FA-75185 / CRDT convergence / Member archive

Visible index translation: negative delete indices wrap to the end · case 05

A delete at index minus one silently removes the last visible character.

Member previewVariant 5 · 3 implementations · 9 checks per implementation

Case contract

A sequence CRDT holds [id, char, deleted]. Local edits use visible indices: ["insert", i, ch] (0 <= i <= visible length) references the visible element at i-1 (None at 0), is placed immediately after it, and gets id "L<k>" from a counter advanced only by accepted inserts; ["delete", i] (0 <= i < visible length) tombstones the visible element at i. Out-of-range edits yield ["reject", i]. Return the emitted ops and visible text.

Why this case matters

Editors translate cursor positions into identifier-based CRDT operations; tombstones make that translation error-prone.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
out-of-range edits are rejected	`{"ops": [["reject", 8], ["reject", 2], ["del", "e1"], ["reject", -1], ["ins", "L1", "e0"]], "text": "ak"}`	`{"ops": [["reject", 8], ["reject", 2], ["reject", -1], ["reject", -1], ["ins", "L1", "e0"]], "text": "akb"}`	Failed

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