FA-72838 / Check-digit algorithms / Member archive

ESR returns the carry as the check digit · case 03

Only references with final carry 0 or 5 get the right check digit.

Member previewVariant 3 · 3 implementations · 8 checks per implementation

Case contract

Swiss ESR/QR reference check digit (modulo 10 recursive): spaces ignored, nonempty ASCII digits required (else None). carry starts at 0 and for each digit becomes T[(carry + digit) mod 10] with T = 0,9,4,6,8,2,7,1,3,5; the check digit is (10 - carry) mod 10.

Why this case matters

Payment slips carry a recursive mod-10 reference that billing systems regenerate for reconciliation.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression ["80"]	`6`	`4`	Failed

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