Re: Performance of getting line number for positions around point?

[Yuan Fu <casouri@gmail.com>]

> On Jan 19, 2025, at 11:18 PM, Yuan Fu <casouri@gmail.com> wrote:
> 
> When sending buffer edits to tree-sitter, we’re supposed to pass it both the byte position and (line, col) position. Up until this point we’ve been only passing the byte position, and just pass a dummy (line, col) position. Most of the time, the (line, col) position just gets carried around in tree-sitter and comes out when tree-sitter reports positions back to us. So as long as we don’t use the (line, col) positions it’s fine.
> 
> However, it turns out that for some languages, the (line, col) information is significant and can affect parsing. As shown in [1]. Most editors do track line and columns so I don’t think tree-sitter will ever track line and column itself just for Emacs.
> 
> So now the question is, is there an existing, performant way to get line numbers? We only need to send line and column positions for buffer edits, so we just need line numbers for positions around point. Also, because narrowing is transparent to tree-sitter, when the buffer is narrowed, the line number also needs to change with it.
> 
> IIUC count_line just counts lines, and redisplay_count_line depends on redisplay?
> 
> [1] https://github.com/tree-sitter/tree-sitter/issues/4001

I haven’t yet started seriously looking into this, but my initial idea is to keep a cache in buffer local variables that stores the line number of a position near point. And in insert/delete functions, we update the line number cache. If we need to get the line number of a point, just scan the content between the cached position and that point.

Does that sound like a good idea? Or there are existing and better ways to do it?

Yuan