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Next: 8. How Aspell Works Up: GNU Aspell 0.50.1 Previous: 6. Writing
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Subsections

  * 7.1 The Language Data File
  * 7.2 Compiling the Word List
  * 7.3 Phonetic Code
      + 7.3.1 Syntax of the transformation array
      + 7.3.2 How do I start finally?
          o 7.3.2.1 Things that come in handy
          o 7.3.2.2 What the phonetic code should do
   
  * 7.4 Controlling the Behavior of Run-together Words

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7. Adding Support For Other Languages

Before you consider adding support for Aspell first make sure that someone
else has not already done it. A good number of dictionaries off the Aspell
home page at http://aspell.net. If your language is not listed above
please send me a note and I will work with you on adding support.

This chapter describes the old manual way of adding support for a new
language in Aspell. The recommended way to add support is through the
aspell-dicts package. The scripts used, as well as some documentation on
what to do, is available in the aspell-gen package which is available in
the same location the dictionary packages are at.

Adding a language to aspell is fairly straightforward. You need to create
the language data file, and compile a new word list.

7.1 The Language Data File

The basic format of the language data data is the same as it for aspell
configuration file. It is named «lang».dat and is located in the
architecture independent data dir for aspell (option data-dir) which is
usually «prefix»/share/aspell. Use "aspell config" to find out where it
is in your installation. By convention the language name should be the two
letter ISO 639 language code.

The language data file has several mandatory fields, and several optional
ones. All fields are case sensitive and should be in all lower case.

The two mandatory fields are name and charset. Name is the name of the
language and should be the same as the file name (without the .dat). 
Charset is the charset aspell will expect the word lists to be formatted
in. You may chose from any of the iso-8859-* character sets as well as,
koi8-f, koi8-r, and viscii. If your language can fit in the plain old
ASCII character set use iso8859-1. If you use some other character set for
your language other than the ones listed here drop me a note and I will
look into adding support for it.

The optional fields are special, soundslike, keyboard and a bunch of
options to specify how run-together words are handles. Special is for non
letter characters that can appear in your language such as the ' and -.
The format for the value is a list separated by spaces. Each item of the
list has the following format

    «char»  «begin»«middle»«end»

«char» is the non letter character in question. «begin»,«middle»,«end» are
either a '-' or a '*'. A star for «begin» means that the character at the
beginning of the word, a '-' means it can't. The same is true for «middle»
and «end». For example the entry for the ' in english is:

    ' -*-

To include more than one middle character just list them one after another
on the same line. for example to make both the ' and the - a middle
character use the following line in the language data file:

    special ' -*- - -*-

The soundslike option, if present, should be the name of the soundslike
data for the language. The data is expected to be in the file «name»
_phonetic.dat.

If the name is generic a really generic soundslike algorithm will be used
which consists of striping all the vowels and removing all accents. I
recommend first using the generic algorithm and then, after aspell is
working with the new language, work on the transformation array.

If the soundslike name is none then no soundslike lookup table will be
used. This will reduce the size of the compiled word list by around 50%
but at the sacrifice of suggestion quality. If the soundslike is none than
the soundslike for the word will simply be the word itself in lowercase,
will all accents stripped. For languages with phonetic spelling the
difference will not be very noticeable. However, for languages with
non-phonetic spelling there will be a noticeable difference. The
difference you notice will depend on the quality of the soundslike data
file. If you do not notice much of a difference for a language with
non-phonetic spelling that is a good indication that the soundslike data
is not rough enough--or the words you are trying are not that badly
misspelled.

The keyboard option specifies the base name of the keyboard definition
file to use. See section 4.4.3 for more information.

The options to control how run-together words are handled are the same as
the are in the normal configurations files. Please see section 7.4 for
more information.

7.2 Compiling the Word List

Once you have a working language data file installed in the right place
you are ready to compile the main word list. See section 5 to find out
what to do. This section also includes instructions for creating the awli
file.

7.3 Phonetic Code

(The following section was written by Björn Jacke, bjoern.jacke at gmx de)

Aspell is in fact the spell checker that comes up with the best
suggestions if it finds an unknown word. One reason is that it does not
just compare the word with other words in the dictionary (like Ispell
does) but also uses phonetic comparisons with other words.

The new table driven phonetic code is very flexible and setting up
phonetic transformation rules for other languages is not difficult but
there can be a number of stumbling stones -- that's why I wrote this
section.

The main phonetic code is free of any language specific code and should be
powerful enough to allow setting up rules for any language. Anything which
is language specific is kept in a plain text file and can easily be
edited. So it's even possible to write phonetic transformation rules if
you don't have any programming skills. All you need to know is how words
of the language are written and how they are pronounced.

7.3.1 Syntax of the transformation array

In the translation array there are two strings on each line; the first one
is the search string (or switch name) and the second one is the
replacement string (or switch parameter). The line

    version   «version»

is also required to appear somewhere in the translation array. The version
string can be anything but it should be changed when ever the a new
version of the translation array is released. This is important because it
will keep Aspell from using a compiled dictionary with the wrong set of
rules. For example if when coming up with suggestion for "hallo" aspell
will use the new rules to come up with the soundslike say "H*L*" but if
hello is stored in the dictionary using the old rules as "HL" instead of
"H*L*" aspell will never be able to come up with hello. So to solve this
problem Aspell checks if the version strings match and abort with an error
if they don't. Thus it is important to update it when ever a new version
of the translation array is releases. This is only a problem with the main
word list as the personal word lists are now stored as simple word lists
with a single header line (ie, no soundslike data).

Each non switch line represents one replacement (transformation) rule.
Words beginning with the same letter must be grouped together; the order
inside this group does not depend on alphabetical issues but it gives
priorities; the higher the rule the higher the priority. That's why the
first rule that matches is applied. In the following example:

    GH     _  
    G      K  
     

"GH to _" has higher priority than "G to K". "_" represents the
empty string "". If "GH to _" would stand after "G to K", the second
rule would never match because the algorithm would stop searching for more
rules after the first match. The above rules transform any "GH" to an
empty string (delete them) and transform any other "G" to "K".

At the end of the first string of a line (the search string) there may
optionally stand a number of characters in brackets. One (only one!) of
these characters must fit. It's comparable with the [ ] brackets in
regular expressions. The rule "DG(EIY) to J" for example would match any
"DGE", "DGI" and "DGY" and replace them with "J". This way you can
reduce several rules to one.

Behind the search string there can stand one or more dashes (-). Those
search strings will be matched totally but only the beginning of the
string will be replaced. Furthermore for these rules no follow-up rule
will be searched (what this is will be explained later). The rule "TCH--
to _" will match any word containing "TCH" (like "match") but will
only replace the first character "T" with an empty string. The number of
dashes determines how many characters from the end will not be replaced.
After the replacement the search for transformation rules continues with
the not replaced "CH"!

If a "<" is appended to the search string, the search for replacement
rules will continue with the replacement string and not with the next
character of the word. The rule "PH< to F" for example would replace
"PH" with "F" and then again start to search for a replacement rule
for "F...". If there would also be rules like "FO to O" and "F to _"
then words like "PHOXYZ" would be transformed to "OXYZ" and any
occurrences of "PH" that are not followed by an "O" will be deleted
like "PHIXYZ to IXYZ". The second replacement however is not applied if
the priority of this rule is lower than the priority of the first rule.

Priorities are added to a rule by putting a number between 0 and 9 at the
end of the search string, for example "ING6 to N". The higher the number
the higher is the priority.

Priorities are especially important for the previously mentioned follow-up
rules. Follow-up rules are searched beginning from the last string of the
first search string. This is a bit complicated but I hope this example
will make it more clear:

    CHS      X 
    CH       G 
     
    HAU--1   H 
     
    SCH      SH 
     

In this example "CHS' in the word "FUCHS" would be transformed to
"X". If we take the word "DURCHSCHNITT" the things look a bit
different. Here "CH" belongs together and "SCH" belongs together and
both are spoken separately. The algorithm however first finds the string
"CHS" which may not be transformed like in the previous word "FUCHS".
At this point the algorithm can find a follow up rule. It takes the last
character of the first matching rule ("CHS") which is "S" and looks
for the next match, beginning from this character. What it finds is clear:
It finds "SCH to SH", which has the same priority (no priority means
standard priority, which is 5). If the priority is the same or higher the
follow-up rule will be applied. Let's take a look at the word
"SCHAUKEL". In this word "SCH" belongs together and may not be torn
apart. After the algorithm has found "SCH to SH" it searches for a
follow-up rule for "H"+"AUKEL". It finds "HAU--1 to H", but does not apply
it because its priority is lower than the one of the first rule. You see
that this is a very powerful feature but it also can easily lead to
mistakes. If you really don't need this feature you can turn it off by
putting the line

    followup      0 

at the beginning of the phonetic table file. As mentioned, for rules
containing a `-' no follow-up rules are searched but giving such rules a
priority is not totally senseless because they self can be follow-up rules
and in that case the priority makes sense again. Follow-up rules of
follow-up rules are not searched because this is in fact not needed very
often.

The control character "^" says that the search string only matches at the
beginning of words so that the rule "RH^to R" will only apply to words
like "RHESUS" but not "PERHAPS". You can append another "^" to the search
string. In that case the algorithm treats the rest of the word totally
separately from first matched string in at beginning. This is useful for
prefixes whose pronunciation does not depend on the rest of the word and
vice versa like "OVER^^" in English for example.

The same way as "^" works does "$" only apply on words that end with the
search string. "GN$ to N" only matches on words like "SIGN" but not
"SIGNUM". If you use "^" and "$" together, both of them must fit "ENOUGH^$
to NF" will only match the word "ENOUGH" and nothing else.

Of course you can combine all of the mentioned control characters but they
must occur in this order: < - priority ^ $. All characters must be written
in CAPITAL letters.

If absolutely no rule can be found -- might happen if you use strange
characters for which you don't have any replacement rule -- the next
character will simply be skipped and the search for replacement rules will
continue with the rest of the word.

If you want double letters to be reduced to one you must set up a rule
like "LL- to L". If double letters in the resulting phonetic word should
be allowed, you must place the line

    collapse_result     0 

at the beginning of your transformation table file; otherwise set the
value to `1'. The English rules for example strip all vowels from words
and so the word "GOGO" would be transformed to "K" and not to "KK" (as
desired) if collapse_result is set to 1. That's why the English rules have
collapse_result set to 0.

7.3.2 How do I start finally?

Before you start to write an array of transformation rules, you should be
aware that you have to do some work to make sure that things you do will
result in correct transformation rules.

7.3.2.1 Things that come in handy

First of all you need to have a large word list of the language you want
to make phonetics for. It should contain about as many words as the
dictionary of the spell checker. If you don't have such a list, you will
probably find an Ispell dictionary at http://fmg-www.cs.ucla.edu/geoff/
ispell-dictionaries.htmlwhich will help you. You can then make affix
expansion via ispell -e and then pipe it trough \tr " " "\n" to put one
word on each line. After that you eventually have to convert special
characters like `é' from Ispell's internal representation to latin1
encoding. sed s/e'/é/g for example would replace all e' with é.

The second is that you know how to use regular expressions and know how to
use grep. You should for example know that

    grep ^[^aeiou]qu[io] wordlist | less

will show you all words that begin with any character but a, e, i, o or u
and then continue with `qui' or `quo'. This stuff is important for example
to find out if a phonetic replacement rule you want to set up is valid for
all words which match the expression you want to replace. Taking a look at
the regex(7) man page is a good idea.

7.3.2.2 What the phonetic code should do

Normal text comparison works well as long as the typer misspells a word
because he pressed one key he didn't really want to press. In this cases
mostly one character differs from the original word.

In cases where the writer didn't know about the correct spelling of the
word however the word may have several characters that differ from the
original word but usually the word would still sound like the original
word. Someone might think for example that `tough' is spelled `taff'. No
spell checker without phonetic code will come to the idea that this might
be `tough' but a spell checker who knows that `taff' would be pronounced
like `tough' will make good suggestions to the user. Another example could
be `funetik' and `phonetic'.

From this examples you can see that the phonetic transformation should not
be too fussy and too precise. If you implement a whole phonetic dictionary
as you can find it in books this will not be very useful because then
there could still be many characters differing from the misspelled and the
desired word. What you should do if you implement the phonetic
transformation table is to reduce the number of used letters to the only
really necessary ones.

Characters that sound similar should be reduced to one. In English
language for example `Z' sounds like `S' and that's why the transformation
rule "Z to S" is present in the replacement table. `PH' is spoken like
`F' and so we have a "PH to F" rule.

If you take a closer look you will even see that vowels sound very similar
in English language: `contradiction', `cuntradiction', `cantradiction' or
`centradiction' in fact sound nearly the same, don't they? Therefore the
English phonetic replacement rules not only reduce all vowels to one but
even remove them all (removing is done by just setting up no rule for
those letters). The phonetic code of `contradiction' is `KNTRTKXN' and if
you try to read this letter-monster loud you will hear that it still sound
a bit like `contradiction'. You also see that `D' is transformed to `T'
because they nearly sound the same.

If you think you have found a regularity you should always take your word
list and grep for the corresponding regular expression you want to make a
transformation rule for. An example: If you come to the idea that all
English words ending on `ough' sound like `AF' at the end because you
think of `enough' and `tough'. If you then grep for the corresponding
regular expression by "grep -i ough$ wordlist" you will see that the
rule you wanted to set up is not correct because the rule doesn't fit to
words like `although' or `bough'. So you have to define your rule more
precisely or you have to set up exceptions if the number of words that
differ from the desired rule is not so big.

Don't forget about follow-up rules which can help in many cases but which
also can lead to many confusions and side effects. It's also important to
write exceptions in front of the more general rules ("GH" before "G"
etc.).

If you think you have set up a number of rules that may produce some good
results try them out! If you run Aspell as "aspell --lang=«your language»
pipe" you get a prompt at which you can type in words. If you just type
words Aspell checks them and eventually makes suggestions if they are
misspelled. If you type in "$$Sw «word»" you will see the phonetic
transformation and you can test out if your work does what you want.

Another good way to control if changes you apply to your rules don't have
any evil side effects is to create another list from your word list which
contains not only the word of the word list but also the corresponding
phonetic version of this word on the same line. If you do this one time
before the change and one time after the change you can make a diff (see
man diff) to see what really changed. To do this use the command "aspell
--lang=«your language» soundslike". In this mode aspell will output the
the original word and then its soundslike separated by a tab character for
each word you give it. If you are interested in seeing how the algorithm
works you can download a set of useful programs from http://
members.xoom.com/maccy/spell/phonet-utils.tar.gz. This includes a program
that produces a list as mentioned above and another program which
illustrates how the algorithm works. It uses the same transformation table
as Aspell and so it helps a lot during the process of creating a phonetic
transformation table for Aspell.

During your work you should write down your basic ideas so that other
people are able to understand what you did (and you still know about it
after a few weeks). The English table has a huge documentation appended
for example.

Now you can start experimenting with all the things you just read and
perhaps set up a nice phonetic transformation table for your language to
help Aspell to come up with the best correction suggestions ever seen also
for your language. Take a look at the Aspell homepage to see if there is
already a transformation table for your language. If there is one you
might also take a look at it to see if it could be improved.

If you think that this section helped you or if you think that this is
just a waste of time you can send any feedback to bjoern.jacke@gmx.de.


7.4 Controlling the Behavior of Run-together Words

Aspell has support for either unconditionally accepting run-together words
or only accepting certain words in compound formation.

Support for unconditionally accepting run-together words can either be
turned on in the language data file or as a normal option via the 
run-together option. The run-together-limit options controls the maximum
number of words that can be strung together, the default is normally 255.
The run-together-min options controls the minimal length the individual
components of the run together word can be, the default is normally 3.
Both the run-together-limit and run-together-min option may be specified
in both the language data file or as a normal. The run-together-mid
option, which may only be specified in the language data file, may be used
to specify up to three optional characters that may appear between
individual words.

In order for aspell to conditionally only accept certain words in
compounds those words must be flagged when the compiled word list is being
created. The format for each entry is

    «word»:C[1][2][3]«middle char»

The 1, 2, and 3 control if the word is allowed to appear in the begging,
middle, or end of the compound, respectfully. More than one position flag
may be specified. If none of them are specified it as assumed that the
word may appear anywhere. The C is optional if 1, 2, or 3 is specified.
The «middle char» represents an optional character that may appear after
the word in the formation of the compound if the word is not at the end of
the compound. If the letter is lowercase than the character may appear
after the word, if it is in uppercase then that letter must appear after
the compound. Only one letter may be specified and it must also be in the
list of middle letters specified via the run-together-mid option. The 
run-together-limit option may also be used to specify the maximum number
of words to string together.

For example the word list:

    beg:1
    mid:2
    end:3
    any:C
    never
    must:CM
    maybe:Cm

Means that the word "beg" may only appear at the begging of a word, the
word "mid" at the middle, the word "end" at the end, and the word
"any" any place. The word "never" is never accepted in a compound
unless the run-together option is set. The word "must" may appear
anywhere however it must be followed by an "m", while the word maybe may
be followed by an "m". Given the above word list the following compounds
or legal:

    begmidend
    begany
    mustmend
    maybeend
    maybemend

are all legal, but the following are not:

    begmid
    mustend
    neverany

Individual words such as "beg" are always accepted.

When the run-together option is not set Aspell will only accept words that
have been flagged in a run-together word. When the run-together option is
set aspell will accept words which are as least as long as the value
specified in the run-together-min option. If the words length is less than
run-together-min then it will only accept the word if it has been flagged.
When the run-together option is not set the run-together-min option is
ignored all together.

Currently Aspell only supports run-together words when checking if a word
is in the dictionary. When coming up with suggestions Aspell treats the
word as a normal word and does not do anything special. This means that
the suggestions will be virtually meaningless when the actual word is a
run-together. I plan on more intelligently supporting run-together words
when coming up with suggestions in a future version of Aspell.

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next up previous contents
Next: 8. How Aspell Works Up: GNU Aspell 0.50.1 Previous: 6. Writing
programs to   Contents
Kevin Atkinson 2002-08-28
