FA-72802 / Check-digit algorithms / Member archive

Missing-digit recovery measures the unknown from the left · case 02

Recovery succeeds only when the number length makes left and right parity agree.

Member previewVariant 2 · 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 ["3077312?96877436"]	`5`	`1`	Failed

MEMBER ARCHIVE

The complete case is available to members.

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

Member access is invitation-based. Sign in with your invited account to inspect the sources.