FA-72440 / Check-digit algorithms / Member archive

ISBN-13 conversion keeps the ISBN-10 check character · case 05

Converted numbers end with the old mod-11 check (sometimes X) instead of an EAN check digit.

Member previewVariant 5 · 3 implementations · 8 checks per implementation

Case contract

Convert a hyphenated or plain ISBN-10 to ISBN-13. The input must be a valid ISBN-10 (mod 11, X allowed as the final character) or None is returned. The result is "978" + the nine ISBN-10 payload digits + a freshly computed EAN-13 check digit (weights 1,3 alternating from the left).

Why this case matters

Library and retail systems migrate legacy ISBN-10 records to 13-digit EAN identifiers.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression ["8449627923"]	`"9788449627923"`	`"9788449627927"`	Failed

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