## Context
`apply_remote_edit` and `apply_local_edit` rebuild the fragment
`SumTree` through `FragmentBuilder`. Since #51941, the builder collapsed
the entire tree into a flat `Vec<Fragment>` (cloning every fragment) and
rebuilt a fresh tree from scratch on every call. The new tree shared no
nodes with the previous one, so each edit cost O(N) in the *total*
number of fragments: clone all fragments, allocate an entirely new tree,
and then free the whole previous tree on assignment — the last part
showing up as a large amount of time spent dropping `SumTree`s.
That batching was a deliberate win for the bulk `replace_all` path
(#51941, one `edit()` carrying millions of ranges), but it penalizes
every other caller — most notably `apply_remote_edit`, which runs once
per op in a loop in `apply_ops` and so rebuilt the whole tree per remote
operation.
## This change
Make `FragmentBuilder` chunk-based. Appended slices are kept as intact
`SumTree` subtrees, so they keep sharing nodes with the previous tree;
only individually pushed fragments are batched into `Vec`s.
`to_sum_tree` appends the shared subtrees (touching just the right
spine) and builds the loose runs in one pass, parallelizing the large
ones.
Net effect:
- Small edits on large/heavily-fragmented buffers (the
`apply_remote_edit` collaborative path) go back to O(edited + log N)
with a cheap drop, instead of O(total fragments).
- The bulk `replace_all` path keeps its batched build and is not
regressed.
## Benchmarks
Using the file from #38927 (619 MB CSV, 10,325,246 matches of `"` →
`""`), `release-fast`:
| Case | flatten (#51941) | this PR |
|---|---|---|
| `replace_all` (all matches) | 50.70 s | 45.16 s |
| replace 1 match (`--single`) | 16.0 ms | 16.0 ms |
`replace_all` is ~11% faster and not regressed. The
single-match-on-a-freshly-loaded-file case is unchanged, because a fresh
`Buffer::local` is barely fragmented, so even the from-scratch rebuild
is cheap there; the structural-sharing win is on heavily-fragmented
buffers receiving small edits.
The `--single` flag added to `editor_benchmarks` makes the latter case
measurable.
Release Notes:
- Improved the performance of applying edits to large buffers
---------
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Helps with https://github.com/zed-industries/zed/issues/38927
- **editor: Add a benchmark for find/replace**
- **text: batch fragment insertions before turning them into a SumTree**
## Context
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## Self-Review Checklist
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- [x] I've reviewed my own diff for quality, security, and reliability
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Release Notes:
- Improved performance of "Replace All" in buffer search
---------
Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>
