USPS Traditional ACS keyline MOD 10 check digit

getUSPSKeylineCheckDigit.spec.ts

const keylinesChecks: [string, number][] = [
  ['JLSTMS6796', 9],
  ['TMS1112/62', 3],
  ['218XN91LMS', 4],
  ['OTUBIKALAM', 8],
  ['ABC999NN//3', 2],
  ['ANIT 028', 1],
  ['0012 ////', 8],
  ['811N IS00 0111', 9],
  ['OTUB IKAL AM', 8],
  ['SEIR AMT', 4],
  ['1402 4981 49', 0],
  ['AEIO UW', 0],
  ['1234 5678 90', 7],
  ['DCS1 402', 0],
  ['0987 6543 21', 3],
];

describe.each(keylinesChecks)('Creating a keyline check digit', (input, expected) => {
  it(`should produce check digit ${expected} for keyline ${input}`, () => {
    expect(getUSPSKeylineCheckDigit(input)).toEqual(expected);
  });
});

getUSPSKeylineCheckDigit.ts

const RADIX_DECIMAL = 10;
const RADIX_HEX = 16;

const isNumber = (n: string): boolean => !isNaN(parseInt(n, RADIX_DECIMAL));

// Convert ascii characters to numeric values by zeroing out all but the lower four bits
const acsCharToInt = (c: string): number =>
  parseInt(c.charCodeAt(0).toString(RADIX_HEX).slice(-1), RADIX_HEX);

const isOdd = (n: number): boolean => n % 2 === 1;

// Reduce by summing figures until single figure
const sumDoubleDigits = (n: number): number => {
  if (n < 10) return n;

  const [a, b] = n
    .toString()
    .split('')
    .map((v) => parseInt(v, RADIX_DECIMAL));

  return sumDoubleDigits(a + b);
};

const lastFigure = (n: number): number =>
  parseInt(n.toString().split('').slice(-1).join(), RADIX_DECIMAL);

/**
 * For generating a USPS Traditional ACS keyline check digit
 * https://postalpro.usps.com/mnt/glusterfs/2019-10/TraditionalACSTechnicalGuide.pdf
 *
 * @param keyline A string for our own reference
 * @returns USPS ACS Traditional keyline MOD 10 check digit
 */
export function getUSPSKeylineCheckDigit(keyline: string) {
  const keySum = keyline
    // Remove whitespace
    .replace(/\s+/g, '')
    .split('')
    // Convert any letters to ACS defined numerical equivalent
    .map((val) => (isNumber(val) ? parseInt(val, RADIX_DECIMAL) : acsCharToInt(val)))
    // Multiply odd position keys by weighting factor of 2 ... a.k.a. double it
    .map((val, idx) => (isOdd(idx + 1) ? val * 2 : val))
    // Reduce double to single digits
    .map((val) => sumDoubleDigits(val))
    // Sum
    .reduce((a, b) => a + b, 0);

  // so we don't subtract 0 from 10
  if (lastFigure(keySum) === 0) return 0;

  return 10 - lastFigure(keySum);
}