damm93: visible ASCII Damm check digits

The Damm algorithm is a check digit algorithm that detects every single-character error and every adjacent transposition. Prepending leading zeros does not change the check digit.

damm93 applies it to the 93 visible ASCII characters other than backslash, with matching implementations in C, PHP and JavaScript. Each one is a library plus a command-line tool, and all three give identical output.

The alphabet runs from 0 to ~ (skipping backslash), then ! to /, so base93 zero is 0 and the digits keep their face values:

0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[]^_`abcdefghijklmnopqrstuvwxyz{|}~!"#$%&'()*+,-./

Input is case-sensitive. Space, backslash, control characters (tab, newline, DEL) and anything outside ASCII are invalid.

Things to know before using damm93:

• The check digit is always visible. Space is not in the alphabet, so trimming whitespace can never remove a check digit, and codes cannot contain spaces.
• No backslash either. Backslash is the escape character in JSON, C, regular expressions and most shells, so leaving it out removes the worst quoting problems. The check digit can still be a quote, $, & or similar, so quote codes on the command line (damm93 -a 'x=1;y=2') and escape them in SQL and shell scripts.
• Damm only guarantees single-character errors and adjacent swaps. That's ideal for human typing errors. For data corrupted in transit, a CRC detects far more kinds of damage.

Usage

npm install damm93

import { checkDigit, isValid, value, INVALID } from 'damm93';

checkDigit('World');  // 'q', or INVALID (null)
isValid('Worldq');    // true
value('H');           // 0-92, or -1 if not in the alphabet

The package is a dependency-free ES module with TypeScript declarations, and includes the damm93 command (npx damm93). C and PHP implementations with identical output are in the GitHub repository.

Licence

BSD 2-Clause