Table of Contents
*****************

Liblouis User's and Programmer's Manual
1 Introduction
2 Test Programs
  2.1 lou_debug
  2.2 lou_checktable
  2.3 lou_allround
  2.4 lou_translate
  2.5 lou_checkhyphens
3 How to Write Translation Tables
  3.1 Hyphenation Tables
  3.2 Character-Definition Opcodes
  3.3 Braille Indicator Opcodes
  3.4 Emphasis Opcodes
  3.5 Special Symbol Opcodes
  3.6 Special Processing Opcodes
  3.7 Translation Opcodes
  3.8 Character-Class Opcodes
  3.9 Swap Opcodes
  3.10 The Context and Multipass Opcodes
  3.11 The correct Opcode
  3.12 Miscellaneous Opcodes
4 Notes on Back-Translation
5 Programming with liblouis
  5.1 License
  5.2 Overview
  5.3 Data structure of liblouis tables
  5.4 lou_version
  5.5 lou_translateString
  5.6 lou_translate
  5.7 lou_backTranslateString
  5.8 lou_backTranslate
  5.9 lou_hyphenate
  5.10 lou_compileString
  5.11 lou_dotsToChar
  5.12 lou_charToDots
  5.13 lou_logFile
  5.14 lou_logPrint
  5.15 lou_getTable
  5.16 lou_readCharFromFile
  5.17 lou_free
  5.18 Python bindings
Opcode Index
Function Index
Program Index


Liblouis User's and Programmer's Manual
***************************************

This manual is for liblouis (version 2.1.1, 27 August 2010), a Braille
Translation and Back-Translation Library derived from the Linux
screenreader BRLTTY.

Copyright (C) 1999-2006 by the BRLTTY Team.

Copyright (C) 2004-2007 ViewPlus Technologies, Inc.  `www.viewplus.com'.

Copyright (C) 2007,2009 Abilitiessoft, Inc.  `www.abilitiessoft.com'.

     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 screenreader 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 maintainer,
<john.boyer@abilitiessoft.com>

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

   Please read the following copyright and warranty information. Note
that this information also applies to all source code, tables and other
files in this distribution of liblouis. It applies similarly to the
sister library liblouisxml.

   This file is maintained by John J. Boyer
<john.boyer@abilitiessoft.com>.

   Persons who wish to program with liblouis but will not be writing
translation tables may want to skip ahead to *note Programming with
liblouis::.

2 Test Programs
***************

Five 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 `xml2brl', which is part of the liblouisxml
package (*note Introduction: (liblouisxml)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.


2.1 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
     `italword', the `firstletterbold' opcode (*note firstletterbold:
     firstletterbold 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.

2.2 lou_checktable
==================

To use this program type the following:

     lou_checktable [OPTIONS] TABLE

   The command line options that are accepted by lou_checktable are
described in *note common options::.

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

2.3 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_debug 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.

2.4 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

   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

2.5 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.

3 How to Write Translation Tables
*********************************

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

`chardefs.cti'
     Character definitions for U.S. tables

`compress.ctb'
     Remove excessive white-space

`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 liblouisxml

`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, 16-bit big-endian Unicode
files or 16-bit little-endian Unicode files. Blank lines are ignored.
Any leading and trailing white-space (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. 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 (`\'). *Note Opcode
Index::, for a list of opcodes, with a link to each one.

   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 pathname 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 necesary for
hyphenation tables to work.

3.1 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 bu
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'.

3.2 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 liblouisxml. 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. The characters may need to be declared with the 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


3.3 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.

`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


3.4 Emphasis Opcodes
====================

These also define braille indicators, but they require more
explanation. There are four sets, for italic, bold, underline and
computer braille. In each of the first three sets there are seven
opcodes, for use before the first word of a phrase, for use before the
last word, for use after the last word, for use before the first letter
(or character) if emphasis starts in the middle of a word, for use
after the last letter (or character) if emphasis ends in the middle of
a word, before a single letter (or character), and to specify the
length of a phrase to which the first-word and last-word-before
indicators apply. This rather elaborate set of emphasis opcodes was
devised to try to meet all contingencies. It is unlikely that a
translation table will contain all of them. The translator checks for
their presence. If they are present, it first looks to see if the
single-letter indicator should be used. Then it looks at the word (or
phrase) indicators and finally at the multi-letter indicators.

   The translator will apply up to two emphasis indicators to each
phrase or string of characters, depending on what the `typeform'
parameter in its calling sequence indicates (*note Programming with
liblouis::).

   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.) or the `literal' opcode (*note literal: literal opcode.).

   Here are the various emphasis opcodes.

`firstwordital dots'
     This is the braille indicator to be placed before the first word
     of an italicized phrase that is longer than the value given in the
     `lenitalphrase' opcode (*note lenitalphrase: lenitalphrase
     opcode.). For example:

          firstwordital 46-46 English indicator

`lastworditalbefore dots'
`italsign dots'
     These two opcodes are synonyms. This is the braille indicator to be
     placed before the last word of an italicized phrase. In addition,
     if `firstwordital' is not used, this braille indicator is doubled
     and placed before the first word. Do not use `lastworditalbefore'
     and `lastworditalafter' in the same table. For example:

          lastworditalbefore 4-6

`lastworditalafter dots'
     This is the braille indicator to be placed after the last word of
     an italicized phrase. Do not use `lastworditalbefore' and
     `lastworditalafter' in the same table. See also the
     `lenitalphrase' opcode (*note lenitalphrase: lenitalphrase
     opcode.) for more information.

`firstletterital dots'
`begital dots'
     These two opcodes are synonyms. This is the braille indicator to be
     placed before the first letter (or character) if italicization
     begins in the middle of a word.

`lastletterital dots'
`endital dots'
     These two opcodes are synonyms. This is the braille indicator to be
     placed after the last letter (or character) when italicization
     ends in the middle of a word.

`singleletterital dots'
     This braille indicator is used if only a single letter (or
     character) is italicized.

`lenitalphrase number'
     If `lastworditalbefore' is used, an italicized phrase is checked
     to see how many words it contains. If this number is less than or
     equal to the number given in the `lenitalphrase' opcode, the
     `lastworditalbefore' sign is placed in front of each word. If it
     is greater, the `firstwordital' indicator is placed before the
     first word and the `lastworditalbefore' indicator is placed after
     the last word. Note that if the `firstwordital' opcode is not used
     its indicator is made up by doubling the dot pattern given in the
     `lastworditalbefore' entry. For example:

          lenitalphrase 4

`firstwordbold dots'
     This is the braille indicator to be placed before the first word
     of a bold phrase. For example:

          firstwordbold 456-456

`lastwordboldbefore dots'
`boldsign dots'
     These two opcodes are synonyms. This is the braille indicator to be
     placed before the last word of a bold phrase. In addition, if
     `firstwordbold' is not used, this braille indicator is doubled and
     placed before the first word. Do not use `lastwordboldbefore' and
     `lastwordboldafter' in the same table. For example:

          lastwordboldbefore 456

`lastwordboldafter dots'
     This is the braille indicator to be placed after the last word of a
     bold phrase. Do not use `lastwordboldbefore' and
     `lastwordboldafter' in the same table.

`firstletterbold dots'
`begbold dots'
     These two opcodes are synonyms. This is the braille indicator to be
     placed before the first letter (or character) if bold emphasis
     begins in the middle of a word.

`lastletterbold dots'
`endbold dots'
     These two opcodes are synonyms. This is the braille indicator to be
     placed after the last letter (or character) when bold emphasis
     ends in the middle of a word.

`singleletterbold dots'
     This braille indicator is used if only a single letter (or
     character) is in boldface.

`lenboldphrase number'
     If `lastwordboldbefore' is used, a bold phrase is checked to see
     how many words it contains. If this number is less than or equal to
     the number given in the `lenboldphrase' opcode, the
     `lastwordboldbefore' sign is placed in front of each word. If it
     is greater, the `firstwordbold' indicator is placed before the
     first word and the `lastwordboldbefore' indicator is placed after
     the last word. Note that if the `firstwordbold' opcode is not used
     its indicator is made up by doubling the dot pattern given in the
     `lastwordboldbefore' entry.

`firstwordunder dots'
     This is the braille indicator to be placed before the first word
     of an underlined phrase.

`lastwordunderbefore dots'
`undersign dots'
     These two opcodes are synonyms. This is the braille indicator to be
     placed before the last word of an underlined phrase. In addition,
     if `firstwordunder' is not used, this braille indicator is doubled
     and placed before the first word.

`lastwordunderafter dots'
     This is the braille indicator to be placed after the last word of
     an underlined phrase.

`firstletterunder dots'
`begunder dots'
     These two opcodes are synonyms. This is the braille indicator to be
     placed before the first letter (or character) if underline emphasis
     begins in the middle of a word.

`lastletterunder dots'
`endunder dots'
     These two opcodes are synonyms. This is the braille indicator to be
     placed after the last letter (or character) when underline emphasis
     ends in the middle of a word.

`singleletterunder dots'
     This braille indicator is used if only a single letter (or
     character) is underlined.

`lenunderphrase number'
     If `lastwordunderbefore' is used, an underlined phrase is checked
     to see how many words it contains. If this number is less than or
     equal to the number given in the `lenunderphrase' opcode, the
     `lastwordunderbefore' sign is placed in front of each word. If it
     is greater, the `firstwordunder' indicator is placed before the
     first word and the `lastwordunderbefore' indicator is placed after
     the last word. Note that if the `firstwordunder' opcode is not
     used its indicator is made up by doubling the dot pattern given in
     the `lastwordunderbefore' entry.

`begcomp dots'
     This braille indicator is placed before a sequence of characters
     translated in computer braille, whether this sequence is indicated
     in the `typeform' parameter (*note Programming with liblouis::) or
     inferred because it contains a subsequence specified by the
     `compbrl' opcode (*note compbrl: compbrl opcode.).

`endcomp dots'
     This braille indicator is placed after a sequence of characters
     translated in computer braille, whether this sequence is indicated
     in the `typeform' parameter (*note Programming with liblouis::) or
     inferred because it contains a subsequence specified by the
     `compbrl' opcode (*note compbrl: compbrl opcode.).


3.5 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.


3.6 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.


3.7 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.

   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 opration 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'
`literal characters'
     These two opcodes are synonyms. 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.

`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 occurence 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
     liblouisxml 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.


3.8 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'
     White-space 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.


3.9 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.


3.10 The Context and Multipass Opcodes
======================================

`context test action'
`pass2 test action'
`pass3 test action'
`pass4 test action'
     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 `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.

    `$ (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. 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.

    `] (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 membar 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'

     Note that if any multipass opcode or the correct opcode is used
     and the `pass1Only' mode bit (*note lou_translateString::) is not
     set input and output positions may be incorrect.


3.11 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

     Note that if the `correct' opcode is used and the `pass1Only' mode
     bit (*note lou_translateString::) is not set input and output
     positions may be incorrect.


3.12 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 pathname 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
     hexidecimal 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:

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

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

     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.

`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.


4 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.

5 Programming with liblouis
***************************

5.1 License
===========

Liblouis may contain code borrowed from the Linux screenreader BRLTTY,
Copyright (C) 1999-2006 by the BRLTTY Team.

Copyright (C) 2004-2007 ViewPlus Technologies, Inc.  `www.viewplus.com'.

Copyright (C) 2007,2009 Abilitiessoft, Inc.  `www.abilitiessoft.com'.

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

   Liblouis 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
General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
License along with Liblouis. If not, see `http://www.gnu.org/licenses/'.

5.2 Overview
============

You use the liblouis library by calling thirteen functions,
`lou_translateString', `lou_backTranslateString', `lou_logFile',
`lou_logPrint', `lou_getTable', `lou_translate', `lou_backTranslate',
`lou_hyphenate', `lou_charToDots', `lou_dotsToChar',
`lou_compileString', `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 just three code modules, `compileTranslationTable.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 liblouisxml.  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_logFile' and `lou_logPrint' functions, 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.

5.3 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.

5.4 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.

5.5 lou_translateString
=======================

     int lou_translateString (
         const char * tableList,
         const widechar * inbuf,
         int *inlen,
         widechar *outbuf,
         int *outlen,
         char *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 typefont.
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'. On receiving a table name liblouis first
checks to see if it can be found on this path. 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 pathname, 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 a string of characters 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 character indicates
the typeform of the corresponding character in the input buffer. The
values are as follows: 0 plain-text; 1 italic; 2 bold; 4 underline; 8
computer braille. These values can be added for multiple emphasis. If
this parameter is `NULL', no checking for typeforms 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.

5.6 lou_translate
=================

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

   This function adds the parameters `outputPos', `inputPos' and
`cursorPos', to facilitate use in screenreader 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 `inbuf'. `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'.

5.7 lou_backTranslateString
===========================

     int lou_backTranslateString (
         const char * tableList,
         const widechar * inbuf,
         int *inlen,
         widechar *outbuf,
         int *outlen,
         char *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.

5.8 lou_backTranslate
=====================

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

   This function is exactly the inverse of `lou_translate'.

5.9 lou_hyphenate
=================

     int lou_hyphenate (
         const char * tableList,
         const widechar * const 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'. 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 retranslated 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
liblouisxml.

5.10 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.

5.11 lou_dotsToChar
===================

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

   This function takes a widechar string in `inbuf' consisting of dot
patterns and converts it to a widechar string in `outbuf' consisting of
characters acording 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.

5.12 lou_charToDots
===================

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

   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 acording to
the specifications in `tableList'. `length' is the length of both
`inbuf' and `outbuf'. The dot patterns in `outbufbuf' are in liblouis
format. The functgion returns 1 on success and 0 on failure.

5.13 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 liblouisxml. Any error messages
generated will be printed to the file given in this call. The entire
pathname of the file must be given.

5.14 lou_logPrint
=================

     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
liblouisxml.

5.15 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 messages are printed to the log file, which is stderr unless a
different filename has been given using the `lou_logFile' function.
Errors result in a `NULL' pointer being returned.

5.16 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'.

5.17 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.

5.18 Python bindings
====================

There are Python bindings for `lou_translateString', `lou_translate'
and `lou_version'. For installation instructions see the the `README'
file in the `python' directory. Usage information is included in the
Python module itself.

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

after:                                         See 3.8.      (line 1185)
always:                                        See 3.7.      (line  965)
before:                                        See 3.8.      (line 1192)
begbold:                                       See 3.4.      (line  768)
begcaps:                                       See 3.3.      (line  604)
begcomp:                                       See 3.4.      (line  837)
begital:                                       See 3.4.      (line  718)
begmidword:                                    See 3.7.      (line 1071)
begnum:                                        See 3.7.      (line 1112)
begunder:                                      See 3.4.      (line  810)
begword:                                       See 3.7.      (line 1067)
boldsign:                                      See 3.4.      (line  753)
capsign:                                       See 3.3.      (line  598)
capsnocont:                                    See 3.6.      (line  875)
class:                                         See 3.8.      (line 1180)
comp6:                                         See 3.7.      (line  938)
compbrl:                                       See 3.7.      (line  926)
context:                                       See 3.10.     (line 1235)
contraction:                                   See 3.7.      (line 1045)
correct:                                       See 3.11.     (line 1402)
decpoint:                                      See 3.5.      (line  858)
digit:                                         See 3.2.      (line  512)
display:                                       See 3.12.     (line 1449)
endbold:                                       See 3.4.      (line  774)
endcaps:                                       See 3.3.      (line  610)
endcomp:                                       See 3.4.      (line  844)
endital:                                       See 3.4.      (line  724)
endnum:                                        See 3.7.      (line 1123)
endunder:                                      See 3.4.      (line  816)
endword:                                       See 3.7.      (line 1083)
exactdots:                                     See 3.7.      (line 1092)
firstletterbold:                               See 3.4.      (line  768)
firstletterital:                               See 3.4.      (line  718)
firstletterunder:                              See 3.4.      (line  810)
firstwordbold:                                 See 3.4.      (line  747)
firstwordital:                                 See 3.4.      (line  693)
firstwordunder:                                See 3.4.      (line  795)
grouping:                                      See 3.2.      (line  535)
hyphen:                                        See 3.5.      (line  864)
include:                                       See 3.12.     (line 1430)
italsign:                                      See 3.4.      (line  701)
joinnum:                                       See 3.7.      (line 1128)
joinword:                                      See 3.7.      (line 1027)
largesign:                                     See 3.7.      (line  990)
lastletterbold:                                See 3.4.      (line  774)
lastletterital:                                See 3.4.      (line  724)
lastletterunder:                               See 3.4.      (line  816)
lastwordboldafter:                             See 3.4.      (line  763)
lastwordboldbefore:                            See 3.4.      (line  753)
lastworditalafter:                             See 3.4.      (line  711)
lastworditalbefore:                            See 3.4.      (line  701)
lastwordunderafter:                            See 3.4.      (line  806)
lastwordunderbefore:                           See 3.4.      (line  799)
lenboldphrase:                                 See 3.4.      (line  784)
lenitalphrase:                                 See 3.4.      (line  734)
lenunderphrase:                                See 3.4.      (line  826)
letsign:                                       See 3.3.      (line  616)
letter:                                        See 3.2.      (line  550)
litdigit:                                      See 3.2.      (line  567)
literal:                                       See 3.7.      (line  926)
locale:                                        See 3.12.     (line 1438)
lowercase:                                     See 3.2.      (line  555)
lowword:                                       See 3.7.      (line 1036)
math:                                          See 3.2.      (line  582)
midendword:                                    See 3.7.      (line 1079)
midnum:                                        See 3.7.      (line 1117)
midword:                                       See 3.7.      (line 1075)
multind:                                       See 3.12.     (line 1466)
noback:                                        See 3.7.      (line  914)
nocont:                                        See 3.7.      (line  948)
nocross:                                       See 3.7.      (line 1017)
nofor:                                         See 3.7.      (line  921)
noletsign:                                     See 3.3.      (line  624)
noletsignafter:                                See 3.3.      (line  641)
noletsignbefore:                               See 3.3.      (line  633)
numsign:                                       See 3.3.      (line  649)
partword:                                      See 3.7.      (line 1087)
pass2:                                         See 3.10.     (line 1235)
pass3:                                         See 3.10.     (line 1235)
pass4:                                         See 3.10.     (line 1235)
postpunc:                                      See 3.7.      (line 1108)
prepunc:                                       See 3.7.      (line 1104)
prfword:                                       See 3.7.      (line 1063)
punctuation:                                   See 3.2.      (line  505)
repeated:                                      See 3.7.      (line  973)
replace:                                       See 3.7.      (line  955)
repword:                                       See 3.7.      (line  981)
sign:                                          See 3.2.      (line  574)
singleletterbold:                              See 3.4.      (line  780)
singleletterital:                              See 3.4.      (line  730)
singleletterunder:                             See 3.4.      (line  822)
space:                                         See 3.2.      (line  499)
sufword:                                       See 3.7.      (line 1059)
swapcc:                                        See 3.9.      (line 1225)
swapcd:                                        See 3.9.      (line 1213)
swapdd:                                        See 3.9.      (line 1218)
syllable:                                      See 3.7.      (line 1005)
undefined:                                     See 3.12.     (line 1442)
undersign:                                     See 3.4.      (line  799)
uplow:                                         See 3.2.      (line  518)
uppercase:                                     See 3.2.      (line  560)
word:                                          See 3.7.      (line 1001)
Function Index
**************

lou_backTranslate:                             See 5.8.      (line 1789)
lou_backTranslateString:                       See 5.7.      (line 1767)
lou_charToDots:                                See 5.12.     (line 1861)
lou_compileString:                             See 5.10.     (line 1836)
lou_dotsToChar:                                See 5.11.     (line 1848)
lou_free:                                      See 5.17.     (line 1921)
lou_getTable:                                  See 5.15.     (line 1895)
lou_hyphenate:                                 See 5.9.      (line 1807)
lou_logFile:                                   See 5.13.     (line 1874)
lou_logPrint:                                  See 5.14.     (line 1885)
lou_readCharFromFile:                          See 5.16.     (line 1908)
lou_translate:                                 See 5.6.      (line 1732)
lou_translateString:                           See 5.5.      (line 1649)
lou_version:                                   See 5.4.      (line 1640)
Program Index
*************

lou_allround:                                  See 2.3.      (line  241)
lou_checkhyphens:                              See 2.5.      (line  285)
lou_checktable:                                See 2.2.      (line  228)
lou_debug:                                     See 2.1.      (line  130)
lou_translate:                                 See 2.4.      (line  259)
