I was pondering improving the logics of the LanguageModel contents, in
order to better handle language with a huge number of characters (far
too much to keep a full frequent list while keeping reasonable memory
consumption and speed).
But then I realize that this happens for languages which have anyway
their own set of characters.
For instance, modern Korean is near full hangul. Of course, we can find
some Chinese characters here and there, but nothing which should really
break confidence if we base it on the hangul ratio. Of course if some
day we want to go further and detect older Korean, we will have to
improve the logics a bit with some statistics, though I wonder if
limiting ourselves to character frequency is not enough here (sequence
frequency is maybe a bit overboard). To be tested.
In any case, this new class gives much more relevant confidence on
Korean texts, compared to the statistics data we previously generated.
For Japanese, it is a mix of kana and Chinese characters. A modern full
text cannot exist without a lot of kanas (probably only old text or very
short texts, such as titles, could have only Chinese characters). We
would still want to add a bit of statistics to differentiate correctly a
Japanese text with a lot of Chinese characters in it and a Chinese
text which quotes a bit of Japanese phrases. It will have to be
improved, but for now it works fairly ok.
A last case where we would want to play with statistics might be if we
want to differentiate between regional variants. For instance,
Simplified Chinese, Taiwan or Hong Kong Chinese… More to experiment
later on. It's already a first good step for UTF-8 support with
language!
Until now, Korean charsets had its own probers as there are no
single-byte encoding for writing Korean. I now added a Korean model only
for the generic character and sequence statistics.
I also improved the generation script (script/BuildLangModel.py) to
allow for languages without single-byte charset generation and to
provide meaningful statistics even when the language script has a lot of
characters (so we can't have a full sequence combination array, just too
much data). It's not perfect yet. For instance our UTF-8 Korean test
file ends up with confidence of 0.38503, which is low for obvious Korean
text. Still it works (correctly detected, with top confidence compared
to others) and is a first step toward more improvement for detection
confidence.
This prober comes from MR !1 on the main branch though it was too
agressive then and could not get merged. On the improved API branch, it
doesn't detect other tests as Johab anymore.
Also fixing it to work with the new API.
Finally adding a Johab/ko unit test.
The Hebrew Model had never been regenerated by my scripts. I now added
the base generation files.
Note that I added 2 charsets: ISO-8859-8 and WINDOWS-1255 but they are
nearly identical. One of the difference is that the generic currency
sign is replaced by the sheqel sign (Israel currency) in Windows-1255.
And though this one lost the "double low line", apparently some Yiddish
characters were added. Basically it looks like most Hebrew text would
work fine with the same confidence on both charsets and detecting both
is likely irrelevant. So I keep the charset file for ISO-8859-8, but
won't actually use it.
The good part is now that Hebrew is also recognized in UTF-8 text thanks
to the new code and newly generated language model.
Returning only the best one has limits, as it doesn't allow to check
very close confidence candidates. Now in particular, the UTF-8 prober
will return all ("UTF-8", lang) candidates for every language with
probable statistical fit.
No functional change yet because all probers still return 1 candidate.
Yet now we add a GetCandidates() method to return a number of
candidates.
GetCharSetName(), GetLanguage() and GetConfidence() now take a parameter
which is the candidate index (which must be below the return value of
GetCandidates()). We can now consider that nsCharSetProber computes a
couple (charset, language) and that the confidence is for this specific
couple, not just the confidence for charset detection.
Since our whole charset detection logics is based on text having meaning
(using actual language statistics), just because a text is valid UTF-8
does not mean it is absolutely the right encoding. It may also fit other
encoding with maybe very high statistical confidence (and therefore a
better candidate).
Therefore instead of just returning 0.99 or other high values, let's
weigh our encoding confidence with the best language confidence.
Now the UTF-8 prober would not only detect valid UTF-8, but would also
detect the most probable language. Using the data generated 2 commits
away, this works very well.
This is still basic and will require even more improvements. In
particular, now the nsUTF8Prober should return an array of ("UTF-8",
language) couple candidate. And nsMBCSGroupProber should itself forward
these candidates as well as other candidates from other multi-byte
detectors. This way, the public-facing API would get more probable
candidates, in case the algorithm is slightly wrong.
Also the UTF-8 confidence is currently stupidly high as soon as we
consider it to be right. We should likely weigh it with language
detection (in particular, if no language is detected, this should
severely weigh down UTF-8 detection; not to 0, but high enough to be a
fallback in case no other encoding+lang is valid and low enough to give
chances to other good candidate couples.
This doesn't work for all probers yet, in particular not for the most
generic probers (such as UTF-8) or WINDOWS-1252. These will return NULL.
It's still a good first step.
Right now, it returns the 2-character language code from ISO 639-1. A
using project could easily get the English language name from the
XML/json files provided by the iso-codes project. This project will also
allow to easily localize the language name in other languages through
gettext (this is what we do in GIMP for instance). I don't add any
dependency though and leave it to downstream projects to implement this.
I was also wondering if we want to support region information for cases
when it would make sense. I especially wondered about it for Chinese
encodings as some of them seem quite specific to a region (according to
Wikipedia at least). For the time being though, these just return "zh".
We'll see later if it makes sense to be more accurate (maybe depending
on reports?).
It was not clear if our naming followed any kind of rules. In particular,
iconv is a widely used encoding conversion API. We will follow its
naming.
At least 1 returned name was found invalid: x-euc-tw instead of EUC-TW.
Other names have been uppercased to follow naming from `iconv --list`
though iconv is mostly case-insensitive so it should not have been a
problem. "Just in case".
Prober names can still have free naming (only used for output display
apparently).
Finally HZ-GB-2312 is absent from my iconv list, but I can still see
this encoding in libiconv master code with this name. So I will
consider it valid.
