Bound common-line pruning to the local window - #46
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Byron
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March 23, 2026 04:13
Fix should_prune_common_line() so the backward scan starts at pos.saturating_sub(WINDOW_SIZE) instead of the absolute WINDOW_SIZE index. The old bound made the scanned range grow with pos, which turned the preprocessing heuristic into an increasingly expensive rescan on highly repetitive inputs and caused the gix-merge clusterfuzz testcase to time out. Add a regression test that proves distant context outside the local window does not affect the pruning decision. Co-authored-by: Sebastian Thiel <sebastian.thiel@icloud.com>
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This PR fixes a performance regression. I validated the work done by Codex, also by running the Gitoxide test-suite against it. It's not the largest in the world for diffing, but it's something.
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Fix should_prune_common_line() so the backward scan starts at pos.saturating_sub(WINDOW_SIZE) instead of the absolute WINDOW_SIZE index.
The old bound made the scanned range grow with pos, which turned the preprocessing heuristic into an increasingly expensive rescan on highly repetitive inputs and caused the gix-merge clusterfuzz testcase to time out.
Add a regression test that proves distant context outside the local window does not affect the pruning decision.