/*  Part of SWI-Prolog

    Author:        Jan Wielemaker
    E-mail:        J.Wielemaker@vu.nl
    WWW:           http://www.swi-prolog.org
    Copyright (c)  2009-2019, University of Amsterdam
                              VU University Amsterdam
                              CWI, Amsterdam
    All rights reserved.

    Redistribution and use in source and binary forms, with or without
    modification, are permitted provided that the following conditions
    are met:

    1. Redistributions of source code must retain the above copyright
       notice, this list of conditions and the following disclaimer.

    2. Redistributions in binary form must reproduce the above copyright
       notice, this list of conditions and the following disclaimer in
       the documentation and/or other materials provided with the
       distribution.

    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
    FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
    COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
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*/

:- module(xpath,
          [ xpath/3,                    % +DOM, +Spec, -Value
            xpath_chk/3,                % +DOM, +Spec, -Value

            op(400, fx, //),
            op(400, fx, /),
            op(200, fy, @)
          ]).
:- use_module(library(record),[record/1, op(_,_,record)]).

:- autoload(library(debug),[assertion/1]).
:- autoload(library(error),[instantiation_error/1,must_be/2]).
:- autoload(library(lists),[member/2]).
:- autoload(library(sgml),[xsd_number_string/2]).

/** <module> Select nodes in an XML DOM

The library xpath.pl provides predicates to select nodes from an XML DOM
tree as produced by library(sgml) based  on descriptions inspired by the
XPath language.

The   predicate   xpath/3   selects   a   sub-structure   of   the   DOM
non-deterministically based on an  XPath-like   specification.  Not  all
selectors of XPath are implemented, but the ability to mix xpath/3 calls
with arbitrary Prolog code  provides  a   powerful  tool  for extracting
information from XML parse-trees.

@see http://www.w3.org/TR/xpath
*/

:- record
    element(name, attributes, content).

%!  xpath_chk(+DOM, +Spec, ?Content) is semidet.
%
%   Semi-deterministic version of xpath/3.

xpath_chk(DOM, Spec, Content) :-
    xpath(DOM, Spec, Content),
    !.

%!  xpath(+DOM, +Spec, ?Content) is nondet.
%
%   Match an element in a DOM structure.   The syntax is inspired by
%   XPath, using () rather than  []   to  select  inside an element.
%   First we can construct paths using / and //:
%
%       $ =|//|=Term :
%       Select any node in the DOM matching term.
%       $ =|/|=Term :
%       Match the root against Term.
%       $ Term :
%       Select the immediate children of the root matching Term.
%
%   The Terms above are of type   _callable_.  The functor specifies
%   the element name. The element name   '*'  refers to any element.
%   The name =self= refers to the   top-element  itself and is often
%   used for processing matches of an  earlier xpath/3 query. A term
%   NS:Term refers to an XML  name   in  the  namespace NS. Optional
%   arguments specify additional  constraints   and  functions.  The
%   arguments are processed from left  to right. Defined conditional
%   argument values are:
%
%       $ index(?Index) :
%       True if the element is the Index-th child of its parent,
%       where 1 denotes the first child. Index can be one of:
%         $ `Var` :
%         `Var` is unified with the index of the matched element.
%         $ =last= :
%         True for the last element.
%         $ =last= - `IntExpr` :
%         True for the last-minus-nth element. For example,
%         `last-1` is the element directly preceding the last one.
%         $ `IntExpr` :
%         True for the element whose index equals `IntExpr`.
%       $ Integer :
%       The N-th element with the given name, with 1 denoting the
%       first element. Same as index(Integer).
%       $ =last= :
%       The last element with the given name. Same as
%       index(last).
%       $ =last= - IntExpr :
%       The IntExpr-th element before the last.
%       Same as index(last-IntExpr).
%
%   Defined function argument values are:
%
%       $ =self= :
%       Evaluate to the entire element
%       $ =content= :
%       Evaluate to the content of the element (a list)
%       $ =text= :
%       Evaluates to all text from the sub-tree as an atom
%       $ `text(As)` :
%       Evaluates to all text from the sub-tree according to
%       `As`, which is either `atom` or `string`.
%       $ =normalize_space= :
%       As =text=, but uses normalize_space/2 to normalise
%       white-space in the output
%       $ =number= :
%       Extract an integer or float from the value.  Ignores
%       leading and trailing white-space
%       $ =|@|=Attribute :
%       Evaluates to the value of the given attribute.  Attribute
%       can be a compound term. In this case the functor name
%       denotes the element and arguments perform transformations
%       on the attribute value.  Defined transformations are:
%
%         - number
%         Translate the value into a number using
%         xsd_number_string/2 from library(sgml).
%         - integer
%         As `number`, but subsequently transform the value
%         into an integer using the round/1 function.
%         - float
%         As `number`, but subsequently transform the value
%         into a float using the float/1 function.
%         - string
%         Translate the value into a Prolog string.
%         - lower
%         Translate the value to lower case, preserving
%         the type.
%         - upper
%         Translate the value to upper case, preserving
%         the type.
%
%   In addition, the argument-list can be _conditions_:
%
%       $ Left = Right :
%       Succeeds if the left-hand unifies with the right-hand.
%       If the left-hand side is a function, this is evaluated.
%       The right-hand side is _never_ evaluated, and thus the
%       condition `content = content` defines that the content
%       of the element is the atom `content`.
%       The functions `lower_case` and `upper_case` can be applied
%       to Right (see example below).
%       $ contains(Haystack, Needle) :
%       Succeeds if Needle is a sub-string of Haystack.
%       $ XPath :
%       Succeeds if XPath matches in the currently selected
%       sub-DOM.  For example, the following expression finds
%       an =h3= element inside a =div= element, where the =div=
%       element itself contains an =h2= child with a =strong=
%       child.
%
%         ==
%         //div(h2/strong)/h3
%         ==
%
%       This is equivalent to the conjunction of XPath goals below.
%
%         ==
%            ...,
%            xpath(DOM, //(div), Div),
%            xpath(Div, h2/strong, _),
%            xpath(Div, h3, Result)
%         ==
%
%   **Examples**:
%
%   Match each table-row in DOM:
%
%       ==
%       xpath(DOM, //tr, TR)
%       ==
%
%   Match the last cell  of  each   tablerow  in  DOM.  This example
%   illustrates that a result can be the input of subsequent xpath/3
%   queries. Using multiple queries  on   the  intermediate  TR term
%   guarantee that all results come from the same table-row:
%
%       ==
%       xpath(DOM, //tr, TR),
%       xpath(TR,  /td(last), TD)
%       ==
%
%   Match each =href= attribute in an <a> element
%
%       ==
%       xpath(DOM, //a(@href), HREF)
%       ==
%
%   Suppose we have a table containing  rows where each first column
%   is the name of a product with a   link to details and the second
%   is the price (a number).  The   following  predicate matches the
%   name, URL and price:
%
%       ==
%       product(DOM, Name, URL, Price) :-
%           xpath(DOM, //tr, TR),
%           xpath(TR, td(1), C1),
%           xpath(C1, /self(normalize_space), Name),
%           xpath(C1, a(@href), URL),
%           xpath(TR, td(2, number), Price).
%       ==
%
%   Suppose we want to select  books   with  genre="thriller" from a
%   tree containing elements =|<book genre=...>|=
%
%       ==
%       thriller(DOM, Book) :-
%           xpath(DOM, //book(@genre=thiller), Book).
%       ==
%
%   Match the elements =|<table align="center">|= _and_ =|<table
%   align="CENTER">|=:
%
%       ```prolog
%           //table(@align(lower) = center)
%       ```
%
%   Get the `width` and `height` of a `div` element as a number,
%   and the `div` node itself:
%
%       ==
%           xpath(DOM, //div(@width(number)=W, @height(number)=H), Div)
%       ==
%
%   Note that `div` is an infix operator, so parentheses must be
%   used in cases like the following:
%
%       ==
%           xpath(DOM, //(div), Div)
%       ==

xpath(DOM, Spec, Content) :-
    in_dom(Spec, DOM, Content).

in_dom(//Spec, DOM, Value) :-
    !,
    element_spec(Spec, Name, Modifiers),
    sub_dom(I, Len, Name, E, DOM),
    modifiers(Modifiers, I, Len, E, Value).
in_dom(/Spec, E, Value) :-
    !,
    element_spec(Spec, Name, Modifiers),
    (   Name == self
    ->  true
    ;   element_name(E, Name)
    ),
    modifiers(Modifiers, 1, 1, E, Value).
in_dom(A/B, DOM, Value) :-
    !,
    in_dom(A, DOM, Value0),
    in_dom(B, Value0, Value).
in_dom(A//B, DOM, Value) :-
    !,
    in_dom(A, DOM, Value0),
    in_dom(//B, Value0, Value).
in_dom(Spec, element(_, _, Content), Value) :-
    element_spec(Spec, Name, Modifiers),
    count_named_elements(Content, Name, CLen),
    CLen > 0,
    nth_element(N, Name, E, Content),
    modifiers(Modifiers, N, CLen, E, Value).

element_spec(Var, _, _) :-
    var(Var),
    !,
    instantiation_error(Var).
element_spec(NS:Term, NS:Name, Modifiers) :-
    !,
    callable_name_arguments(Term, Name0, Modifiers),
    star(Name0, Name).
element_spec(Term, Name, Modifiers) :-
    !,
    callable_name_arguments(Term, Name0, Modifiers),
    star(Name0, Name).

callable_name_arguments(Atom, Name, Arguments) :-
    atom(Atom),
    !,
    Name = Atom, Arguments = [].
callable_name_arguments(Compound, Name, Arguments) :-
    compound_name_arguments(Compound, Name, Arguments).


star(*, _) :- !.
star(Name, Name).


%!  sub_dom(-Index, -Count, +Name, -Sub, +DOM) is nondet.
%
%   Sub is a node in DOM with Name.
%
%   @param Count    is the total number of nodes in the content
%                   list Sub appears that have the same name.
%   @param Index    is the 1-based index of Sub of nodes with
%                   Name.

sub_dom(1, 1, Name, DOM, DOM) :-
    element_name(DOM, Name0),
    \+ Name \= Name0.
sub_dom(N, Len, Name, E, element(_,_,Content)) :-
    !,
    sub_dom_2(N, Len, Name, E, Content).
sub_dom(N, Len, Name, E, Content) :-
    is_list(Content),
    sub_dom_2(N, Len, Name, E, Content).

sub_dom_2(N, Len, Name, Element, Content) :-
    (   count_named_elements(Content, Name, Len),
        nth_element(N, Name, Element, Content)
    ;   member(element(_,_,C2), Content),
        sub_dom_2(N, Len, Name, Element, C2)
    ).


%!  count_named_elements(+Content, +Name, -Count) is det.
%
%   Count is the number of nodes with Name in Content.

count_named_elements(Content, Name, Count) :-
    count_named_elements(Content, Name, 0, Count).

count_named_elements([], _, Count, Count).
count_named_elements([element(Name,_,_)|T], Name0, C0, C) :-
    \+ Name \= Name0,
    !,
    C1 is C0+1,
    count_named_elements(T, Name0, C1, C).
count_named_elements([_|T], Name, C0, C) :-
    count_named_elements(T, Name, C0, C).


%!  nth_element(?N, +Name, -Element, +Content:list) is nondet.
%
%   True if Element is the N-th element with name in Content.

nth_element(N, Name, Element, Content) :-
    nth_element_(1, N, Name, Element, Content).

nth_element_(I, N, Name, E, [H|T]) :-
    element_name(H, Name0),
    \+ Name \= Name0,
    !,
    (   N = I,
        E = H
    ;   I2 is I + 1,
        (   nonvar(N), I2 > N
        ->  !, fail
        ;   true
        ),
        nth_element_(I2, N, Name, E, T)
    ).
nth_element_(I, N, Name, E, [_|T]) :-
    nth_element_(I, N, Name, E, T).


%!  modifiers(+Modifiers, +I, +Clen, +DOM, -Value)
%
%

modifiers([], _, _, Value, Value).
modifiers([H|T], I, L, Value0, Value) :-
    modifier(H, I, L, Value0, Value1),
    modifiers(T, I, L, Value1, Value).

modifier(M, _, _, _, _) :-
    var(M),
    !,
    instantiation_error(M).
modifier(Index, I, L, Value0, Value) :-
    implicit_index_modifier(Index),
    !,
    Value = Value0,
    index_modifier(Index, I, L).
modifier(index(Index), I, L, Value, Value) :-
    !,
    index_modifier(Index, I, L).
modifier(Function, _, _, In, Out) :-
    xpath_function(Function),
    !,
    xpath_function(Function, In, Out).
modifier(Function, _, _, In, Out) :-
    xpath_condition(Function, In),
    Out = In.

implicit_index_modifier(I) :-
    integer(I),
    !.
implicit_index_modifier(last).
implicit_index_modifier(last-_Expr).

index_modifier(Var, I, _L) :-
    var(Var),
    !,
    Var = I.
index_modifier(last, I, L) :-
    !,
    I =:= L.
index_modifier(last-Expr, I, L) :-
    !,
    I =:= L-Expr.
index_modifier(N, I, _) :-
    N =:= I.

xpath_function(self, DOM, Value) :-                            % self
    !,
    Value = DOM.
xpath_function(content, Element, Value) :-                     % content
    !,
    element_content(Element, Value).
xpath_function(text, DOM, Text) :-                             % text
    !,
    text_of_dom(DOM, atom, Text).
xpath_function(text(As), DOM, Text) :-                         % text(As)
    !,
    text_of_dom(DOM, As, Text).
xpath_function(normalize_space, DOM, Text) :-                  % normalize_space
    !,
    text_of_dom(DOM, string, Text0),
    normalize_space(atom(Text), Text0).
xpath_function(number, DOM, Number) :-                         % number
    !,
    text_of_dom(DOM, string, Text0),
    normalize_space(string(Text), Text0),
    catch(xsd_number_string(Number, Text), _, fail).
xpath_function(@Name, element(_, Attrs, _), Value) :-          % @Name
    !,
    (   atom(Name)
    ->  memberchk(Name=Value, Attrs)
    ;   compound(Name)
    ->  compound_name_arguments(Name, AName, AOps),
        memberchk(AName=Value0, Attrs),
        translate_attribute(AOps, Value0, Value)
    ;   member(Name=Value, Attrs)
    ).
xpath_function(quote(Value), _, Value).                         % quote(Value)

xpath_function(self).
xpath_function(content).
xpath_function(text).
xpath_function(text(_)).
xpath_function(normalize_space).
xpath_function(number).
xpath_function(@_).
xpath_function(quote(_)).

translate_attribute([], Value, Value).
translate_attribute([H|T], Value0, Value) :-
    translate_attr(H, Value0, Value1),
    translate_attribute(T, Value1, Value).

translate_attr(number, Value0, Value) :-
    xsd_number_string(Value, Value0).
translate_attr(integer, Value0, Value) :-
    xsd_number_string(Value1, Value0),
    Value is round(Value1).
translate_attr(float, Value0, Value) :-
    xsd_number_string(Value1, Value0),
    Value is float(Value1).
translate_attr(string, Value0, Value) :-
    atom_string(Value0, Value).
translate_attr(lower, Value0, Value) :-
    (   atom(Value0)
    ->  downcase_atom(Value0, Value)
    ;   string_lower(Value0, Value)
    ).
translate_attr(upper, Value0, Value) :-
    (   atom(Value0)
    ->  upcase_atom(Value0, Value)
    ;   string_upper(Value0, Value)
    ).

xpath_condition(Left = Right, Value) :-                        % =
    !,
    var_or_function(Left, Value, LeftValue),
    process_equality(LeftValue, Right).
xpath_condition(contains(Haystack, Needle), Value) :-          % contains(Haystack, Needle)
    !,
    val_or_function(Haystack, Value, HaystackValue),
    val_or_function(Needle, Value, NeedleValue),
    atom(HaystackValue), atom(NeedleValue),
    (   sub_atom(HaystackValue, _, _, _, NeedleValue)
    ->  true
    ).
xpath_condition(Spec, Dom) :-
    in_dom(Spec, Dom, _).


%!  process_equality(+Left, +Right) is semidet.
%
%   Provides (very) partial support for XSLT   functions that can be
%   applied according to the XPath 2 specification.
%
%   For example the XPath expression  in   [1],  and  the equivalent
%   Prolog expression in [2], would both   match the HTML element in
%   [3].
%
%     ==
%     [1] //table[align=lower-case(center)]
%     [2] //table(@align=lower_case(center))
%     [3] <table align="CENTER">
%     ==

process_equality(Left, Right) :-
    var(Right),
    !,
    Left = Right.
process_equality(Left, lower_case(Right)) :-
    !,
    downcase_atom(Left, Right).
process_equality(Left, upper_case(Right)) :-
    !,
    upcase_atom(Left, Right).
process_equality(Left, Right) :-
    Left = Right.


var_or_function(Arg, _, Arg) :-
    var(Arg),
    !.
var_or_function(Func, Value0, Value) :-
    xpath_function(Func),
    !,
    xpath_function(Func, Value0, Value).
var_or_function(Value, _, Value).

val_or_function(Arg, _, Arg) :-
    var(Arg),
    !,
    instantiation_error(Arg).
val_or_function(Func, Value0, Value) :-                         % TBD
    xpath_function(Func, Value0, Value),
    !.
val_or_function(Value, _, Value).


%!  text_of_dom(+DOM, +As, -Text:atom) is det.
%
%   Text is the joined textual content of DOM.

text_of_dom(DOM, As, Text) :-
    phrase(text_of(DOM), Tokens),
    (   As == atom
    ->  atomic_list_concat(Tokens, Text)
    ;   As == string
    ->  atomics_to_string(Tokens, Text)
    ;   must_be(oneof([atom,string]), As)
    ).

text_of(element(_,_,Content)) -->
    text_of_list(Content).
text_of([]) -->
    [].
text_of([H|T]) -->
    text_of(H),
    text_of(T).


text_of_list([]) -->
    [].
text_of_list([H|T]) -->
    text_of_1(H),
    text_of_list(T).


text_of_1(element(_,_,Content)) -->
    !,
    text_of_list(Content).
text_of_1(Data) -->
    { assertion(atom_or_string(Data)) },
    [Data].

atom_or_string(Data) :-
    (   atom(Data)
    ->  true
    ;   string(Data)
    ).
