FA-72587 / Check-digit algorithms / Member archive

Resident ID maps the remainder straight to a digit · case 02

Check characters are wrong for all remainders except 10-to-X style coincidences.

Member previewVariant 2 · 3 implementations · 8 checks per implementation

Case contract

Check character of an 18-character resident identity number: seventeen ASCII digits followed by a digit or X (lowercase x accepted). Weights are 2^(17-i) mod 11 for 0-based i; the check character is "10X98765432"[sum % 11]. Return [check character, whether the 18th character matches case-insensitively].

Why this case matters

Account opening flows validate national identity numbers before identity verification.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression ["445793197510028320"]	`["2", false]`	`["X", false]`	Failed

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