Check Digit Calculator: Luhn, Verhoeff, Damm

A check digit is one or two characters appended to an identifier so that a typing mistake produces an invalid number instead of a different valid one. This calculator runs nine schemes side by side on whatever you type: Luhn (the mod 10 algorithm of payment cards and IMEI numbers), Verhoeff, Damm, the ISO/IEC 7064 pure systems MOD 11-2 (ORCID, ISNI), MOD 37-2 and MOD 97-10 (LEI; IBAN applies it after moving the country code), the hybrid systems MOD 11,10 and MOD 37,36, and Luhn mod 36 for alphanumeric codes. For each it shows the check character to append, the complete number, and whether your input already ends in a correct check. It then does what the textbooks only claim: it takes the complete number, tries every single-character substitution and every swap of two neighbouring characters, and counts how many each scheme would notice, listing a few it would not. On most numbers that makes the differences concrete — Luhn misses the 09↔90 swap and certain twin errors, Verhoeff and Damm catch every single error and every adjacent swap using digits only, and MOD 11-2 does too at the price of an 'X' check character. The census covers those two error types only; jump transpositions (1x3 ↔ 3x1) and phonetic slips such as 15 ↔ 50 are not counted. Spaces, hyphens and dots are ignored, letters are case-insensitive, and schemes whose alphabet does not fit the input are marked not applicable. MOD 97-10 always produces 02–98, as IBAN does, but also accepts the equivalent 00 or 01 when validating. A check digit guards against slips, not forgery: anyone can compute a valid one.