FA-75652 / Text diff and three-way merge / Member archive

Line mapping through hunks: the size change is applied with the wrong sign · case 02

Comments move up after insertions and down after deletions.

Member previewVariant 2 · 3 implementations · 8 checks per implementation

Case contract

Hunks [old_start, old_len, new_start, new_len] use unified conventions (start is the line before when the length is 0) and are sorted. An old line maps to None if it lies in a hunk's old range; otherwise it moves by the sum of new_len - old_len over hunks entirely before it. A zero-length hunk after line s affects lines from s+1 on. Return the mapped line numbers.

Why this case matters

Review tools re-anchor comments on changed files by mapping old line numbers through the diff.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
lines before and after a replacement	`[4, null, null, 6, 7]`	`[4, null, null, 8, 9]`	Failed

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This record includes three runnable implementations, regression fixtures, execution results, and source hashes.

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