FA-75343 / Text diff and three-way merge / Member archive

LCS edit script: a match extends the wrong neighbouring cell · case 03

Diffs contain more insertions and deletions than necessary around repeated lines.

Member previewVariant 3 · 3 implementations · 10 checks per implementation

Case contract

Given old and new line lists, fill a suffix LCS table L[i][j] for a[i:], b[j:], then walk from the start: equal lines are kept, otherwise delete when L[i+1][j] >= L[i][j+1] and insert otherwise; flush remaining old lines as deletions and new lines as insertions. Return [tag, line] ops with tags " ", "-", "+".

Why this case matters

Line diffs are built from a longest-common-subsequence table; the walk decides how edits are ordered and presented.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
repeated line after a replaced one	`[["+", "q"], [" ", "r3"], ["-", "r3"]]`	`[["-", "r3"], ["+", "q"], [" ", "r3"]]`	Failed

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