FA-72799 / Check-digit algorithms / Member archive

Missing-digit recovery ignores the doubling at the unknown position · case 04

Unknown digits in doubled positions are recovered incorrectly.

Member previewVariant 4 · 3 implementations · 8 checks per implementation

Case contract

Recover the single unknown digit (marked "?") in a Luhn-protected number: input must be at least two characters, ASCII digits and exactly one "?" (else None). Return the digit 0-9 that makes the whole number Luhn-valid; when the unknown sits in a doubled position the needed contribution must be mapped back through the doubling map.

Why this case matters

Customer-service tools reconstruct a smudged or misheard digit of an account number.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression ["6?0334555"]	`6`	`3`	Failed

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