sz0 < 0 ->
    case init_merge_join(M) of
        ok -> 
            Sz = M#m.max_list,
            C = fun() -> 
                        same_keys_file(E1, K1, L1, C1, L2, C2, [], Sz, M) 
                end,
            write_same_keys(E1, E2s, M, C);
        Error ->
            Error
    end;
same_keys_cache(E1, K1, L1, C1, [E2 | L2], C2, E2s, Sz0, M) ->
    K2 = element(C2, E2),
    if
        K1 == K2 ->
            Sz = decr_list_size(Sz0, E2),
            same_keys_cache(E1, K1, L1, C1, L2, C2, [E2 | E2s], Sz, M);
        K1 < K2 ->
            Cont = fun(L1b) -> merge_join1(E2, K2, L1b, C1, L2, C2, M) end,
            loop_same_keys(E1, K1, L1, C1, E2s, Cont, M)
    end;
same_keys_cache(E1, K1, L1, C1, F2, C2, E2s, Sz, M) ->
    case F2() of
        L2 when is_list(L2) ->
            same_keys_cache(E1, K1, L1, C1, L2, C2, E2s, Sz, M);
        T2 ->
            Cont = fun(_L1b) -> T2 end,
            loop_same_keys(E1, K1, L1, C1, E2s, Cont, M)
    end.

%% E2s holds all elements E2 in L2 such that element(E2, C2) == K1.
loop_same_keys(E1, _K1, [], _C1, E2s, _Cont, M) ->
    end_merge_join(loop_same(E1, E2s, []), M);
loop_same_keys(E1, K1, L1, C1, E2s, Cont, M) ->
    loop_same(E1, E2s, fun() -> loop_keys(K1, L1, C1, E2s, Cont, M) end).

loop_same(_E1, [], L) ->
    L;
loop_same(E1, [E2 | E2s], L) ->
    loop_same(E1, E2s, [?JWRAP(E1, E2) | L]).

loop_keys(K, [E1 | L1]=L1_0, C1, E2s, Cont, M) ->
    K1 = element(C1, E1),
    if 
        K1 == K ->
            loop_same_keys(E1, K1, L1, C1, E2s, Cont, M);
        K1 > K -> 
            Cont(L1_0)
    end;
loop_keys(_K, []=L1, _C1, _Es2, Cont, _M) ->
    Cont(L1);
loop_keys(K, F1, C1, E2s, Cont, M) ->
    case F1() of
        L1 when is_list(L1) ->
            loop_keys(K, L1, C1, E2s, Cont, M);
        T1 ->
            T1
    end.

%% This is for the case when a temporary file has to be used.
same_keys_file(E1, K1, L1, C1, [], _C2, E2s, _Sz, M) ->
    Cont = fun(_L1b) -> end_merge_join([], M) end,
    same_keys_file_write(E1, K1, L1, C1, E2s, M, Cont);    
same_keys_file(E1, K1, L1, C1, L2, C2, E2s, Sz0, M) when Sz0 < 0 ->
    Sz = M#m.max_list,
    C = fun() -> same_keys_file(E1, K1, L1, C1, L2, C2, [], Sz, M) end,
    write_same_keys(E1, E2s, M, C);
same_keys_file(E1, K1, L1, C1, [E2 | L2], C2, E2s, Sz0, M) ->
    K2 = element(C2, E2),
    if
        K1 == K2 ->
            Sz = decr_list_size(Sz0, E2),
            same_keys_file(E1, K1, L1, C1, L2, C2, [E2 | E2s], Sz, M);
        K1 < K2 ->
            Cont = fun(L1b) ->
                           %% The temporary file could be truncated here.
                           merge_join1(E2, K2, L1b, C1, L2, C2, M)
                   end,
            same_keys_file_write(E1, K1, L1, C1, E2s, M, Cont)
    end;
same_keys_file(E1, K1, L1, C1, F2, C2, E2s, Sz, M) ->
    case F2() of
        L2 when is_list(L2) ->
            same_keys_file(E1, K1, L1, C1, L2, C2, E2s, Sz, M);
        T2 ->
            Cont = fun(_L1b) -> T2 end,
            same_keys_file_write(E1, K1, L1, C1, E2s, M, Cont)
    end.

same_keys_file_write(E1, K1, L1, C1, E2s, M, Cont) ->
    C = fun() -> loop_keys_file(K1, L1, C1, Cont, M) end,
    write_same_keys(E1, E2s, M, C).

write_same_keys(_E1, [], _M, Cont) ->
    Cont();
write_same_keys(E1, Es2, M, Cont) ->
    write_same_keys(E1, Es2, M, [], Cont).

%% Avoids one (the first) traversal of the temporary file.
write_same_keys(_E1, [], M, E2s, Objs) ->
    case write_merge_join(M, E2s) of
        ok -> Objs;
        Error -> Error
    end;
write_same_keys(E1, [E2 | E2s0], M, E2s, Objs) ->
    BE2 = term_to_binary(E2),
    write_same_keys(E1, E2s0, M, [BE2 | E2s], [?JWRAP(E1, E2) | Objs]).

loop_keys_file(K, [E1 | L1]=L1_0, C1, Cont, M) ->
    K1 = element(C1, E1),
    if
        K1 == K ->
            C = fun() -> loop_keys_file(K1, L1, C1, Cont, M) end,
            read_merge_join(M, E1, C);
        K1 > K ->
            Cont(L1_0)
    end;
loop_keys_file(_K, []=L1, _C1, Cont, _M) ->
    Cont(L1);
loop_keys_file(K, F1, C1, Cont, M) ->
    case F1() of
        L1 when is_list(L1) ->
            loop_keys_file(K, L1, C1, Cont, M);
        T1 ->
            T1
    end.

end_merge_join(Reply, M) ->
    end_merge_join(M),
    Reply.

%% Normally post_funs() cleans up temporary files by calling funs in
%% Post. It seems impossible to do that with the temporary file(s)
%% used when many objects have the same key--such a file is created
%% after the setup when Post is prepared. There seems to be no real
%% alternative to using the process dictionary, at least as things
%% have been implemented so far. Probably all of Post could have been
%% put in the process dictionary...

-define(MERGE_JOIN_FILE, '$_qlc_merge_join_tmpfiles_').

init_merge_join(#m{id = MergeId, tmpdir = TmpDir, tmp_usage = TmpUsage}) ->
    case tmp_merge_file(MergeId) of
        {Fd, FileName} ->
            case file:position(Fd, bof) of
                {ok, 0} ->
                    case file:truncate(Fd) of
                        ok ->
                            ok;
                        Error ->
                            file_error(FileName, Error)
                    end;
                Error ->
                    file_error(FileName, Error)
            end;
        none when TmpUsage =:= not_allowed ->
            error({tmpdir_usage, joining});
        none ->
            maybe_error_logger(TmpUsage, joining),
            FName = tmp_filename(TmpDir),
            case file:open(FName, [raw, binary, read, write]) of
                {ok, Fd} ->
                    TmpFiles = get(?MERGE_JOIN_FILE),
                    put(?MERGE_JOIN_FILE, [{MergeId, Fd, FName} | TmpFiles]),
                    ok;
                Error ->
                    file_error(FName, Error)
            end
    end.

write_merge_join(#m{id = MergeId}, BTerms) ->
    {Fd, FileName} = tmp_merge_file(MergeId),
    write_binary_terms(BTerms, Fd, FileName).

read_merge_join(#m{id = MergeId}, E1, Cont) ->
    {Fd, FileName} = tmp_merge_file(MergeId),
    case file:position(Fd, bof) of
        {ok, 0} -> 
            Fun = fun([], _) ->
                          Cont();
                     (Ts, C) when is_list(Ts) ->
                          join_read_terms(E1, Ts, C)
                  end,
            file_loop_read(<<>>, ?CHUNK_SIZE, {Fd, FileName}, Fun);
        Error -> 
            file_error(FileName, Error)
    end.

join_read_terms(_E1, [], Objs) ->
    Objs;
join_read_terms(E1, [E2 | E2s], Objs) ->
    join_read_terms(E1, E2s, [?JWRAP(E1, E2) | Objs]).

end_merge_join(#m{id = MergeId}) ->
    case tmp_merge_file(MergeId) of
        none -> 
            ok;
        {Fd, FileName}  ->
            _ = file:close(Fd),
            _ = file:delete(FileName),
            put(?MERGE_JOIN_FILE, 
                lists:keydelete(MergeId, 1, get(?MERGE_JOIN_FILE)))
    end.

end_all_merge_joins() ->
    lists:foreach(
      fun(Id) -> end_merge_join(#m{id = Id}) end, 
      [Id || {Id, _Fd, _FileName} <- lists:flatten([get(?MERGE_JOIN_FILE)])]),
    erase(?MERGE_JOIN_FILE).

merge_join_id() ->
    case get(?MERGE_JOIN_FILE) of
        undefined ->
            put(?MERGE_JOIN_FILE, []);
        _ -> 
            ok
    end,
    make_ref().

tmp_merge_file(MergeId) ->
    TmpFiles = get(?MERGE_JOIN_FILE),
    case lists:keyfind(MergeId, 1, TmpFiles) of
        {MergeId, Fd, FileName} ->
            {Fd, FileName};
        false ->
            none
    end.

decr_list_size(Sz0, E) when is_integer(Sz0) ->
    Sz0 - erlang:external_size(E).

%%% End of merge join.    

lookup_join([E1 | L1], C1, LuF, C2, Rev) ->
    K1 = element(C1, E1),
    case LuF(C2, [K1]) of
        [] ->
            lookup_join(L1, C1, LuF, C2, Rev);
        [E2] when Rev ->
            [?JWRAP(E2, E1) | fun() -> lookup_join(L1, C1, LuF, C2, Rev) end];
        [E2] ->
            [?JWRAP(E1, E2) | fun() -> lookup_join(L1, C1, LuF, C2, Rev) end];
        E2s when is_list(E2s), Rev ->
            [?JWRAP(E2, E1) || E2 <- E2s] ++
                fun() -> lookup_join(L1, C1, LuF, C2, Rev) end;
        E2s when is_list(E2s) ->
            [?JWRAP(E1, E2) || E2 <- E2s] ++
                fun() -> lookup_join(L1, C1, LuF, C2, Rev) end;
        Term ->
            Term
    end;
lookup_join([]=Cont, _C1, _LuF, _C2, _Rev) ->
    Cont;
lookup_join(F1, C1, LuF, C2, Rev) ->
    case F1() of
        L1 when is_list(L1) ->
            lookup_join(L1, C1, LuF, C2, Rev);
        T1 ->
            T1
    end.

maybe_error_logger(allowed, _) ->
    ok;
maybe_error_logger(Name, Why) ->
    [_, _, {?MODULE,maybe_error_logger,_,_} | Stacktrace] =
	expand_stacktrace(),
    Trimmer = fun(M, _F, _A) -> M =:= erl_eval end,
    Formater = fun(Term, I) -> io_lib:print(Term, I, 80, -1) end,
    X = erl_error:format_stacktrace(1, Stacktrace, Trimmer, Formater),
    error_logger:Name("qlc: temporary file was needed for ~w\n~ts\n",
                      [Why, lists:flatten(X)]).

expand_stacktrace() ->
    D = erlang:system_flag(backtrace_depth, 8),
    try
        %% Compensate for a bug in erlang:system_flag/2:
        expand_stacktrace(erlang:max(1, D))
    after
        erlang:system_flag(backtrace_depth, D)
    end.

expand_stacktrace(D) ->
    _ = erlang:system_flag(backtrace_depth, D),
    {'EXIT', {foo, Stacktrace}} = (catch erlang:error(foo)),
    L = lists:takewhile(fun({M,_,_,_}) -> M =/= ?MODULE
                        end, lists:reverse(Stacktrace)),
    if
        length(L) < 3 andalso length(Stacktrace) =:= D ->
            expand_stacktrace(D + 5);
        true ->
            Stacktrace
    end.

write_binary_terms(BTerms, Fd, FileName) ->
    case file:write(Fd, size_bin(BTerms, [])) of
        ok ->
            ok;
        Error ->
            file_error(FileName, Error)
    end.

post_funs(L) ->
    end_all_merge_joins(),
    local_post(L).

local_post(L) ->
    lists:foreach(fun(undefined) -> ok;
                     (F) -> catch (F)() 
                  end, L).

call(undefined, _Arg, Default, _Post) ->
    Default;
call(Fun, Arg, _Default, Post) ->
    try
        Fun(Arg) 
    catch Class:Reason:Stacktrace ->
        post_funs(Post),
        erlang:raise(Class, Reason, Stacktrace)
    end.

grd(undefined, _Arg) ->
    false;
grd(Fun, Arg) ->
    case Fun(Arg) of
        true -> 
            true;
        _ ->
            false
    end.

anno0() ->
    anno(0).

anno1() ->
    anno(1).

anno(L) ->
    erl_anno:new(L).

family(L) ->
    sofs:to_external(sofs:relation_to_family(sofs:relation(L))).

family_union(L) ->
    R = sofs:relation(L,[{atom,[atom]}]),
    sofs:to_external(sofs:family_union(sofs:relation_to_family(R))).

file_error(File, {error, Reason}) ->
    error({file_error, File, Reason}).

-spec throw_file_error(string(), {'error',atom()}) -> no_return().

throw_file_error(File, {error, Reason}) ->
    throw_reason({file_error, File, Reason}).

-spec throw_reason(term()) -> no_return().

throw_reason(Reason) ->
    throw_error(error(Reason)).

-spec throw_error(term()) -> no_return().

throw_error(Error) ->
    throw(Error).

error(Reason) ->
    {error, ?MODULE, Reason}.
