How does Chrome decide what to highlight when you double-click Japanese text?
JavascriptGoogle ChromeCjkJavascript Problem Overview
If you double-click English text in Chrome, the whitespace-delimited word you clicked on is highlighted. This is not surprising. However, the other day I was clicking while reading some text in Japanese and noticed that some words were highlighted at word boundaries, even though Japanese doesn't have spaces. Here's some example text:
> どこで生れたかとんと見当がつかぬ。何でも薄暗いじめじめした所でニャーニャー泣いていた事だけは記憶している。
For example, if you click on 薄暗い, Chrome will correctly highlight it as a single word, even though it's not a single character class (this is a mix of kanji and hiragana). Not all the highlights are correct, but they don't seem random.
How does Chrome decide what to highlight here? I tried searching the Chrome source for "japanese word" but only found tests for an experimental module that doesn't seem active in my version of Chrome.
Javascript Solutions
Solution 1 - Javascript
So it turns out v8 has a non-standard multi-language word segmenter and it handles Japanese.
function tokenizeJA(text) {
var it = Intl.v8BreakIterator(['ja-JP'], {type:'word'})
it.adoptText(text)
var words = []
var cur = 0, prev = 0
while (cur < text.length) {
prev = cur
cur = it.next()
words.push(text.substring(prev, cur))
}
return words
}
console.log(tokenizeJA('どこで生れたかとんと見当がつかぬ。何でも薄暗いじめじめした所でニャーニャー泣いていた事だけは記憶している。'))
// ["どこ", "で", "生れ", "たか", "とんと", "見当", "が", "つ", "か", "ぬ", "。", "何でも", "薄暗い", "じめじめ", "した", "所", "で", "ニャーニャー", "泣", "い", "て", "いた事", "だけ", "は", "記憶", "し", "て", "いる", "。"]
I also made a jsfiddle that shows this.
The quality is not amazing but I'm surprised this is supported at all.
Solution 2 - Javascript
Based on links posted by JonathonW, the answer basically boils down to: "There's a big list of Japanese words and Chrome checks to see if you double-clicked in a word."
Specifically, v8 uses ICU to do a bunch of Unicode-related text processing things, including breaking text up into words. The ICU boundary-detection code includes a "Dictionary-Based BreakIterator" for languages that don't have spaces, including Japanese, Chinese, Thai, etc.
And for your specific example of "薄暗い", you can find that word in the combined Chinese-Japanese dictionary shipped by ICU (line 255431). There are currently 315,671 total Chinese/Japanese words in the list. Presumably if you find a word that Chrome doesn't split properly, you could send ICU a patch to add that word.