GiellaLT Documentation

GiellaLT provides rule-based language technology aimed at minority and indigenous languages

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Unicode pain points

TLDR: Make sure that the surface side of fst’s do not contain any multichars, only sequences of individual code points, also in the case of combining diacritics. Exception: spellers. Details below.

Combining diacritics background

Many minority and indigenous languages have orthographies that use various diacritics to mark features like length, tone, nasalisation and more. But unlike the majority languages, quite often the combination of base character and diacritic(s) is not available in Unicode as a precomposed combo, and instead one has to use the fallback mechanism of combining diacritics. While nice on paper, it leads to various issues that only arise for these languages, the most relevant for our work are:

Tokenisation and text analysis

When working with the lexicon and phonological rules, it is natural to either implicitly or explicitly treat sequences of base characters and combining diacritics as one entity, preferably explicitly defined as a multichar symbol in lexc, and in the alphabet section in twolc files (in xfst rewrite rules such symbols tend to be more implicilty defined).

As part of the internal processing and rule writing, this is fine. But for the final analysers it can be problematic. Using hfst-lookup, it works fine, and the tool will parse the input string either way (as multichars or individual symbols) in accordance with the fst. But when using hfst-tokenise, this does not work: the tool assumes a sequence of code point characters. There has been an attempt at working around this restriction, but in the end the simplest solution is to ensure that all multichars on the surface side of an fst are expanded into individual character symbols. For consistency’s sake, and to avoid issues separating lemma letters from multichar tags in hfst-tokenise, it should be done on both sides of the fst.

The regex

To ensure that this is done consistently for all fst’s, this is best done as the last processing step of the raw fst. Here’s an example from lang-lut/src/orthography/split-composed-chars.regex:

# Regex to expand multichar symbols (in the fst sense) to a string of
# individual letters. Will make sure that various tools do not choke
# on parsing input strings.

"b̓"  -> {b̓} ,
"c̓"  -> {c̓} ,
"dᶻ" -> {dᶻ},
"gʷ" -> {gʷ},
"kʷ" -> {kʷ},
"k̓"  -> {k̓} ,
"k̓ʷ" -> {k̓ʷ},
"l̕"  -> {l̕} ,
"m̓"  -> {m̓} ,
"n̓"  -> {n̓} ,
"p̓"  -> {p̓} ,
"qʷ" -> {qʷ},
"q̓"  -> {q̓} ,
"q̓ʷ" -> {q̓ʷ},
"t̕"  -> {t̕} ,
"w̓"  -> {w̓} ,
"xʷ" -> {xʷ},
"x̌"  -> {x̌} ,
"x̌ʷ" -> {x̌ʷ},
"y̓"  -> {y̓} ,
"č̓"  -> {č̓} ,
"ƛ̕"  -> {ƛ̕} ,
"ə́"  -> {ə́} ;

The build rule

Then in the src/ file:

### Split multichar letters early, to avoid repetitive code. ###
### Multichar letters must be split on both sides, and then  ###
### the alphabet pruned, for hfst-tokenise to work without   ###
### issues.                                                  ###
generator-raw-gt-desc.hfst: generator-raw-gt-desc.tmp.hfst \
	$(AM_V_XFST_TOOL)$(PRINTF) "read regex            \
	        @\"orthography/split-composed-chars.compose.hfst\".i \
	    .o. @\"$<\"                               \
	    .o. @\"orthography/split-composed-chars.compose.hfst\" \
	 save stack $@.tmp\n\
	 quit\n" | $(XFST_TOOL)
	 $(AM_V_HPRUNE)$(HFST_PRUNE_ALPHABET) -i $@.tmp -o $@
	 $(AM_V_at)rm -f $@.tmp

# Xfst and Foma version:
analyser-raw-gt-desc.%: analyser-raw-gt-desc.tmp.% \
	$(AM_V_XFST_TOOL)$(PRINTF) "read regex            \
	        @\"orthography/split-composed-chars.compose.$*\".i \
	    .o. @\"$<\"                               \
	    .o. @\"orthography/split-composed-chars.compose.$*\" \
	 save stack $@\n\
	 quit\n" | $(XFST_TOOL)


When working with spellers, the main concern is very different from that of text tokenisation: we want to modell and correct spelling errors, and to keep a sane mind, it is important to make sure that fst “letters” are the same as our mental “letters”. That is, we want sequences of base characters and combining diacritics to be encoded as multichar symbols in the fst, so that we can manipulate them as single entities.

The regex

Since this only concerns spellers, we device a regex to revert the operation above, for spellers only. Here is another example from lang-lut/tools/spellcheckers/filters/join-composed-chars.regex:

# Regex to turn a string of individual letters including combining diacritics
# into multichar symbols (in the fst sense).
# Needed to stay sane when working with speller error models.

{b̓}  -> "b̓" ,
{c̓}  -> "c̓" ,
{dᶻ} -> "dᶻ",
{gʷ} -> "gʷ",
{kʷ} -> "kʷ",
{k̓}  -> "k̓" ,
{k̓ʷ} -> "k̓ʷ",
{l̕}  -> "l̕" ,
{m̓}  -> "m̓" ,
{n̓}  -> "n̓" ,
{p̓}  -> "p̓" ,
{qʷ} -> "qʷ",
{q̓}  -> "q̓" ,
{q̓ʷ} -> "q̓ʷ",
{t̕}  -> "t̕" ,
{w̓}  -> "w̓" ,
{xʷ} -> "xʷ",
{x̌}  -> "x̌" ,
{x̌ʷ} -> "x̌ʷ",
{y̓}  -> "y̓" ,
{č̓}  -> "č̓" ,
{ƛ̕}  -> "ƛ̕" ,
{ə́}  -> "ə́" ;

The build rule

To make use of the regex, add something like the following to tools/spellcheckers/

# To be able to stay sane while working with spellers, error models, etc
# we convert back combining diacritic sequences to multichars, so that the
# mental letters are single fst symbols and not several. For consistensy's
# sake we do the same on the lemma side.
generator-speller-gt-norm.%: generator-speller-gt-norm.tmp.% \
	$(AM_V_XFST_TOOL)$(PRINTF) "read regex            \
	            @\"filters/join-composed-chars.hfst\".i \
	        .o. @\"$<\"                                 \
	        .o. @\"filters/join-composed-chars.hfst\"   \
	     save stack $@.tmp\n\
	     quit\n" | $(XFST_TOOL)
	     $(AM_V_HPRUNE)$(HFST_PRUNE_ALPHABET) -i $@.tmp -o $@
	     $(AM_V_at)rm -f $@.tmp

Given this, a letter in the common sense will also be a letter in the technical sense, and working with the error model and correcting algorithm becomes less frustrating.

Text rendering

For the idea of combining diacritics to work, they need to be properly rendered. And for that the Unicode consortium has developed an algorithm that software developers are supposed to implement. And they have - sort of, and buggy. The end result is in many cases unreadable text. And this is of course an issue only hitting minority and indigenous languages, because all majority languages can use letters wth precomposed diacritics.

So what is the problem? Essentially one of (lack of) stacking, so that multiple diacritics render on top of each other or on the base character. Another issue is inconsistent implementations, so that one and the same text using exactly the same font does not render the same in different environments.

Examples can be found for many languages, below the issue is documented with Kildin Sámi letters.

The problems described here can not be solved using the GiellaLT infrastructure, that is the responsibility of system and office software developers. But documenting it with clear examples is one important step.

The test text used in the images below is as follows:

О̄ о̄, А̄ а̄, Е̄ е̄, Ё ё, Ё̄ ё̄ - Calibri
О̄ о̄, А̄ а̄, Е̄ е̄, Ё ё, Ё̄ ё̄ - Palatino
О̄ о̄, А̄ а̄, Е̄ е̄, Ё ё, Ё̄ ё̄ - Times New Roman
О̄ о̄, А̄ а̄, Е̄ е̄, Ё ё, Ё̄ ё̄ - Arial Unicode MS

The macron is a combining diacritic, the diaeresis is precomposed. All base characters are Cyrillic. None of these letters exist as precomposed, so using a combining diacritic macron is the only option.

All screen shots below were taken September 7, 2021.

Pages, macOS 11

Kildin Sami letters with macron, Pages 11 on Mac

Most things look good, although the macron on top of the diaeresis in Times New Roman is a bit close. Otherwise fine. This app is probably using the default macOS font renderer, and is thus representative for most apps on the platform.

MS Word 365, macOS

Kildin Sami letters with macron, MSWord 365

Only Times New Roman is ok, with the same issue as for Pages/macOS system renderer above. The other fonts have various strange issues:

LibreOffice 7, macOS

Kildin Sami letters with macron, LibreOffice 7

Only Times New Roman is ok, with the same issue as for Pages/macOS system renderer above. The other fonts have various strange issues:

MS Word 365, Windows 10

Kildin Sami letters with macron, MSWord 365 Win

Only Times New Roman is ok, and on Windows even the macron over the diaeresis has ok spacing. The other fonts have various strange issues:

MS WordPad, Windows 10

Kildin Sami letters with macron, WordPad Win

Text rendering summary

Using a small sample of letters with a set of popular fonts, only two combinations were fully or close to fully acceptable:

The issue was first described about ten years ago, and the situation has not improved at all.

The fact that the macOS system renderer gets it nearly correct for all fonts shows that it is possible.