FA-72750 / Check-digit algorithms / Member archive

CPF second check digit reuses the first weight range · case 05

Most valid CPFs fail the second check digit.

Member previewVariant 5 · 3 implementations · 8 checks per implementation

Case contract

Brazilian CPF taxpayer number: dots and hyphens removed, then 11 ASCII digits that are not all the same digit (else "malformed"). k1 = (sum of digits 1-9 weighted 10..2) * 10 mod 11 mod 10; k2 = (digits 1-9 and the computed k1 weighted 11..2) * 10 mod 11 mod 10. Return [k1, k2, whether digits 10-11 equal them].

Why this case matters

Onboarding and invoicing flows validate CPF numbers before tax reporting.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression ["29622049718"]	`[0, 0, false]`	`[0, 2, false]`	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.