Monday, February 13, 2017 [Tweets] [Favorites]

Optimizations in Syntax Highlighting

Alexandru Dima (via Hacker News):

Tokenization in VS Code (and in the Monaco Editor) runs line-by-line, from top to bottom, in a single pass. A tokenizer can store some state at the end of a tokenized line, which will be passed back when tokenizing the next line. This is a technique used by many tokenization engines, including TextMate grammars, that allows an editor to retokenize only a small subset of the lines when the user makes edits.

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Holding on to that tokens array takes 648 bytes in Chrome and so storing such an object is quite costly in terms of memory (each object instance must reserve space for pointing to its prototype, to its properties list, etc). Our current machines do have a lot of RAM, but storing 648 bytes for a 15 characters line is unacceptable.

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Perhaps the biggest breakthrough we've had is that we don't need to store tokens, nor their scopes, since tokens only produce effects in terms of a theme matching them or in terms of bracket matching skipping strings.

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When pushing a new scope onto the scope stack, we will look up the new scope in the theme trie. We can then compute immediately the fully resolved desired foreground or font style for a scope list, based on what we inherit from the scope stack and on what the theme trie returns.

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