FA-75443 / Text diff and three-way merge / Member archive

diff3 chunk merge: a change made only on their side is discarded · case 03

Edits from the other branch silently vanish when our side left the region untouched.

Member previewVariant 3 · 3 implementations · 9 checks per implementation

Case contract

Match base against ours and against theirs with difflib.SequenceMatcher (autojunk off). Base lines matched on both sides are stable anchors. Between consecutive anchors (and after the last) compare the three regions: unchanged ours takes theirs, unchanged theirs takes ours, identical changes take ours, anything else is a conflict rendered as "<<<<<<< ours", ours, "||||||| base", base, "=======", theirs, ">>>>>>> theirs". Return merged lines and the conflict count.

Why this case matters

Three-way merge tools classify each region between stable lines to decide between clean merges and conflicts.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
only theirs deleted a line	`{"conflicts": 0, "merged": ["a", "b", "c", "d", "e"]}`	`{"conflicts": 0, "merged": ["a", "b", "d", "e"]}`	Failed

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