The side-by-side diff heavily relies on a primitive from `buffer_diff`
that converts a point on one side of the diff to a range of points on
the other side. The way this primitive is set up on main is pretty
naive--every time we call `points_to_base_text_points` (or
`base_text_points_to_points`), we need to iterate over all hunks in the
diff. That's particularly bad for the case of constructing a new
side-by-side diff starting from a multibuffer, because we call those
APIs once per excerpt, and the number of excerpts is ~equal to the
number of hunks.
This PR changes the point translation APIs exposed by `buffer_diff` to
make it easier to use them efficiently in `editor`. The new shape is a
pair of functions that return a patch that can be used to translate from
the main buffer to the base text or vice versa. When syncing edits
through the block map that touch several excerpts for the same buffer,
we can reuse this patch for excerpts after the first--so when building a
new side-by-side diff, we'll iterate over each hunk just once.
The shape of the new APIs also sets us up to scale down to cases like
editing on the right-hand side of the diff: we can pass in a point range
and give them permission to return an approximate patch that's only
guaranteed to give the correct results when used with points in that
range. For edits that only affect one excerpt, and given how the project
diff is set up, that should allow us to skip iterating over most of the
hunks in a buffer.
Release Notes:
- N/A
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Co-authored-by: cameron <cameron.studdstreet@gmail.com>
