fix(diff): correct overlap detection and bounds in linear space Myers

[KimNorgaard]

This commit resolves two critical pathfinding bugs in the linear space Myers algorithm (findMiddleSnake) that caused it to produce suboptimal diffs or silently fail to explore valid search spaces.

1. Fix forward search overlap detection: During the forward search overlap check (when delta is odd), the algorithm incorrectly checked the reverse vector vr at offset + (delta - k). However, the reverse search already maps its furthest-reaching x bounds to the same forward diagonal k. Checking the wrong index caused the algorithm to miss the true optimal middle snake, leading to longer, suboptimal edit scripts. This is fixed by correctly checking reverseIdx := offset + k.

2. Fix vector sizing and offset for negative delta: When sequence A is shorter than sequence B (delta < 0), the forward-diagonal k in the reverse search can reach down to -maxDiff + delta. Because offset was previously only adjusted for positive delta, calculating offset + k would result in negative indices. This caused the reverse search to silently trigger bounds checks and skip exploring half of its required search grid. The vectorSize and offset are now correctly adjusted when delta < 0 to accommodate the full bounds.

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses two significant bugs in the linear space Myers diff algorithm's findMiddleSnake function. The changes ensure that the algorithm correctly identifies the optimal middle snake and explores the entire necessary search space, preventing suboptimal diff results and silent failures due to incorrect array indexing.

Highlights

• Corrected overlap detection in Myers algorithm: Fixed an incorrect index calculation (offset + (delta - k) to offset + k) during the forward search overlap check, which previously led to suboptimal diffs by missing the true middle snake.
• Improved vector sizing for negative delta: Adjusted the vectorSize and offset calculations for cases where delta < 0 to prevent negative array indices and ensure the full search space is explored, resolving silent failures in the reverse search.

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[gemini-code-assist]

Code Review

This pull request addresses two critical bugs in the linear space Myers diff algorithm (findMiddleSnake). The first fix correctly calculates the vector size and offset when the first sequence is shorter than the second (delta < 0), preventing silent out-of-bounds errors and ensuring the entire search space is covered. The second fix corrects the overlap detection logic during the forward search, ensuring the algorithm finds the truly optimal edit script by checking the correct index in the reverse search vector. The changes are well-explained and appear to correctly resolve the bugs. I have one minor suggestion to improve the code's readability for future maintenance.

[ChBLA]

Looks good