Liblouis User’s and Programmer’s Manual
1 Introduction
  1.1 Who is this manual for
  1.2 How to read this manual
2 How to Write Translation Tables
  2.1 Overview
  2.2 Hyphenation Tables
  2.3 Character-Definition Opcodes
  2.4 Braille Indicator Opcodes
  2.5 Emphasis Opcodes
    2.5.1 Emphasis class
    2.5.2 Contexts
      2.5.2.1 None
      2.5.2.2 Letter
      2.5.2.3 Word
      2.5.2.4 Phrase
      2.5.2.5 Symbol
    2.5.3 Fallback behavior
    2.5.4 Computer braille
  2.6 Special Symbol Opcodes
  2.7 Special Processing Opcodes
  2.8 Translation Opcodes
  2.9 Character-Class Opcodes
  2.10 Swap Opcodes
  2.11 The Context and Multipass Opcodes
  2.12 The correct Opcode
  2.13 Miscellaneous Opcodes
3 Notes on Back-Translation
4 Testing Translation Tables interactively
  4.1 lou_debug
  4.2 lou_trace
  4.3 lou_checktable
  4.4 lou_allround
  4.5 lou_translate
  4.6 lou_checkhyphens
5 Automated Testing of Translation Tables
  5.1 YAML Tests
    5.1.1 Optional test description
  5.2 Doctests
6 Programming with liblouis
  6.1 Overview
  6.2 Data structure of liblouis tables
  6.3 lou_version
  6.4 lou_translateString
  6.5 lou_translate
  6.6 lou_backTranslateString
  6.7 lou_backTranslate
  6.8 lou_hyphenate
  6.9 lou_compileString
  6.10 lou_getTypeformForEmphClass
  6.11 lou_dotsToChar
  6.12 lou_charToDots
  6.13 lou_registerLogCallback
  6.14 lou_setLogLevel
  6.15 lou_logFile
  6.16 lou_logPrint (deprecated)
  6.17 lou_logEnd
  6.18 lou_setDataPath
  6.19 lou_getDataPath
  6.20 lou_getTable
  6.21 lou_checkTable
  6.22 lou_readCharFromFile
  6.23 lou_free
  6.24 Python bindings
Concept Index
Opcode Index
Function Index
Program Index
Liblouis User’s and Programmer’s Manual
***************************************

This manual is for liblouis (version 3.0.0, 14 July 2016), a Braille
Translation and Back-Translation Library derived from the Linux screen
reader BRLTTY.

Copyright © 1999-2006 by the BRLTTY Team.

Copyright © 2004-2007 ViewPlus Technologies, Inc.  <www.viewplus.com>.

Copyright © 2007, 2009 Abilitiessoft, Inc.  <www.abilitiessoft.org>.

Copyright © 2014, 2016 Swiss Library for the Blind, Visually Impaired
and Print Disabled.  <www.sbs.ch>.

     This file is free software; you can redistribute it and/or modify
     it under the terms of the GNU Lesser (or library) General Public
     License (LGPL) as published by the Free Software Foundation; either
     version 3, or (at your option) any later version.

     This file is distributed in the hope that it will be useful, but
     WITHOUT ANY WARRANTY; without even the implied warranty of
     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
     Lesser (or Library) General Public License LGPL for more details.

     You should have received a copy of the GNU Lesser (or Library)
     General Public License (LGPL) along with this program; see the file
     COPYING. If not, write to the Free Software Foundation, 51 Franklin
     Street, Fifth Floor, Boston, MA 02110-1301, USA.

1 Introduction
**************

Liblouis is an open-source braille translator and back-translator
derived from the translation routines in the BRLTTY screen reader for
Linux.  It has, however, gone far beyond these routines.  It is named in
honor of Louis Braille.  In Linux and Mac OSX it is a shared library,
and in Windows it is a DLL. For installation instructions see the README
file.  Please report bugs and oddities to the mailing list,
<liblouis-liblouisxml@freelists.org>

   This documentation is derived from Chapter 7 of the BRLTTY manual,
but it has been extensively rewritten to cover new features.

1.1 Who is this manual for
==========================

This manual has two main audiences: People who want to write or improve
a braille translation table and people who want to use the braille
translator library in their own programs.  This manual is probably not
for people who are looking for some turn-key braille translation
software.

1.2 How to read this manual
===========================

If you are mostly interested in writing braille translation tables then
you want to focus on *note How to Write Translation Tables::.  You might
want to look at *note Notes on Back-Translation:: if you are interested
in back-translation.  Finally *note Testing Translation Tables
interactively:: and *note Automated Testing of Translation Tables:: will
show how your braille translation tables can be tested interactively and
also in an automated fashion.

   If you want to use the braille translation library in your own
program or you are interested in enhancing the braille translation
library itself then you will want to look at *note Programming with
liblouis::.

2 How to Write Translation Tables
*********************************

For many languages there is already a translation table, so before
creating a new table start by looking at existing tables to modify them
as needed.

   Typically, a braille translation table consists of several parts.
First are header and includes, in which you write what the table is for,
license information and include tables you need for your table.

   Following this, you’ll write various translation rules and lastly you
write special rules to handle certain situations.

   A translation rule is composed of at least three parts: the opcode
(translation command), character(s) and braille dots.  An opcode is a
command you give to a machine or a program to perform something on your
behalf.  In liblouis, an opcode tells it which rule to use when
translating characters into braille.  An operand can be thought of as
parameters for the translation rule and is composed of two parts: the
character or word to be translated and the braille dots.

   For example, suppose you want to read the word ‘world’ using braille
dots ‘456’, followed by the letter ‘W’ all the time.  Then you’d write:

     always world 456-2456

   The word ‘always’ is an opcode which tells liblouis to always honor
this translation, that is to say when the word ‘world’ (an operand) is
encountered, always show braille dots ‘456’ followed by the letter ‘w’
(‘2456’).

   When you write any braille table for any language, we’d recommend
working from some sort of official standard, and have a device or a
program in which you can test your work.

2.1 Overview
============

Many translation (contraction) tables have already been made up.  They
are included in the distribution in the tables directory and can be
studied as part of the documentation.  Some of the more helpful (and
normative) are listed in the following table:

‘chardefs.cti’
     Character definitions for U.S. tables
‘compress.ctb’
     Remove excessive whitespace
‘en-us-g1.ctb’
     Uncontracted American English
‘en-us-g2.ctb’
     Contracted or Grade 2 American English
‘en-us-brf.dis’
     Make liblouis output conform to BRF standard
‘en-us-comp8.ctb’
     8-dot computer braille for use in coding examples
‘en-us-comp6.ctb’
     6-dot computer braille
‘nemeth.ctb’
     Nemeth Code translation for use with liblouisutdml
‘nemeth_edit.ctb’
     Fixes errors at the boundaries of math and text

   The names used for files containing translation tables are completely
arbitrary.  They are not interpreted in any way by the translator.
Contraction tables may be 8-bit ASCII files, UTF-8, 16-bit big-endian
Unicode files or 16-bit little-endian Unicode files.  Blank lines are
ignored.  Any leading and trailing whitespace (any number of blanks
and/or tabs) is ignored.  Lines which begin with a number sign or hatch
mark (‘#’) are ignored, i.e.  they are comments.  If the number sign is
not the first non-blank character in the line, it is treated as an
ordinary character.  If the first non-blank character is less-than (‘<’)
the line is also treated as a comment.  This makes it possible to mark
up tables as xhtml documents.  Lines which are not blank or comments
define table entries.  The general format of a table entry is:

     opcode operands comments

   Table entries may not be split between lines.  The opcode is a
mnemonic that specifies what the entry does.  The operands may be
character sequences, braille dot patterns or occasionally something
else.  They are described for each opcode, please *note Opcode Index::.
With some exceptions, opcodes expect a certain number of operands.  Any
text on the line after the last operand is ignored, and may be a
comment.  A few opcodes accept a variable number of operands.  In this
case a number sign (‘#’) begins a comment unless it is preceded by a
backslash (‘\’).

   Here are some examples of table entries.

     # This is a comment.
     always world 456-2456 A word and the dot pattern of its contraction

   Most opcodes have both a "characters" operand and a "dots" operand,
though some have only one and a few have other types.

   The characters operand consists of any combination of characters and
escape sequences proceeded and followed by whitespace.  Escape sequences
are used to represent difficult characters.  They begin with a backslash
(‘\’).  They are:

‘\’
     backslash
‘\f’
     form feed
‘\n’
     new line
‘\r’
     carriage return
‘\s’
     blank (space)
‘\t’
     horizontal tab
‘\v’
     vertical tab
‘\e’
     "escape" character (hex 1b, dec 27)
‘\xhhhh’
     4-digit hexadecimal value of a character

   If liblouis has been compiled for 32-bit Unicode the following are
also recognized.

‘\yhhhhh’
     5-digit (20 bit) character
‘\zhhhhhhhh’
     Full 32-bit value.

   The dots operand is a braille dot pattern.  The real braille dots, 1
through 8, must be specified with their standard numbers.

   liblouis recognizes _virtual dots_, which are used for special
purposes, such as distinguishing accent marks.  There are seven virtual
dots.  They are specified by the number 9 and the letters ‘a’ through
‘f’.

   For a multi-cell dot pattern, the cell specifications must be
separated from one another by a dash (‘-’).  For example, the
contraction for the English word ‘lord’ (the letter ‘l’ preceded by dot
5) would be specified as ‘5-123’.  A space may be specified with the
special dot number 0.

   An opcode which is helpful in writing translation tables is
‘include’.  Its format is:

     include filename

   It reads the file indicated by ‘filename’ and incorporates or
includes its entries into the table.  Included files can include other
files, which can include other files, etc.  For an example, see what
files are included by the entry ‘include en-us-g1.ctb’ in the table
‘en-us-g2.ctb’.  If the included file is not in the same directory as
the main table, use a full path name for filename.  Tables can also be
specified in a table list, in which the table names are separated by
commas and given as a single table name in calls to the translation
functions.

   The order of the various types of opcodes or table entries is
important.  Character-definition opcodes should come first.  However, if
the optional ‘display’ opcode (*note display: display opcode.) is used
it should precede character-definition opcodes.  Braille-indicator
opcodes should come next.  Translation opcodes should follow.  The
‘context’ opcode (*note context: context opcode.) is a translation
opcode, even though it is considered along with the multipass opcodes.
These latter should follow the translation opcodes.  The ‘correct’
opcode (*note correct: correct opcode.) can be used anywhere after the
character-definition opcodes, but it is probably a good idea to group
all ‘correct’ opcodes together.  The ‘include’ opcode (*note include:
include opcode.) can be used anywhere, but the order of entries in the
combined table must conform to the order given above.  Within each type
of opcode, the order of entries is generally unimportant.  Thus the
translation entries can be grouped alphabetically or in any other order
that is convenient.  Hyphenation tables may be specified either with an
‘include’ opcode or as part of a table list.  They should come after
everything else.  Character-definition opcodes are necessary for
hyphenation tables to work.

2.2 Hyphenation Tables
======================

Hyphenation tables are necessary to make opcodes such as the ‘nocross’
opcode (*note nocross: nocross opcode.) function properly.  There are no
opcodes for hyphenation table entries because these tables have a
special format.  Therefore, they cannot be specified as part of an
ordinary table.  Rather, they must be included using the ‘include’
opcode (*note include: include opcode.) or as part of a table list.  The
liblouis hyphenation algorithm was adopted from the one used by
OpenOffice.  Note that Hyphenation tables must follow character
definitions and should preferably be the last.  For an example of a
hyphenation table, see ‘hyph_en_US.dic’.

2.3 Character-Definition Opcodes
================================

These opcodes are needed to define attributes such as digit,
punctuation, letter, etc.  for all characters and their dot patterns.
liblouis has no built-in character definitions, but such definitions are
essential to the operation of the ‘context’ opcode (*note context:
context opcode.), the ‘correct’ opcode (*note correct: correct opcode.),
the multipass opcodes and the back-translator.  If the dot pattern is a
single cell, it is used to define the mapping between dot patterns and
characters, unless a ‘display’ opcode (*note display: display opcode.)
for that character-dot-pattern pair has been used previously.  If only a
single-cell dot pattern has been given for a character, that dot pattern
is defined with the character’s own attributes.  If more than one cell
is given and some of them have not previously been defined as single
cells, the undefined cells are entered into the dots table with the
space attribute.  This is done for backward compatibility with old
tables, but it may cause problems with the above opcodes or
back-translation.  For this reason, every single-cell dot pattern should
be defined before it is used in a multi-cell character representation.
The best way to do this is to use the 8-dot computer braille
representation for the particular braille code.  If a character or dot
pattern used in any rule, except those with the ‘display’ opcode, the
‘repeated’ opcode (*note repeated: repeated opcode.) or the ‘replace’
opcode (*note replace: replace opcode.), is not defined by one of the
character-definition opcodes, liblouis will give an error message and
refuse to continue until the problem is fixed.  If the translator or
back-translator encounters an undefined character in its input it
produces a succinct error indication in its output, and the character is
treated as a space.

‘space character dots’
     Defines a character as a space and also defines the dot pattern as
     such.  for example:

          space \s 0 \s is the escape sequence for blank; 0 means no dots.

‘punctuation character dots’
     Associates a punctuation mark in the particular language with a
     braille representation and defines the character and dot pattern as
     punctuation.  For example:

          punctuation . 46 dot pattern for period in NAB computer braille

‘digit character dots’
     Associates a digit with a dot pattern and defines the character as
     a digit.  For example:

          digit 0 356 NAB computer braille

‘uplow characters dots [,dots]’
     The characters operand must be a pair of letters, of which the
     first is uppercase and the second lowercase.  The first dots
     suboperand indicates the dot pattern for the upper-case letter.  It
     may have more than one cell.  The second dots suboperand must be
     separated from the first by a comma and is optional, as indicated
     by the square brackets.  If present, it indicates the dot pattern
     for the lower-case letter.  It may also have more than one cell.
     If the second dots suboperand is not present the first is used for
     the lower-case letter as well as the upper-case letter.  This
     opcode is needed because not all languages follow a consistent
     pattern in assigning Unicode codes to upper and lower case letters.
     It should be used even for languages that do.  The distinction is
     important in the forward translator.  for example:

          uplow Aa 17,1

‘grouping name characters dots ,dots’
     This opcode is used to indicate pairs of grouping symbols used in
     processing mathematical expressions.  These symbols are usually
     generated by the MathML interpreter in liblouisutdml.  They are
     used in multipass opcodes.  The name operand must contain only
     letters, but they may be upper- or lower-case.  The characters
     operand must contain exactly two Unicode characters.  The dots
     operand must contain exactly two braille cells, separated by a
     comma.  Note that grouping dot patterns also need to be declared
     with the ‘exactdots’ opcode (*note exactdots: exactdots opcode.).
     The characters may need to be declared with the ‘math’ opcode
     (*note math: math opcode.).

          grouping mrow \x0001\x0002 1e,2e
          grouping mfrac \x0003\x0004 3e,4e

‘letter character dots’
     Associates a letter in the language with a braille representation
     and defines the character as a letter.  This is intended for
     letters which are neither uppercase nor lowercase.

‘lowercase character dots’
     Associates a character with a dot pattern and defines the character
     as a lowercase letter.  Both the character and the dot pattern have
     the attributes lowercase and letter.

‘uppercase character dots’
     Associates a character with a dot pattern and defines the character
     as an uppercase letter.  Both the character and the dot pattern
     have the attributes uppercase and letter.  ‘lowercase’ and
     ‘uppercase’ should be used when a letter has only one case.
     Otherwise use the ‘uplow’ opcode (*note uplow: uplow opcode.).

‘litdigit digit dots’
     Associates a digit with the dot pattern which should be used to
     represent it in literary texts.  For example:

          litdigit 0 245
          litdigit 1 1

‘sign character dots’
     Associates a character with a dot pattern and defines both as a
     sign.  This opcode should be used for things like at sign (‘@’),
     percent (‘%’), dollar sign (‘$’), etc.  Do not use it to define
     ordinary punctuation such as period and comma.  For example:

          sign % 4-25-1234 literary percent sign

‘math character dots’
     Associates a character and a dot pattern and defines them as a
     mathematical symbol.  It should be used for less than (‘<’),
     greater than(‘>’), equals(‘=’), plus(‘+’), etc.  For example:

          math + 346 plus

2.4 Braille Indicator Opcodes
=============================

Braille indicators are dot patterns which are inserted into the braille
text to indicate such things as capitalization, italic type, computer
braille, etc.  The opcodes which define them are followed only by a dot
pattern, which may be one or more cells.

‘capsign dots’
     The dot pattern which indicates capitalization of a single letter.
     In English, this is dot 6.  For example:

          capsign 6

‘begcaps dots’
     The dot pattern which begins a block of capital letters.  For
     example:

          begcaps 6-6

‘endcaps dots’
     The dot pattern which ends a block of capital letters within a
     word.  For example:

          endcaps 6-3

‘letsign dots’
     This indicator is needed in Grade 2 to show that a single letter is
     not a contraction.  It is also used when an abbreviation happens to
     be a sequence of letters that is the same as a contraction.  For
     example:

          letsign 56

‘noletsign letters’

     The letters in the operand will not be proceeded by a letter sign.
     More than one ‘noletsign’ opcode can be used.  This is equivalent
     to a single entry containing all the letters.  In addition, if a
     single letter, such as ‘a’ in English, is defined as a ‘word’
     (*note word: word opcode.) or ‘largesign’ (*note largesign:
     largesign opcode.), it will be treated as though it had also been
     specified in a ‘noletsign’ entry.

‘noletsignbefore characters’
     If any of the characters proceeds a single letter without a space a
     letter sign is not used.  By default the characters apostrophe
     (‘'’) and period (‘.’) have this property.  Use of a
     ‘noletsignbefore’ entry cancels the defaults.  If more than one
     ‘noletsignbefore’ entry is used, the characters in all entries are
     combined.

‘noletsignafter characters’
     If any of the characters follows a single letter without a space a
     letter sign is not used.  By default the characters apostrophe
     (‘'’) and period (‘.’) have this property.  Use of a
     ‘noletsignafter’ entry cancels the defaults.  If more than one
     ‘noletsignafter’ entry is used the characters in all entries are
     combined.

‘nocontractsign dots’

     The dots in this opcode are used to indicate a letter or a sequence
     of letters that are not a contraction, e.g.  ‘CD’.  The opcode is
     similar to the ‘letsign’ opcode (*note letsign: letsign opcode.).

‘numsign dots’
     The translator inserts this indicator before numbers made up of
     digits defined with the ‘litdigit’ opcode (*note litdigit: litdigit
     opcode.) to show that they are a number and not letters or some
     other symbols.  For example:

          numsign 3456

2.5 Emphasis Opcodes
====================

In many braille systems emphasis such as bold, italics or underline is
indicated using special dot patterns that mark the start and often also
the end.  For some languages these braille indicators differ depending
on the context, i.e.  here is an separate indicator for an emphasized
word and another one for an emphasized phrase.  To accommodate for all
these usage scenarios liblouis provides a number of opcodes for various
contexts.

   At the same time some braille systems use different indicators for
different kinds of emphasis while others know only one kind of emphasis.
For that reason liblouis doesn’t hard code any emphasis but the table
author defines which kind of emphasis exist for a specific language
using the ‘emphclass’ opcode (*note emphclass: emphclass opcode.)
opcode.

2.5.1 Emphasis class
--------------------

The ‘emphclass’ opcode defines the classes of emphasis that are relevant
for a particular language.  For all emphasis that need special
indicators an emphasis class has to be declared.

‘emphclass <emphasis class>’
     Define an emphasis class to be used later in other emphasis related
     opcodes in the table.

          emphclass italic
          emphclass underline
          emphclass bold
          emphclass transnote

2.5.2 Contexts
--------------

In order to understand the capabilities of Liblouis for emphasis
handling we have to look at the different contexts that are supported.

2.5.2.1 None
............

For some languages there is no such concept as contexts.  Emphasis is
always handled the same regardless of context.  There is simply an
indicator for the beginning of emphasis and another one for the end of
the emphasis.

‘begemph <emphasis class> <dot pattern>’
     Braille dot pattern to indicate the beginning of emphasis.

          begemph italic 46-3

‘endemph <emphasis class> <dot pattern>’
     Braille dot pattern to indicate the end of emphasis.

          endemph italic 46-36

2.5.2.2 Letter
..............

Some languages have special indicators for single letter emphasis.

‘emphletter <emphasis class> <dot pattern>’
     Braille dot pattern to indicate that the next character is
     emphasized.

          emphletter italic 46-25

2.5.2.3 Word
............

Many languages have special indicators for emphasized words.  Usually
they start at the beginning of the word and and implicitly, i.e.
without a closing indicator at the end of the word.  There are also use
cases where the emphasis starts in the middle of the word and an
explicit closing indicator is required.

‘begemphword <emphasis class> <dot pattern>’
     Braille dot pattern to indicate the beginning of an emphasized word
     or the beginning of emphasized characters within a word.

          begemphword underline 456-36

‘endemphword <emphasis class> <dot pattern>’
     Generally emphasis with word context ends when the word ends.
     However when an indication is required to close a word emphasis
     then this opcode defines the Braille dot pattern that indicates the
     end of a word emphasis.

          endemphword transnote 6-3

     If emphasis ends in the middle of a word the Braille dot pattern
     defined in this opcode is also used.

2.5.2.4 Phrase
..............

Many languages have a concept of a phrase where the emphasis is valid
for a number of words.  The beginning of the phase is indicated with a
braille dot pattern and a closing indicator is put before or after the
last word of the phrase.  To define how many words are considered a
phrase in your language use the ‘lenemphphrase’ opcode (*note
lenemphphrase: lenemphphrase opcode.).

‘begemphphrase <emphasis class> <dot pattern>’
     Braille dot pattern to indicate the beginning of a phrase.

          begemphphrase bold 456-46-46

‘endemphphrase before <emphasis class> before <dot pattern>’
     Braille dot pattern to indicate the end of a phrase.  The closing
     indicator will be placed before the last word of the phrase.

          endemphphrase bold before 456-46

‘endemphphrase after <emphasis class> after <dot pattern>’
     Braille dot pattern to indicate the end of a phrase.  The closing
     indicator will be placed after the last word of the phrase.  If
     both ‘endemphphrase <emphasis class> before’ and ‘endemphphrase
     <emphasis class> after’ are defined an error will be signaled.

          endemphphrase underline after 6-3

‘lenemphphrase <emphasis class> <number>’
     Define how many words are required before a sequence of words is
     considered a phrase.

          lenemphphrase underline 3

2.5.2.5 Symbol
..............

UEB has a concept of symbols that need special indication.  When the
translator detects an emphasis sequence that needs to be indicated with
the rules for a symbol then it will use the dots defined with the
‘emphletter’ opcode (*note emphletter: emphletter opcode.).  To indicate
the end of the symbol it will use the dots defined in the ‘endemphword’
opcode (*note endemphword: endemphword opcode.).

2.5.3 Fallback behavior
-----------------------

Many braille systems either handle emphasis using no contexts or
otherwise by employing a combination of the letter, word and phrase
contexts.  So if a table defines any opcodes for the letter, word or
phrase contexts then liblouis will signal an error for opcodes that
define emphasis with no context.  In other words contrary to previous
versions of liblouis there is no fallback behavior.

   As a consequence, there will only be emphasis for a context when the
table defines it.  So for example when defining a braille dot pattern
for phrases and not for words liblouis will not indicate emphasis on
words that aren’t part of a phrase.

2.5.4 Computer braille
----------------------

For computer braille there are only two braille indicators, for the
beginning and end of a sequence of characters to be rendered in computer
braille.  Such a sequence may also have other emphasis.  The computer
braille indicators are applied not only when computer braille is
indicated in the ‘typeform’ parameter, but also when a sequence of
characters is determined to be computer braille because it contains a
subsequence defined by the ‘compbrl’ opcode (*note compbrl: compbrl
opcode.).

2.6 Special Symbol Opcodes
==========================

These opcodes define certain symbols, such as the decimal point, which
require special treatment.

‘decpoint character dots’
     This opcode defines the decimal point.  The character operand must
     have only one character.  For example, in ‘en-us-g1.ctb’ we have:

          decpoint . 46

‘hyphen character dots’
     This opcode defines the hyphen, that is, the character used in
     compound words such as ‘have-nots’.  The back-translator uses it to
     determine the end of individual words.

2.7 Special Processing Opcodes
==============================

These opcodes cause special processing to be carried out.

‘capsnocont’
     This opcode has no operands.  If it is specified, words or parts of
     words in all caps are not contracted.  This is needed for languages
     such as Norwegian.

2.8 Translation Opcodes
=======================

These opcodes define the braille representations for character
sequences.  Each of them defines an entry within the contraction table.
These entries may be defined in any order except, as noted below, when
they define alternate representations for the same character sequence.

   Each of these opcodes specifies a condition under which the
translation is legal, and each also has a characters operand and a dots
operand.  The text being translated is processed strictly from left to
right, character by character, with the most eligible entry for each
position being used.  If there is more than one eligible entry for a
given position in the text, then the one with the longest character
string is used.  If there is more than one eligible entry for the same
character string, then the one defined first is is tested for legality
first.  (This is the only case in which the order of the entries makes a
difference.)

   The characters operand is a sequence or string of characters preceded
and followed by whitespace.  Each character can be entered in the normal
way, or it can be defined as a four-digit hexadecimal number preceded by
‘\x’.

   The dots operand defines the braille representation for the
characters operand.  It may also be specified as an equals sign (‘=’).
This means that the the default representation for each character (*note
Character-Definition Opcodes::) within the sequence is to be used.  Note
however that the ‘=’ shortcut for dot patterns is deprecated.  Dot
patterns should be written out.  Otherwise back-translation may not be
correct.

   In what follows the word ‘characters’ means a sequence of one or more
consecutive letters between spaces and/or punctuation marks.

‘noback opcode ...’
     This is an opcode prefix, that is to say, it modifies the operation
     of the opcode that follows it on the same line.  noback specifies
     that no back-translation is to be done using this line.

          noback always ;\s; 0

‘nofor opcode ...’
     This is an opcode prefix which modifies the operation of the opcode
     following it on the same line.  nofor specifies that forward
     translation is not to use the information on this line.

‘compbrl characters’
     If the characters are found within a block of text surrounded by
     whitespace the entire block is translated according to the default
     braille representations defined by the *note Character-Definition
     Opcodes::, if 8-dot computer braille is enabled or according to the
     dot patterns given in the ‘comp6’ opcode (*note comp6: comp6
     opcode.), if 6-dot computer braille is enabled.  For example:

          compbrl www translate URLs in computer braille

‘comp6 character dots’
     This opcode specifies the translation of characters in 6-dot
     computer braille.  It is necessary because the translation of a
     single character may require more than one cell.  The first operand
     must be a character with a decimal representation from 0 to 255
     inclusive.  The second operand may specify as many cells as
     necessary.  The opcode is somewhat of a misnomer, since any dots,
     not just dots 1 through 6, can be specified.  This even includes
     virtual dots (*note virtual dots::).

‘nocont characters’
     Like ‘compbrl’, except that the string is uncontracted.  ‘prepunc’
     opcode (*note prepunc: prepunc opcode.) and ‘postpunc’ opcode
     (*note postpunc: postpunc opcode.) rules are applied, however.
     This is useful for specifying that foreign words should not be
     contracted in an entire document.

‘replace characters {characters}’
     Replace the first set of characters, no matter where they appear,
     with the second.  Note that the second operand is _NOT_ a dot
     pattern.  It is also optional.  If it is omitted the character(s)
     in the first operand will be discarded.  This is useful for
     ignoring characters.  It is possible that the "ignored" characters
     may still affect the translation indirectly.  Therefore, it is
     preferable to use ‘correct’ opcode (*note correct: correct
     opcode.).

‘always characters dots’
     Replace the characters with the dot pattern no matter where they
     appear.  Do _NOT_ use an entry such as ‘always a 1’.  Use the
     ‘uplow’, ‘letter’, etc.  character definition opcodes instead.  For
     example:

          always world 456-2456 unconditional translation

‘repeated characters dots’
     Replace the characters with the dot pattern no matter where they
     appear.  Ignore any consecutive repetitions of the same character
     sequence.  This is useful for shortening long strings of spaces or
     hyphens or periods.  For example:

          repeated --- 36-36-36 shorten separator lines made with hyphens

‘repword characters dots’
     When characters are encountered check to see if the word before
     this string matches the word after it.  If so, replace characters
     with dots and eliminate the second word and any word following
     another occurrence of characters that is the same.  This opcode is
     used in Malaysian braille.  In this case the rule is:

          repword - 123456

‘largesign characters dots’
     Replace the characters with the dot pattern no matter where they
     appear.  In addition, if two words defined as large signs follow
     each other, remove the space between them.  For example, in
     ‘en-us-g2.ctb’ the words ‘and’ and ‘the’ are both defined as large
     signs.  Thus, in the phrase ‘the cat and the dog’ the space would
     be deleted between ‘and’ and ‘the’, with the result ‘the cat andthe
     dog’.  Of course, ‘and’ and ‘the’ would be properly contracted.
     The term ‘largesign’ is a bit of braille jargon that pleases
     braille experts.

‘word characters dots’
     Replace the characters with the dot pattern if they are a word,
     that is, are surrounded by whitespace and/or punctuation.

‘syllable characters dots’
     As its name indicates, this opcode defines a "syllable" which must
     be represented by exactly the dot patterns given.  Contractions may
     not cross the boundaries of this "syllable" either from left or
     right.  The character string defined by this opcode need not be a
     lexical syllable, though it usually will be.  The equal sign in the
     following example means that the the default representation for
     each character within the sequence is to be used (*note Translation
     Opcodes::):

          syllable horse = sawhorse, horseradish

‘nocross characters dots’
     Replace the characters with the dot pattern if the characters are
     all in one syllable (do not cross a syllable boundary).  For this
     opcode to work, a hyphenation table must be included.  If this is
     not done, ‘nocross’ behaves like the ‘always’ opcode (*note always:
     always opcode.).  For example, if the English Grade 2 table is
     being used and the appropriate hyphenation table has been included
     ‘nocross sh 146’ will cause the ‘sh’ in ‘monkshood’ not to be
     contracted.

‘joinword characters dots’
     Replace the characters with the dot pattern if they are a word
     which is followed by whitespace and a letter.  In addition remove
     the whitespace.  For example, ‘en-us-g2.ctb’ has ‘joinword to 235’.
     This means that if the word ‘to’ is followed by another word the
     contraction is to be used and the space is to be omitted.  If these
     conditions are not met, the word is translated according to any
     other opcodes that may apply to it.

‘lowword characters dots’
     Replace the characters with the dot pattern if they are a word
     preceded and followed by whitespace.  No punctuation either before
     or after the word is allowed.  The term ‘lowword’ derives from the
     fact that in English these contractions are written in the lower
     part of the cell.  For example:

          lowword were 2356

‘contraction characters’
     If you look at ‘en-us-g2.ctb’ you will see that some words are
     actually contracted into some of their own letters.  A famous
     example among braille transcribers is ‘also’, which is contracted
     as ‘al’.  But this is also the name of a person.  To take another
     example, ‘altogether’ is contracted as ‘alt’, but this is the
     abbreviation for the alternate key on a computer keyboard.
     Similarly ‘could’ is contracted into ‘cd’, but this is the
     abbreviation for compact disk.  To prevent confusion in such cases,
     the letter sign (see ‘letsign’ opcode (*note letsign: letsign
     opcode.)) is placed before such letter combinations when they
     actually are abbreviations, not contractions.  The ‘contraction’
     opcode tells the translator to do this.

‘sufword characters dots’
     Replace the characters with the dot pattern if they are either a
     word or at the beginning of a word.

‘prfword characters dots’
     Replace the characters with the dot pattern if they are either a
     word or at the end of a word.

‘begword characters dots’
     Replace the characters with the dot pattern if they are at the
     beginning of a word.

‘begmidword characters dots’
     Replace the characters with the dot pattern if they are either at
     the beginning or in the middle of a word.

‘midword characters dots’
     Replace the characters with the dot pattern if they are in the
     middle of a word.

‘midendword characters dots’
     Replace the characters with the dot pattern if they are either in
     the middle or at the end of a word.

‘endword characters dots’
     Replace the characters with the dot pattern if they are at the end
     of a word.

‘partword characters dots’
     Replace the characters with the dot pattern if the characters are
     anywhere in a word, that is, if they are proceeded or followed by a
     letter.

‘exactdots @dots’
     Note that the operand must begin with an at sign (‘@’).  The dot
     pattern following it is evaluated for validity.  If it is valid,
     whenever an at sign followed by this dot pattern appears in the
     source document it is replaced by the characters corresponding to
     the dot pattern in the output.  This opcode is intended for use in
     liblouisutdml semantic-action files to specify exact dot patterns,
     as in mathematical codes.  For example:

          exactdots @4-46-12356
     will produce the characters with these dot patterns in the output.

‘prepunc characters dots’
     Replace the characters with the dot pattern if they are part of
     punctuation at the beginning of a word.

‘postpunc characters dots’
     Replace the characters with the dot pattern if they are part of
     punctuation at the end of a word.

‘begnum characters dots’
     Replace the characters with the dot pattern if they are at the
     beginning of a number, that is, before all its digits.  For
     example, in ‘en-us-g1.ctb’ we have ‘begnum # 4’.

‘midnum characters dots’
     Replace the characters with the dot pattern if they are in the
     middle of a number.  For example, ‘en-us-g1.ctb’ has ‘midnum . 46’.
     This is because the decimal point has a different dot pattern than
     the period.

‘endnum characters dots’
     Replace the characters with the dot pattern if they are at the end
     of a number.  For example ‘en-us-g1.ctb’ has ‘endnum th 1456’.
     This handles things like ‘4th’.  A letter sign is _NOT_ inserted.

‘joinnum characters dots’
     Replace the characters with the dot pattern.  In addition, if
     whitespace and a number follows omit the whitespace.  This opcode
     can be used to join currency symbols to numbers for example:

          joinnum \x20AC 15 (EURO SIGN)
          joinnum \x0024 145 (DOLLAR SIGN)
          joinnum \x00A3 1234 (POUND SIGN)
          joinnum \x00A5 13456 (YEN SIGN)

2.9 Character-Class Opcodes
===========================

These opcodes define and use character classes.  A character class
associates a set of characters with a name.  The name then refers to any
character within the class.  A character may belong to more than one
class.

   The basic character classes correspond to the character definition
opcodes, with the exception of the ‘uplow’ opcode (*note uplow: uplow
opcode.), which defines characters belonging to the two classes
‘uppercase’ and ‘lowercase’.  These classes are:

‘space’
     Whitespace characters such as blank and tab
‘digit’
     Numeric characters
‘letter’
     Both uppercase and lowercase alphabetic characters
‘lowercase’
     Lowercase alphabetic characters
‘uppercase’
     Uppercase alphabetic characters
‘punctuation’
     Punctuation marks
‘sign’
     Signs such as percent (‘%’)
‘math’
     Mathematical symbols
‘litdigit’
     Literary digit
‘undefined’
     Not properly defined

   The opcodes which define and use character classes are shown below.
For examples see ‘fr-abrege.ctb’.

‘class name characters’
     Define a new character class.  The characters operand must be
     specified as a string.  A character class may not be used until it
     has been defined.

‘after class opcode ...’
     The specified opcode is further constrained in that the matched
     character sequence must be immediately preceded by a character
     belonging to the specified class.  If this opcode is used more than
     once on the same line then the union of the characters in all the
     classes is used.

‘before class opcode ...’
     The specified opcode is further constrained in that the matched
     character sequence must be immediately followed by a character
     belonging to the specified class.  If this opcode is used more than
     once on the same line then the union of the characters in all the
     classes is used.

2.10 Swap Opcodes
=================

The swap opcodes are needed to tell the ‘context’ opcode (*note context:
context opcode.), the ‘correct’ opcode (*note correct: correct opcode.)
and multipass opcodes which dot patterns to swap for which characters.
There are three, ‘swapcd’, ‘swapdd’ and ‘swapcc’.  The first swaps dot
patterns for characters.  The second swaps dot patterns for dot patterns
and the third swaps characters for characters.  The first is used in the
‘context’ opcode and the second is used in the multipass opcodes.  Dot
patterns are separated by commas and may contain more than one cell.

‘swapcd name characters dots, dots, dots, ...’
     See above paragraph for explanation.  For example:

          swapcd dropped 0123456789 356,2,23,...

‘swapdd name dots, dots, dots ... dotpattern1, dotpattern2, dotpattern3, ...’
     The ‘swapdd’ opcode defines substitutions for the multipass
     opcodes.  In the second operand the dot patterns must be single
     cells, but in the third operand multi-cell dot patterns are
     allowed.  This is because multi-cell patterns in the second operand
     would lead to ambiguities.

‘swapcc name characters characters’
     The ‘swapcc’ opcode swaps characters in its second operand for
     characters in the corresponding places in its third operand.  It is
     intended for use with ‘correct’ opcodes and can solve problems such
     as formatting phone numbers.

2.11 The Context and Multipass Opcodes
======================================

The ‘context’ and multipass opcodes (‘pass2’, ‘pass3’ and ‘pass4’)
provide translation capabilities beyond those of the basic translation
opcodes (*note Translation Opcodes::) discussed previously.  The
multipass opcodes cause additional passes to be made over the string to
be translated.  The number after the word ‘pass’ indicates in which pass
the entry is to be applied.  If no multipass opcodes are given, only the
first translation pass is made.  The ‘context’ opcode is basically a
multipass opcode for the first pass.  It differs slightly from the
multipass opcodes per se.  The format of all these opcodes is ‘opcode
test action’.  The specific opcodes are invoked as follows:

‘context test action’
‘pass2 test action’
‘pass3 test action’
‘pass4 test action’

   The ‘test’ and ‘action’ operands have suboperands.  Each suboperand
begins with a non-alphanumeric character and ends when another
non-alphanumeric character is encountered.  The suboperands and their
initial characters are as follows.

‘" (double quote)’
     a string of characters.  This string must be terminated by another
     double quote.  It may contain any characters.  If a double quote is
     needed within the string, it must be preceded by a backslash (‘\’).
     If a space is needed, it must be represented by the escape sequence
     \s.  This suboperand is valid only in the test part of the
     ‘context’ opcode.

‘@ (at sign)’
     a sequence of dot patterns.  Cells are separated by hyphens as
     usual.  This suboperand is not valid in the test part of the
     context and correct opcodes.

‘` (accent mark)’
     If this is the beginning of the string being translated this
     suboperand is true.  It is valid only in the test part and must be
     the first thing in this operand.

‘~ (tilde)’
     If this is the end of the string being translated this suboperand
     is true.  It is valid only in the test part and must be the last
     thing in this operand.

‘$ (dollar sign)’
     a string of attributes, such as ‘d’ for digit, ‘l’ for letter, etc.
     More than one attribute can be given.  If you wish to check
     characters with any attribute, use the letter ‘a’.  Input
     characters are checked to see if they have at least one of the
     attributes.  The attribute string can be followed by numbers
     specifying how many characters are to be checked.  If no numbers
     are given, 1 is assumed.  If two numbers separated by a hyphen are
     given, the input is checked to make sure that at least the first
     number of characters with the attributes are present, but no more
     than the second number.  If only one number is present, then
     exactly that many characters must have the attributes.  A period
     instead of the numbers indicates an indefinite number of characters
     (for technical reasons the number of characters that are actually
     matched is limited to 65535).

     This suboperand is valid in all test parts but not in action parts.
     For the characters which can be used in attribute strings, see the
     following table.

‘! (exclamation point)’
     reverses the logical meaning of the suboperand which follows.  For
     example, !$d is true only if the character is _NOT_ a digit.  This
     suboperand is valid in test parts only.

‘% (percent sign)’
     the name of a class defined by the ‘class’ opcode (*note class:
     class opcode.) or the name of a swap set defined by the swap
     opcodes (*note Swap Opcodes::).  Names may contain only letters.
     The letters may be upper or lower-case.  The case matters.  Class
     names may be used in test parts only.  Swap names are valid
     everywhere.

‘{ (left brace)’
     Name: the name of a grouping pair.  The left brace indicates that
     the first (or left) member of the pair is to be used in matching.
     If this is between replacement brackets it must be the only item.
     This is also valid in the action part.

‘} (right brace)’
     Name: the name of a grouping pair.  The right brace indicates that
     the second (or right) member is to be used in matching.  See the
     remarks on the left brace immediately above.

‘/ (slash)’
     Search the input for the expression following the slash and return
     true if found.  This can be used to set a variable.

‘_ (underscore)’
     Move backward.  If a number follows, move backward that number of
     characters.  The program never moves backward beyond the beginning
     of the input string.  This suboperand is valid only in test parts.

‘[ (left bracket)’
     start replacement here.  This suboperand must always be paired with
     a right bracket and is valid only in test parts.  Multiple pairs of
     square brackets in a single expression are not allowed.

‘] (right bracket)’
     end replacement here.  This suboperand must always be paired with a
     left bracket and is valid only in test parts.

‘# (number sign or crosshatch)’
     test or set a variable.  Variables are referred to by numbers 1 to
     50, for example, ‘#1’, ‘#2’, ‘#25’.  Variables may be set by one
     ‘context’ or multipass opcode and tested by another.  Thus, an
     operation that occurs at one place in a translation can tell an
     operation that occurs later about itself.  This feature will be
     used in math translation, and it may also help to alleviate the
     need for new opcodes.  This suboperand is valid everywhere.

     Variables are set in the action part.  To set a variable use an
     expression like ‘#1=1’, ‘#2=5’, etc.  Variables are also
     incremented and decremented in the action part with expressions
     like ‘#1+’, ‘#3-’, etc.  These operators increment or decrement the
     variable by 1.

     Variables are tested in the test part with expressions like ‘#1=2’,
     ‘#3<4’, ‘#5>6’, etc.

‘* (asterisk)’
     Copy the characters or dot patterns in the input within the
     replacement brackets into the output and discard anything else that
     may match.  This feature is used, for example, for handling numeric
     subscripts in Nemeth.  This suboperand is valid only in action
     parts.

‘? (question mark)’
     Valid only in the action part.  The characters to be replaced are
     simply ignored.  That is, they are replaced with nothing.  If
     either member of a grouping pair is in the replace brackets the
     other member at the same level is also removed.

   The characters which can be used in attribute strings are as follows:

‘a’
     any attribute
‘d’
     digit
‘D’
     literary digit
‘l’
     letter
‘m’
     math
‘p’
     punctuation
‘S’
     sign
‘s’
     space
‘U’
     uppercase
‘u’
     lowercase
‘w’
     first user-defined class
‘x’
     second user-defined class
‘y’
     third user-defined class
‘z’
     fourth user-defined class

   The following illustrates the algorithm how text is evaluated with
multipass expressions:

Loop over context, pass2, pass3 and pass4 and do the following for each
pass:

  a. Match the text following the cursor against all expressions in the
     current pass
  b. If there is no match: shift the cursor one position to the right
     and continue the loop
  c. If there is a match: choose the longest match
  d. Do the replacement (everything between square brackets)
  e. Place the cursor after the replaced text
  f. continue loop

2.12 The correct Opcode
=======================

‘correct test action’
     Because some input (such as that from an OCR program) may contain
     systematic errors, it is sometimes advantageous to use a
     pre-translation pass to remove them.  The errors and their
     corrections are specified by the ‘correct’ opcode.  If there are no
     ‘correct’ opcodes in a table, the pre-translation pass is not used.
     The format of the ‘correct’ opcode is very similar to that of the
     ‘context’ opcode (*note context: context opcode.).  The only
     difference is that in the action part strings may be used and dot
     patterns may not be used.  Some examples of ‘correct’ opcode
     entries are:

          correct "\\" ? Eliminate backslashes
          correct "cornf" "comf" fix a common "scano"
          correct "cornm" "comm"
          correct "cornp" "comp"
          correct "*" ? Get rid of stray asterisks
          correct "|" ? ditto for vertical bars
          correct "\s?" "?" drop space before question mark

2.13 Miscellaneous Opcodes
==========================

‘include filename’
     Read the file indicated by ‘filename’ and incorporate or include
     its entries into the table.  Included files can include other
     files, which can include other files, etc.  For an example, see
     what files are included by the entry include ‘en-us-g1.ctb’ in the
     table ‘en-us-g2.ctb’.  If the included file is not in the same
     directory as the main table, use a full path name for filename.

‘locale characters’
     Not implemented, but recognized and ignored for backward
     compatibility.

‘undefined dots’
     If this opcode is used in a table any characters which have not
     been defined in the table but are encountered in the text will be
     replaced by the dot pattern.  If this opcode is not used, any
     undefined characters are replaced by ‘'\xhhhh'’, where the h’s are
     hexadecimal digits.

‘display character dots’
     Associates dot patterns with the characters which will be sent to a
     braille embosser, display or screen font.  The character must be in
     the range 0-255 and the dots must specify a single cell.  Here are
     some examples:

          # When the character a is sent to the embosser or display,
          # it will produce a dot 1.
          display a 1

          # When the character L is sent to the display or embosser
          # it will produce dots 1-2-3.
          display L 123

     The ‘display’ opcode is optional.  It is used when the embosser or
     display has a different mapping of characters to dot patterns than
     that given in *note Character-Definition Opcodes::.  If used,
     display entries must proceed character-definition entries.

     A possible use case would be to define display opcodes so that the
     result is Unicode braille for use on a display and a second set of
     display opcodes (in a different file) to produce plain ASCII
     braille for use with an embosser.

‘multind dots opcode opcode ...’
     The ‘multind’ opcode tells the back-translator that a sequence of
     braille cells represents more than one braille indicator.  For
     example, in ‘en-us-g1.ctb’ we have ‘multind 56-6 letsign capsign’.
     The back-translator can generally handle single braille indicators,
     but it cannot apply them when they immediately follow each other.
     It recognizes the letter sign if it is followed by a letter and
     takes appropriate action.  It also recognizes the capital sign if
     it is followed by a letter.  But when there is a letter sign
     followed by a capital sign it fails to recognize the letter sign
     unless the sequence has been defined with ‘multind’.  A ‘multind’
     entry may not contain a comment because liblouis would attempt to
     interpret it as an opcode.

3 Notes on Back-Translation
***************************

Back-translation is carried out by the function
‘lou_backTranslateString’.  Its calling sequence is described in *note
Programming with liblouis::.  Tables containing no ‘context’ opcode
(*note context: context opcode.), ‘correct’ opcode (*note correct:
correct opcode.) or multipass opcodes can be used for both forward and
backward translation.  If these opcodes are needed different tables will
be required.  ‘lou_backTranslateString’ first performs ‘pass4’, if
present, then ‘pass3’, then ‘pass2’, then the backtranslation, then
corrections.  Note that this is exactly the inverse of forward
translation.

4 Testing Translation Tables interactively
******************************************

A number of test programs are provided as part of the liblouis package.
They are intended for testing liblouis and for debugging tables.  None
of them is suitable for braille transcription.  An application that can
be used for transcription is ‘file2brl’, which is part of the
liblouisutdml package (*note Introduction: (liblouisutdml)Top.).  The
source code of the test programs can be studied to learn how to use the
liblouis library and they can be used to perform the following
functions.

   All of these programs recognize the ‘--help’ and ‘--version’ options.

‘--help’
‘-h’
     Print a usage message listing all available options, then exit
     successfully.

‘--version’
‘-v’
     Print the version number, then exit successfully.

4.1 lou_debug
=============

The ‘lou_debug’ tool is intended for debugging liblouis translation
tables.  The command line for ‘lou_debug’ is:

     lou_debug [OPTIONS] TABLE[,TABLE,...]

   The command line options that are accepted by ‘lou_debug’ are
described in *note common options::.

   The table (or comma-separated list of tables) is compiled.  If no
errors are found a brief command summary is printed, then the prompt
‘Command:’.  You can then input one of the command letters and get
output, as described below.

   Most of the commands print information in the various arrays of
‘TranslationTableHeader’.  Since these arrays are pointers to chains of
hashed items, the commands first print the hash number, then the first
item, then the next item chained to it, and so on.  After each item
there is a prompt indicated by ‘=>’.  You can then press enter (‘<RET>’)
to see the next item in the chain or the first item in the next chain.
Or you can press ‘h’ (for next-(h)ash) to skip to the next hash chain.
You can also press ‘e’ to exit the command and go back to the ‘command:’
prompt.

‘h’
     Brings up a screen of somewhat more extensive help.

‘f’
     Display the first forward-translation rule in the first non-empty
     hash bucket.  The number of the bucket is displayed at the
     beginning of the chain.  Each rule is identified by the word
     ‘Rule:’.  The fields are displayed by phrases consisting of the
     name of the field, an equal sign, and its value.  The before and
     after fields are displayed only if they are nonzero.  Special
     opcodes such as the ‘correct’ opcode (*note correct: correct
     opcode.) and the multipass opcodes are shown with the code that
     instructs the virtual machine that interprets them.  If you want to
     see only the rules for a particular character string you can type
     ‘p’ at the ‘command:’ prompt.  This will take you to the
     ‘particular:’ prompt, where you can press ‘f’ and then type in the
     string.  The whole hash chain containing the string will be
     displayed.

‘b’
     Display back-translation rules.  This display is very similar to
     that of forward translation rules except that the dot pattern is
     displayed before the character string.

‘c’
     Display character definitions, again within their hash chains.

‘d’
     Displays single-cell dot definitions.  If a character-definition
     opcode gives a multi-cell dot pattern, it is displayed among the
     back-translation rules.

‘C’
     Display the character-to-dots map.  This is set up by the
     character-definition opcodes and can also be influenced by the
     ‘display’ opcode (*note display: display opcode.).

‘D’
     Display the dot to character map, which shows which single-cell dot
     patterns map to which characters.

‘z’
     Show the multi-cell dot patterns which have been assigned to the
     characters from 0 to 255 to comply with computer braille codes such
     as a 6-dot code.  Note that the character-definition opcodes should
     use 8-dot computer braille.

‘p’
     Bring up a secondary (‘particular:’) prompt from which you can
     examine particular character strings, dot patterns, etc.  The
     commands (given in its own command summary) are very similar to
     those of the main ‘command:’ prompt, but you can type a character
     string or dot pattern.  They include ‘h’, ‘f’, ‘b’, ‘c’, ‘d’, ‘C’,
     ‘D’, ‘z’ and ‘x’ (to exit this prompt), but not ‘p’, ‘i’ and ‘m’.

‘i’
     Show braille indicators.  This shows the dot patterns for various
     opcodes such as the ‘capsign’ opcode (*note capsign: capsign
     opcode.) and the ‘numsign’ opcode (*note numsign: numsign opcode.).
     It also shows emphasis dot patterns, such as those for the
     ‘begemphword’ opcode (*note begemphword: begemphword opcode.), the
     ‘begemphphrase’ opcode (*note begemphphrase: begemphphrase
     opcode.), etc.  If a given opcode has not been used nothing is
     printed for it.

‘m’
     Display various miscellaneous information about the table, such as
     the number of passes, whether certain opcodes have been used, and
     whether there is a hyphenation table.

‘q’
     Exit the program.

4.2 lou_trace
=============

When working on translation tables it is sometimes useful to determine
what rules were applied when translating a string.  ‘lou_trace’ helps
with exactly that.  It list all the the applied rules for a given
translation table and an input string.

     lou_trace [OPTIONS] TABLE[,TABLE,...]

   Aside from the standard options (*note common options::) ‘lou_trace’
also accepts the following options:

‘--forward’
‘-f’
     Trace a forward translation.

‘--backward’
‘-b’
     Trace a backward translation.

   If no options are given forward translation is assumed.

   Once started you can type an input string followed by ‘<RET>’.
‘lou_trace’ will print the braille translation followed by list of rules
that were applied to produce the translation.  A possible invocation is
listed in the following example:

     $ lou_trace tables/en-us-g2.ctb
     the u.s. postal service
     ! u4s4 po/al s}vice
     1.      largesign       the     2346
     2.      repeated                0
     3.      lowercase       u       136
     4.      punctuation     .       46
     5.      context _$l["."]$l      @256
     6.      lowercase       s       234
     7.      postpunc        .       256
     8.      repeated                0
     9.      begword post    1234-135-34
     10.     largesign       a       1
     11.     lowercase       l       123
     12.     repeated                0
     13.     lowercase       s       234
     14.     always  er      12456
     15.     lowercase       v       1236
     16.     lowercase       i       24
     17.     lowercase       c       14
     18.     lowercase       e       15
     19.     pass2   $s1-10  @0
     20.     pass2   $s1-10  @0
     21.     pass2   $s1-10  @0

4.3 lou_checktable
==================

To use this program type the following:

     lou_checktable [OPTIONS] TABLE

   Aside from the standard options (*note common options::)
‘lou_checktable’ also accepts the following options:

‘--quiet’
‘-q’
     Do not write to standard error if there are no errors.

   If the table contains errors, appropriate messages will be displayed.
If there are no errors the message ‘no errors found.’ will be shown.

4.4 lou_allround
================

This program tests every capability of the liblouis library.  It is
completely interactive.  Invoke it as follows:

     lou_allround [OPTIONS]

   The command line options that are accepted by ‘lou_allround’ are
described in *note common options::.

   You will see a few lines telling you how to use the program.
Pressing one of the letters in parentheses and then enter will take you
to a message asking for more information or for the answer to a yes/no
question.  Typing the letter ‘r’ and then <RET> will take you to a
screen where you can enter a line to be processed by the library and
then view the results.

4.5 lou_translate
=================

This program translates whatever is on the standard input unit and
prints it on the standard output unit.  It is intended for large-scale
testing of the accuracy of translation and back-translation.  The
command line for ‘lou_translate’ is:

     lou_translate [OPTION] TABLE[,TABLE,...]

   Aside from the standard options (*note common options::) this program
also accepts the following options:

‘--forward’
‘-f’
     Do a forward translation.

‘--backward’
‘-b’
     Do a backward translation.

   To use it to translate or back-translate a file use a line like

     lou_translate --forward en-us-g2.ctb <liblouis.txt >testtrans

4.6 lou_checkhyphens
====================

This program checks the accuracy of hyphenation in Braille translation
for both translated and untranslated words.  It is completely
interactive.  Invoke it as follows:

     lou_checkhyphens [OPTIONS]

   The command line options that are accepted by ‘lou_checkhyphens’ are
described in *note common options::.

   You will see a few lines telling you how to use the program.

5 Automated Testing of Translation Tables
*****************************************

There are a number of automated tests for liblouis and they are proving
to be of tremendous value.  When changing the code the developers can
run the tests to see if anything broke.

   The easiest way to test the translation tables is to write a YAML
file where you define the table that is to be tested and any number of
words or phrases to translate together with their respective expected
translation.

   The YAML based tests replace older methods for testing the
translation tables which only work with Python and when compiled with
UCS4.  They are deprecated and will be removed in a future release.

YAML tests
     The YAML tests are data driven, i.e.  you give the test data, a
     string to translate and the expected output.  The data is in a
     standard format namely YAML. If you have ‘libyaml’ installed they
     will automatically be invoked as part of the standard ‘make check’
     command.

Doctests
     The doctests are based on a technology used in Python where you
     define your tests as if you were sitting at a terminal session with
     a Python interpreter.  They only work with either Python2 and
     Python3 but not from plain C, so you need Python and you will have
     to compile with UCS4.

5.1 YAML Tests
==============

YAML (http://yaml.org/) is a human readable data serialization format
that allows for an easy and compact way to define tests.

   A YAML file first defines which tables are to be used for the tests.
Then it optionally defines flags such as the ‘testmode’.  Finally all
the tests are defined.

   Let’s just look at a simple example how tests could be defined:

     # comments start with '#' anywhere on a line
     # first define which tables will be used for your tests
     tables: [unicode.dis, en-ueb-g1.ctb]

     # then optionally define flags such as testmode. If no flags are
     # defined forward translation is assumed

     # now define the tests
     tests:
       - # each test is a list.
         # The first item is the string to translate. Quoting of strings is
         # optional
         - hello
         # The second item is the expected translation
         - ⠓⠑⠇⠇⠕
       - # optionally you can define additional parameters in a third
         # item such as typeform or expected failure, etc
         - Hello
         - ⠨⠶⠠⠓⠑⠇⠇⠕⠨⠄
         - {typeform: {italic: '++++ '}, xfail: true}
       - # a simple, no-frills test
         - Good bye
         - ⠠⠛⠕⠕⠙ ⠃⠽⠑
       # same as above using "flow style" notation
       - [Good bye,  ⠠⠛⠕⠕⠙ ⠃⠽⠑]

   The three basic components of a test file are as follows:

‘tables’
     A list containing table names, which the tests should be run
     against.  This is usually just one table, but for some situations
     more than one table can be required.

     To test the ‘en-ueb-g1.ctb’ table using unicode braille you could
     use the following definition:

          tables: [unicode.dis, en-ueb-g1.ctb]

     If you wanted to test the ‘eo-g1.ctb’ table using brf notation then
     you would use the following definition:

          tables: [en-us-brf.dis, eo-g1.ctb]

‘flags’
     The flags that apply for all tests in this file.  At the moment
     only the ‘testmode’ flag is supported.  It can have three possible
     values:

     ‘forward’
          This indicates that the tests are for forward translation
     ‘backward’
          This indicates that the tests are for backward translation
     ‘hyphenate’
          This indicates that the tests are for hyphenation

     If no flags are defined forward translation is assumed.

‘tests’
     A list of tests.  Each test consists of a list of two, three or in
     some cases even four items.  The first item is the unicode text to
     be tested.  The second item is the expected braille output.  This
     can be either unicode braille or an ASCII-braille like encoding.
     Quoting strings is optional.  Comments can be inserted almost
     anywhere using the ‘#’ sign.  A simple test would look at follows:

            - # a simple, no-frills test
              - Good bye
              - ⠠⠛⠕⠕⠙ ⠃⠽⠑

     Using the more compact “flow style” notation it would look like the
     following:

            - [Good bye, ⠠⠛⠕⠕⠙ ⠃⠽⠑]

     An optional third item can contain additional options for a test
     such as the typeform, or whether a test is expected to fail.  The
     following shows a typical example:

            -
              - Hello
              - ⠨⠶⠠⠓⠑⠇⠇⠕⠨⠄
              - {typeform: {italic: '++++ '}, xfail: true}
            # same test more compact
            - [Hello, ⠨⠶⠠⠓⠑⠇⠇⠕⠨⠄, {typeform: {italic: '++++ '}, xfail: true}]

     The valid additional options for a test are as follows:

     ‘xfail’
          Whether a test is expected to fail.  If you expect a test to
          fail, set this to ‘true’.  If you prefer you can also specify
          a reason for the failure:

                 - [Hello, ⠨, {xfail: Test case is not complete}]

          If you expect a test case to pass then just don’t mark it with
          ‘xfail’ or if you really have to, set ‘xfail’ to ‘false’ or
          ‘off’.

     ‘typeform’
          The typeform used for a translation.  It consists of one or
          more emphasis specifications.  For each character in the
          specifications that is not a space the corresponding emphasis
          will be set.  Valid options for emphasis are ‘italic’,
          ‘underline’, ‘bold’, ‘computer_braille’, ‘passage_break’,
          ‘word_reset’, ‘script’, ‘trans_note’, ‘trans_note_1’,
          ‘trans_note_2’, ‘trans_note_3’, ‘trans_note_4’ or
          ‘trans_note_5’.  The following shows an example where both
          ‘italic’ and ‘underline’ are specified:

                 -
                   - Hello
                   - ⠨⠶⠠⠓⠑⠇⠇⠕⠨⠄
                   - typeform:
                       italic:    '++++ '
                       underline: '    +'

     ‘cursorPos’
          A list of cursor positions for each input position.  Useful
          when simulating screen reader interaction, to debug
          contraction and cursor behavior as in the following example
          from the ‘en-GB-g2.ctb’ test.  Also note that in this example
          the additional options are not passed in using the “flow
          style” notation.

                 -
                   - you went to
                   - ⠽ ⠺⠑⠝⠞ ⠞⠕
                   - mode: [compbrlAtCursor]
                     cursorPos: [0,1,2,3,4,5,6,7,8,9,10]

     ‘mode’
          A list of translation modes that should be used for this test.
          If not defined defaults to 0.  Valid mode values are
          ‘noContractions’, ‘compbrlAtCursor’, ‘dotsIO’, ‘comp8Dots’,
          ‘pass1Only’, ‘compbrlLeftCursor’, ‘otherTrans’or ‘ucBrl’.

5.1.1 Optional test description
-------------------------------

When a test contains three or four items the first item is assumed to be
a test description, the second item is the unicode text to be tested and
the third item is the expected braille output.  Again an optional fourth
item can contain additional options for the test.  The following shows
an example:

       -
         - Number-text-transitions with italic
         - 123abc
         - ⠼⠁⠃⠉⠨⠶⠰⠁⠃⠉⠨⠄
         - {typeform: '000111'}

   In case the test fails the description will be printed together with
the expected and the actual braille output.

   For more examples and inspiration please see the YAML tests
(‘*.yaml’) in the ‘tests’ directory of the source distribution.

5.2 Doctests
============

A doctest looks like you typed a command at the Python command line and
got some output, e.g.

     >>> translate(['table.ctb'], "Hello", mode=compbrlLeftCursor)
     ("HELLO", [0,1,2,3], [0,1,2,3], 0)

   There is a convenience wrapper which hides away much of the
complexity of above example so you can write stuff like

     >>> t.braille('the cat sat on the mat')
     u'! cat sat on ! mat'

   Essentially you are writing code, so the doctests allow you to do
more flexible tests that are much closer to the raw iron than any of the
other tests.  However the doctests will only work in either Python2 or
Python3.

   For examples on how to create doctests please see ‘*_test.txt’ in the
doctest directory in the source distribution.

6 Programming with liblouis
***************************

6.1 Overview
============

You use the liblouis library by calling the following functions,
‘lou_translateString’, ‘lou_backTranslateString’, ‘lou_translate’,
‘lou_backTranslate’, ‘lou_registerLogCallback’, ‘lou_setLogLevel’,
‘lou_logFile’, ‘lou_logPrint’, ‘lou_logEnd’, ‘lou_getTable’,
‘lou_checkTable’, ‘lou_hyphenate’, ‘lou_charToDots’, ‘lou_dotsToChar’,
‘lou_compileString’, ‘lou_getTypeformForEmphClass’,
‘lou_readCharFromFile’, ‘lou_version’ and ‘lou_free’.  These are
described below.  The header file, ‘liblouis.h’, also contains brief
descriptions.  Liblouis is written in straight C. It has four code
modules, ‘compileTranslationTable.c’, ‘logging.c’,
‘lou_translateString.c’ and ‘lou_backTranslateString.c’.  In addition,
there are two header files, ‘liblouis.h’, which defines the API, and
‘louis.h’, used only internally and by liblouisutdml.  The latter
includes ‘liblouis.h’.

   Persons who wish to use liblouis from Python may want to skip ahead
to *note Python bindings::.

   ‘compileTranslationTable.c’ keeps track of all translation tables
which an application has used.  It is called by the translation,
hyphenation and checking functions when they start.  If a table has not
yet been compiled ‘compileTranslationTable.c’ checks it for correctness
and compiles it into an efficient internal representation.  The main
entry point is ‘lou_getTable’.  Since it is the module that keeps track
of memory usage, it also contains the ‘lou_free’ function.  In addition,
it contains the ‘lou_checkTable’ function, plus some utility functions
which are used by the other modules.

   By default, liblouis handles all characters internally as 16-bit
unsigned integers.  It can be compiled for 32-bit characters as
explained below.  The meanings of these integers are not hard-coded.
Rather they are defined by the character-definition opcodes.  However,
the standard printable characters, from decimal 32 to 126 are recognized
for the purpose of processing the opcodes.  Hence, the following
definition is included in ‘liblouis.h’.  It is correct for computers
with at least 32-bit processors.

     #define widechar unsigned short int

   To make liblouis handle 32-bit Unicode simply remove the word ‘short’
in the above ‘define’.  This will cause the translate and back-translate
functions to expect input in 32-bit form and to deliver their output in
this form.  The input to the compiler (tables) is unaffected except that
two new escape sequences for 20-bit and 32-bit characters are
recognized.

   Here are the definitions of the eleven liblouis functions and their
parameters.  They are given in terms of 16-bit Unicode.  If liblouis has
been compiled for 32-bit Unicode simply read 32 instead of 16.

6.2 Data structure of liblouis tables
=====================================

The data structure ‘TranslationTableHeader’ is defined by a ‘typedef’
statement in ‘louis.h’.  To find the beginning, search for the word
‘header’.  As its name implies, this is actually the table header.  Data
are placed in the ‘ruleArea’ array, which is the last item defined in
this structure.  This array is declared with a length of 1 and is
expanded as needed.  The table header consists mostly of arrays of
pointers of size ‘HASHNUM’.  These pointers are actually offsets into
‘ruleArea’ and point to chains of items which have been placed in the
same hash bucket by a simple hashing algorithm.  ‘HASHNUM’ should be a
prime and is currently 1123.  The structure of the table was chosen to
optimize speed rather than memory usage.

   The first part of the table contains miscellaneous information, such
as the number of passes and whether various opcodes have been used.  It
also contains the amount of memory allocated to the table and the amount
actually used.

   The next section contains pointers to various braille indicators and
begins with ‘capitalSign’.  The rules pointed to contain the dot pattern
for the indicator and an opcode which is used by the back-translator but
does not appear in the list of opcodes.  The braille indicators also
include various kinds of emphasis, such as italic and bold and
information about the length of emphasized phrases.  The latter is
contained directly in the table item instead of in a rule.

   After the braille indicators comes information about when a letter
sign should be used.

   Next is an array of size ‘HASHNUM’ which points to character
definitions.  These are created by the character-definition opcodes.

   Following this is a similar array pointing to definitions of
single-cell dot patterns.  This is also created from the
character-definition opcodes.  If a character definition contains a
multi-cell dot pattern this is compiled into ordinary forward and
backward rules.  If such a multi-cell dot pattern contains a single cell
which has not previously been defined that cell is placed in this array,
but is given the attribute ‘space’.

   Next come arrays that map characters to single-cell dot patterns and
dots to characters.  These are created from both character-definition
opcodes and display opcodes.

   Next is an array of size 256 which maps characters in this range to
dot patterns which may consist of multiple cells.  It is used, for
example, to map ‘{’ to dots 456-246.  These mappings are created by the
‘compdots’ or the ‘comp6’ opcode (*note comp6: comp6 opcode.).

   Next are two small arrays that held pointers to chains of rules
produced by the ‘swapcd’ opcode (*note swapcd: swapcd opcode.) and the
‘swapdd’ opcode (*note swapdd: swapdd opcode.) and by some multipass,
‘context’ and ‘correct’ opcodes.

   Now we get to an array of size ‘HASHNUM’ which points to chains of
rules for forward translation.

   Following this is a similar array for back-translation.

   Finally is the ‘ruleArea’, an array of variable size to which various
structures are mapped and to which almost everything else points.

6.3 lou_version
===============

     char *lou_version ()

   This function returns a pointer to a character string containing the
version of liblouis, plus other information, such as the release date
and perhaps notable changes.

6.4 lou_translateString
=======================

     int lou_translateString(
       const char *tableList,
       const widechar *inbuf,
       int *inlen,
       widechar *outbuf,
       int *outlen,
       formtype *typeform,
       char *spacing,
       int mode);

   This function takes a string of 16-bit Unicode characters in ‘inbuf’
and translates it into a string of 16-bit characters in ‘outbuf’.  Each
16-bit character produces a particular dot pattern in one braille cell
when sent to an embosser or braille display or to a screen type font.
Which 16-bit character represents which dot pattern is indicated by the
character-definition and display opcodes in the translation table.

   The ‘tableList’ parameter points to a list of translation tables
separated by commas.  If only one table is given, no comma should be
used after it.  It is these tables which control just how the
translation is made, whether in Grade 2, Grade 1, or something else.

   liblouis knows where to find all the tables that have been
distributed with it.  So you can just give a table name such as
‘en-us-g2.ctb’ and liblouis will load it.  You can also give a table
name which includes a path.  If this is the first table in a list, all
the tables in the list must be on the same path.  You can specify a path
on which liblouis will look for table names by setting the environment
variable ‘LOUIS_TABLEPATH’.  This environment variable can contain one
or more paths separated by commas.  On receiving a table name liblouis
first checks to see if it can be found on any of these paths.  If not,
it then checks to see if it can be found in the current directory, or,
if the first (or only) name in a table list, if it contains a path name,
can be found on that path.  If not, it checks to see if it can be found
on the path where the distributed tables have been installed.  If a
table has already been loaded and compiled this path-checking is
skipped.

   The tables in a list are all compiled into the same internal table.
The list is then regarded as the name of this table.  As explained in
*note How to Write Translation Tables::, each table is a file which may
be plain text, big-endian Unicode or little-endian Unicode.  A table (or
list of tables) is compiled into an internal representation the first
time it is used.  Liblouis keeps track of which tables have been
compiled.  For this reason, it is essential to call the ‘lou_free’
function at the end of your application to avoid memory leaks.  Do _NOT_
call ‘lou_free’ after each translation.  This will force liblouis to
compile the translation tables each time they are used, leading to great
inefficiency.

   Note that both the ‘*inlen’ and ‘*outlen’ parameters are pointers to
integers.  When the function is called, these integers contain the
maximum input and output lengths, respectively.  When it returns, they
are set to the actual lengths used.

   The ‘typeform’ parameter is used to indicate italic type, boldface
type, computer braille, etc.  It is an array of ‘formtype’ with the same
length as the input buffer pointed to by ‘*inbuf’.  However, it is used
to pass back character-by-character results, so enough space must be
provided to match the ‘*outlen’ parameter.  Each element indicates the
typeform of the corresponding character in the input buffer.  The values
and their meaning can be consulted in the ‘typeforms’ enum in
‘liblouis.h’.  These values can be added for multiple emphasis.  If this
parameter is ‘NULL’, no checking for type forms is done.  In addition,
if this parameter is not ‘NULL’, it is set on return to have an 8 at
every position corresponding to a character in ‘outbuf’ which was
defined to have a dot representation containing dot 7, dot 8 or both,
and to 0 otherwise.

   The ‘spacing’ parameter is used to indicate differences in spacing
between the input string and the translated output string.  It is also
of the same length as the string pointed to by ‘*inbuf’.  If this
parameter is ‘NULL’, no spacing information is computed.

   The ‘mode’ parameter specifies how the translation should be done.
The valid values of mode are listed in ‘liblouis.h’.  They are all
powers of 2, so that a combined mode can be specified by adding up
different values.

   The function returns 1 if no errors were encountered and 0 if a
complete translation could not be done.

6.5 lou_translate
=================

     int lou_translate(
       const char *tableList,
       const widechar *inbuf,
       int *inlen,
       widechar *outbuf,
       int *outlen,
       formtype *typeform,
       char *spacing,
       int *outputPos,
       int *inputPos,
       int *cursorPos,
       int mode);

   This function adds the parameters ‘outputPos’, ‘inputPos’ and
‘cursorPos’, to facilitate use in screen reader programs.  The
‘outputPos’ parameter must point to an array of integers with at least
‘inlen’ elements.  On return, this array will contain the position in
‘outbuf’ corresponding to each input position.  Similarly, ‘inputPos’
must point to an array of integers of at least ‘outlen’ elements.  On
return, this array will contain the position in ‘inbuf’ corresponding to
each position in ‘outbuf’.  ‘cursorPos’ must point to an integer
containing the position of the cursor in the input.  On return, it will
contain the cursor position in the output.  Any parameter after ‘outlen’
may be ‘NULL’.  In this case, the actions corresponding to it will not
be carried out.  The ‘mode’ parameter, however, must be present and must
be an integer, not a pointer to an integer.  If the ‘compbrlAtCursor’
bit is set in the ‘mode’ parameter the space-bounded characters
containing the cursor will be translated in computer braille.  If the
‘compbrlLeftCursor’ bit is set only the characters to the left of the
cursor will be in computer braille.  This bit overrides
‘compbrlAtCursor’.  When the ‘dotsIO’ bit is set, during translation,
produce output as dot patterns.  During back-translation accept input as
dot patterns.  Note that the produced dot patterns are affected if you
have any ‘display’ opcode (*note display: display opcode.) defined in
any of your tables.  The ‘ucBrl’ (Unicode Braille) bit is used by
‘lou_charToDots’ and ‘lou_translate’.  It causes the dot patterns to be
Unicode Braille rather than the liblouis representation.  Note that you
will not notice any change when setting ‘ucBrl’ unless ‘dotsIO’ is also
set.  ‘lou_dotsToChar’ and ‘lou_backTranslate’ recognize Unicode braille
automatically.

   The ‘otherTrans’ mode needs special description.  If it is set
liblouis will attempt to call a wrapper for another translator.  These
other translators are usually for Asian languages.  The calling sequence
is the same as for liblouis itself except that the ‘trantab’ parameter
gives the name of the other translator, possibly abbreviated, followed
by a colon, followed by whatever other information the other translator
needs.  This is specific for each translator.  If no such information is
needed the colon should be omitted.  The result of calling either the
translate or back-translate functions with this mode bit set will be the
same as calling without it set.  That is, the wrapper for the other
translator simulates a call to liblouis.  Note that the wrappers are not
implemented at this time.  Setting this mode bit will result in failure
(return value of 0).

6.6 lou_backTranslateString
===========================

     int lou_backTranslateString(
       const char *tableList,
       const widechar *inbuf,
       int *inlen,
       widechar *outbuf,
       int *outlen,
       formtype *typeform,
       char *spacing,
       int mode);

   This is exactly the opposite of ‘lou_translateString’.  ‘inbuf’ is a
string of 16-bit Unicode characters representing braille.  ‘outbuf’ will
contain a string of 16–bit Unicode characters.  ‘typeform’ will indicate
any emphasis found in the input string, while ‘spacing’ will indicate
any differences in spacing between the input and output strings.  The
‘typeform’ and ‘spacing’ parameters may be ‘NULL’ if this information is
not needed.  ‘mode’ again specifies how the back-translation should be
done.

6.7 lou_backTranslate
=====================

     int lou_backTranslate(
       const char *tableList,
       const widechar *inbuf,
       int *inlen,
       widechar *outbuf,
       int *outlen,
       formtype *typeform,
       char *spacing,
       int *outputPos,
       int *inputPos,
       int *cursorPos,
       int mode);

   This function is exactly the inverse of ‘lou_translate’.

6.8 lou_hyphenate
=================

     int lou_hyphenate (
       const char *tableList,
       const widechar *inbuf,
       int inlen,
       char *hyphens,
       int mode);

   This function looks at the characters in ‘inbuf’ and if it finds a
sequence of letters attempts to hyphenate it as a word.  Note that
lou_hyphenate operates on single words only, and spaces or punctuation
marks between letters are not allowed.  Leading and trailing punctuation
marks are ignored.  The table named by the ‘tableList’ parameter must
contain a hyphenation table.  If it does not, the function does nothing.
‘inlen’ is the length of the character string in ‘inbuf’.  ‘hyphens’ is
an array of characters and must be of size ‘inlen’ + 1 (to account for
the NULL terminator).  If hyphenation is successful it will have a 1 at
the beginning of each syllable and a 0 elsewhere.  If the ‘mode’
parameter is 0 ‘inbuf’ is assumed to contain untranslated characters.
Any nonzero value means that ‘inbuf’ contains a translation.  In this
case, it is back-translated, hyphenation is performed, and it is
re-translated so that the hyphens can be placed correctly.  The
‘lou_translate’ and ‘lou_backTranslate’ functions are used in this
process.  ‘lou_hyphenate’ returns 1 if hyphenation was successful and 0
otherwise.  In the latter case, the contents of the ‘hyphens’ parameter
are undefined.  This function was provided for use in liblouisutdml.

6.9 lou_compileString
=====================

     int lou_compileString (const char *tableList, const char *inString)

   This function enables you to compile a table entry on the fly at
run-time.  The new entry is added to ‘tableList’ and remains in force
until ‘lou_free’ is called.  If ‘tableList’ has not previously been
loaded it is loaded and compiled.  ‘inString’ contains the table entry
to be added.  It may be anything valid.  Error messages will be produced
if it is invalid.  The function returns 1 on success and 0 on failure.

6.10 lou_getTypeformForEmphClass
================================

     int lou_getTypeformForEmphClass (const char *tableList, const char *emphClass);

   This function returns the typeform bit associated with the given
emphasis class.  If the emphasis class is undefined this function
returns ‘0’.  If errors are found error messages are logged to the log
callback (see ‘lou_registerLogCallback’) and the return value is ‘0’.
‘tableList’ is a list of names of table files separated by commas, as
explained previously (*note ‘tableList’ parameter in
‘lou_translateString’: translation-tables.).  ‘emphClass’ is the name of
an emphasis class.

6.11 lou_dotsToChar
===================

     int lou_dotsToChar (
       const char *tableList,
       const widechar *inbuf,
       widechar *outbuf,
       int length,
       int mode)

   This function takes a widechar string in ‘inbuf’ consisting of dot
patterns and converts it to a widechar string in ‘outbuf’ consisting of
characters according to the specifications in ‘tableList’.  ‘length’ is
the length of both ‘inbuf’ and ‘outbuf’.  The dot patterns in ‘inbuf’
can be in either liblouis format or Unicode braille.  The function
returns 1 on success and 0 on failure.

6.12 lou_charToDots
===================

     int lou_charToDots (
       const char *tableList,
       const widechar *inbuf,
       widechar *outbuf,
       int length,
       int mode)

   This function is the inverse of ‘lou_dotsToChar’.  It takes a
widechar string in ‘inbuf’ consisting of characters and converts it to a
widechar string in ‘outbuf’ consisting of dot patterns according to the
specifications in ‘tableList’.  ‘length’ is the length of both ‘inbuf’
and ‘outbuf’.  The dot patterns in ‘outbufbuf’ are in liblouis format if
the mode bit ‘ucBrl’ is not set and in Unicode format if it is set.  The
function returns 1 on success and 0 on failure.

6.13 lou_registerLogCallback
============================

     typedef void (*logcallback) (
       int level,
       const char *message);

     void lou_registerLogCallback (
       logcallback callback);

   This function can be used to register a custom logging callback.  The
callback must take a single argument, the message string.  By default
log messages are printed to stderr, or if a filename was specified with
‘lou_logFile’ then messages are logged to that file.
‘lou_registerLogCallback’ overrides the default callback.  Passing
‘NULL’ resets to the default callback.

6.14 lou_setLogLevel
====================

     typedef enum
     {
       LOG_ALL = 0,
       LOG_DEBUG = 10000,
       LOG_INFO = 20000,
       LOG_WARN = 30000,
       LOG_ERROR = 40000,
       LOG_FATAL = 50000,
       LOG_OFF = 6000
     } logLevels;
     void lou_setLogLevel (
       logLevels level);

   This function can be used to influence the amount of logging, from
fatal error messages only to detailed debugging messages.  Supported
values are ‘LOG_DEBUG’, ‘LOG_INFO’, ‘LOG_WARN’, ‘LOG_ERROR’, ‘LOG_FATAL’
and ‘LOG_OFF’.  Enabling logging at a given level also enables logging
at all higher levels.  Setting the level to ‘LOG_OFF’ disables logging.
The default level is ‘LOG_INFO’.

6.15 lou_logFile
================

     void lou_logFile (
       char *fileName);

   This function is used when it is not convenient either to let
messages be printed on stderr or to use redirection, as when liblouis is
used in a GUI application or in liblouisutdml.  Any error messages
generated will be printed to the file given in this call.  The entire
path name of the file must be given.

6.16 lou_logPrint (deprecated)
==============================

     void lou_logPrint (
       char *format,
       ...);

   This function is called like ‘fprint’.  It can be used by other
libraries to print messages to the file specified by the call to
‘lou_logFile’.  In particular, it is used by the companion library
liblouisutdml.

   This function is deprecated as of version 2.6.0.

6.17 lou_logEnd
===============

     lou_logEnd ();

   This function is used at the end of processing a document to close
the log file, so that it can be read by the rest of the program.

6.18 lou_setDataPath
====================

     char *lou_setDataPath (
       char *path);

   This function is used to tell liblouis and liblouisutdml where tables
and files are located.  It thus makes them completely relocatable, even
on Linux.  The ‘path’ is the directory where the subdirectories
‘liblouis/tables’ and ‘liblouisutdml/lbu_files’ are rooted or located.
The function returns a pointer to the ‘path’.

6.19 lou_getDataPath
====================

     char *lou_getDataPath ();

   This function returns a pointer to the path set by ‘lou_setDataPath’.
If no path has been set it returns ‘NULL’.

6.20 lou_getTable
=================

     void *lou_getTable (
       char *tableList);

   ‘tableList’ is a list of names of table files separated by commas, as
explained previously (*note ‘tableList’ parameter in
‘lou_translateString’: translation-tables.).  If no errors are found
this function returns a pointer to the compiled table.  If errors are
found error messages are logged to the log callback (see
‘lou_registerLogCallback’).  Errors result in a ‘NULL’ pointer being
returned.

6.21 lou_checkTable
===================

     int lou_checkTable (const char *tableList);

   This function does the same as ‘lou_getTable’ but does not return a
pointer to the resulting table.  It is to be preferred if only the
validity of a table needs to be checked.  ‘tableList’ is a list of names
of table files separated by commas, as explained previously (*note
‘tableList’ parameter in ‘lou_translateString’: translation-tables.).
If no errors are found this function returns a non-zero.  If errors are
found error messages are logged to the log callback (see
‘lou_registerLogCallback’) and the return value is ‘0’.

6.22 lou_readCharFromFile
=========================

     int lou_readCharFromFile (
       const char *fileName,
       int *mode);

   This function is provided for situations where it is necessary to
read a file which may contain little-endian or big-endian 16-bit Unicode
characters or ASCII8 characters.  The return value is a little-endian
character, encoded as an integer.  The ‘fileName’ parameter is the name
of the file to be read.  The ‘mode’ parameter is a pointer to an integer
which must be set to 1 on the first call.  After that, the function
takes care of it.  On end-of-file the function returns ‘EOF’.

6.23 lou_free
=============

     void lou_free ();

   This function should be called at the end of the application to free
all memory allocated by liblouis.  Failure to do so will result in
memory leaks.  Do _NOT_ call ‘lou_free’ after each translation.  This
will force liblouis to compile the translation tables every time they
are used, resulting in great inefficiency.

6.24 Python bindings
====================

There are Python bindings for ‘lou_translateString’, ‘lou_translate’,
‘lou_backTranslateString’, ‘lou_backTranslate’, ‘lou_hyphenate’,
‘checkTable’, ‘lou_compileString’ and ‘lou_version’.  For installation
instructions see the the ‘README’ file in the ‘python’ directory.  Usage
information is included in the Python module itself.

Concept Index
*************

* Menu:

* characters operand:                    Overview.            (line 234)
* dots operand:                          Overview.            (line 266)
* multi-cell dot pattern:                Overview.            (line 274)
* opcode:                                How to Write Translation Tables.
                                                              (line 152)
* virtual dots:                          Overview.            (line 269)

Opcode Index
************

* Menu:

* after:                                 Character-Class Opcodes.
                                                             (line 1018)
* always:                                Translation Opcodes.
                                                             (line  803)
* before:                                Character-Class Opcodes.
                                                             (line 1025)
* begcaps:                               Braille Indicator Opcodes.
                                                             (line  465)
* begemph:                               None.               (line  573)
* begemphphrase:                         Phrase.             (line  630)
* begemphword:                           Word.               (line  603)
* begmidword:                            Translation Opcodes.
                                                             (line  909)
* begnum:                                Translation Opcodes.
                                                             (line  950)
* begword:                               Translation Opcodes.
                                                             (line  905)
* capsign:                               Braille Indicator Opcodes.
                                                             (line  459)
* capsnocont:                            Special Processing Opcodes.
                                                             (line  714)
* class:                                 Character-Class Opcodes.
                                                             (line 1013)
* comp6:                                 Translation Opcodes.
                                                             (line  776)
* compbrl:                               Translation Opcodes.
                                                             (line  766)
* context:                               The Context and Multipass Opcodes.
                                                             (line 1076)
* contraction:                           Translation Opcodes.
                                                             (line  883)
* correct:                               The correct Opcode. (line 1251)
* decpoint:                              Special Symbol Opcodes.
                                                             (line  698)
* digit:                                 Character-Definition Opcodes.
                                                             (line  373)
* display:                               Miscellaneous Opcodes.
                                                             (line 1293)
* emphclass:                             Emphasis class.     (line  550)
* emphletter:                            Letter.             (line  588)
* endcaps:                               Braille Indicator Opcodes.
                                                             (line  471)
* endemph:                               None.               (line  578)
* endemphphrase after:                   Phrase.             (line  641)
* endemphphrase before:                  Phrase.             (line  635)
* endemphword:                           Word.               (line  609)
* endnum:                                Translation Opcodes.
                                                             (line  961)
* endword:                               Translation Opcodes.
                                                             (line  921)
* exactdots:                             Translation Opcodes.
                                                             (line  930)
* grouping:                              Character-Definition Opcodes.
                                                             (line  396)
* hyphen:                                Special Symbol Opcodes.
                                                             (line  704)
* include:                               Miscellaneous Opcodes.
                                                             (line 1274)
* joinnum:                               Translation Opcodes.
                                                             (line  966)
* joinword:                              Translation Opcodes.
                                                             (line  865)
* largesign:                             Translation Opcodes.
                                                             (line  828)
* lenemphphrase:                         Phrase.             (line  649)
* letsign:                               Braille Indicator Opcodes.
                                                             (line  477)
* letter:                                Character-Definition Opcodes.
                                                             (line  412)
* litdigit:                              Character-Definition Opcodes.
                                                             (line  429)
* locale:                                Miscellaneous Opcodes.
                                                             (line 1282)
* lowercase:                             Character-Definition Opcodes.
                                                             (line  417)
* lowword:                               Translation Opcodes.
                                                             (line  874)
* math:                                  Character-Definition Opcodes.
                                                             (line  444)
* midendword:                            Translation Opcodes.
                                                             (line  917)
* midnum:                                Translation Opcodes.
                                                             (line  955)
* midword:                               Translation Opcodes.
                                                             (line  913)
* multind:                               Miscellaneous Opcodes.
                                                             (line 1317)
* noback:                                Translation Opcodes.
                                                             (line  754)
* nocont:                                Translation Opcodes.
                                                             (line  786)
* nocontractsign:                        Braille Indicator Opcodes.
                                                             (line  511)
* nocross:                               Translation Opcodes.
                                                             (line  855)
* nofor:                                 Translation Opcodes.
                                                             (line  761)
* noletsign:                             Braille Indicator Opcodes.
                                                             (line  485)
* noletsignafter:                        Braille Indicator Opcodes.
                                                             (line  503)
* noletsignbefore:                       Braille Indicator Opcodes.
                                                             (line  495)
* numsign:                               Braille Indicator Opcodes.
                                                             (line  517)
* partword:                              Translation Opcodes.
                                                             (line  925)
* pass2:                                 The Context and Multipass Opcodes.
                                                             (line 1076)
* pass3:                                 The Context and Multipass Opcodes.
                                                             (line 1076)
* pass4:                                 The Context and Multipass Opcodes.
                                                             (line 1076)
* postpunc:                              Translation Opcodes.
                                                             (line  946)
* prepunc:                               Translation Opcodes.
                                                             (line  942)
* prfword:                               Translation Opcodes.
                                                             (line  901)
* punctuation:                           Character-Definition Opcodes.
                                                             (line  366)
* repeated:                              Translation Opcodes.
                                                             (line  811)
* replace:                               Translation Opcodes.
                                                             (line  793)
* repword:                               Translation Opcodes.
                                                             (line  819)
* sign:                                  Character-Definition Opcodes.
                                                             (line  436)
* space:                                 Character-Definition Opcodes.
                                                             (line  360)
* sufword:                               Translation Opcodes.
                                                             (line  897)
* swapcc:                                Swap Opcodes.       (line 1056)
* swapcd:                                Swap Opcodes.       (line 1044)
* swapdd:                                Swap Opcodes.       (line 1049)
* syllable:                              Translation Opcodes.
                                                             (line  843)
* undefined:                             Miscellaneous Opcodes.
                                                             (line 1286)
* uplow:                                 Character-Definition Opcodes.
                                                             (line  379)
* uppercase:                             Character-Definition Opcodes.
                                                             (line  422)
* word:                                  Translation Opcodes.
                                                             (line  839)

Function Index
**************

* Menu:

* lou_backTranslate:                     lou_backTranslate.  (line 2113)
* lou_backTranslateString:               lou_backTranslateString.
                                                             (line 2091)
* lou_charToDots:                        lou_charToDots.     (line 2203)
* lou_checkTable:                        lou_checkTable.     (line 2329)
* lou_compileString:                     lou_compileString.  (line 2160)
* lou_dotsToChar:                        lou_dotsToChar.     (line 2186)
* lou_free:                              lou_free.           (line 2358)
* lou_getDataPath:                       lou_getDataPath.    (line 2307)
* lou_getTable:                          lou_getTable.       (line 2315)
* lou_getTypeformForEmphClass:           lou_getTypeformForEmphClass.
                                                             (line 2172)
* lou_hyphenate:                         lou_hyphenate.      (line 2131)
* lou_logEnd:                            lou_logEnd.         (line 2287)
* lou_logFile:                           lou_logFile.        (line 2261)
* lou_logPrint:                          lou_logPrint.       (line 2273)
* lou_readCharFromFile:                  lou_readCharFromFile.
                                                             (line 2343)
* lou_registerLogCallback:               lou_registerLogCallback.
                                                             (line 2221)
* lou_setDataPath:                       lou_setDataPath.    (line 2295)
* lou_setLogLevel:                       lou_setLogLevel.    (line 2238)
* lou_translate:                         lou_translate.      (line 2033)
* lou_translateString:                   lou_translateString.
                                                             (line 1948)
* lou_version:                           lou_version.        (line 1939)

Program Index
*************

* Menu:

* lou_allround:                          lou_allround.       (line 1540)
* lou_checkhyphens:                      lou_checkhyphens.   (line 1583)
* lou_checktable:                        lou_checktable.     (line 1523)
* lou_debug:                             lou_debug.          (line 1371)
* lou_trace:                             lou_trace.          (line 1470)
* lou_translate:                         lou_translate (program).
                                                             (line 1558)

