solution.ts

/**
 * ====================================================================================================
 *
 * Luhn Algorithm:
 *
 * - Given a number as input
 * - Split the number into two parts:
 *   - the last digit (called the "check digit")
 *   - all other digits (called the "payload digits")
 *
 * On the payload digits, run this logic:
 * - Starting from the last digit and working backwards, double every other digit
 * - If doubling a digit produces a number >= 10, replace it with the sum of its digits
 * - Sum all the digits
 * - Take the remainder of dividing this sum by 10
 * - Subtract the remainder from 10
 *
 * If the number you receive matches the "check digit", the input is valid
 *
 * ====================================================================================================
 */

const luhnAlgorithm = (number: number) => {
  const strNum = String(number);
  const payloadDigits = strNum.slice(0, strNum.length - 1);
  const checkDigit = parseInt(strNum[strNum.length - 1]);

  const payloadResult = payloadDigits
    .split("")
    .reverse()
    .reduce((result: Array<number>, str, idx) => {
      let num = parseInt(str);
      if (idx % 2 === 0) {
        num *= 2;

        if (num >= 10) {
          const nextNum = String(num)
            .split("")
            .reduce((r, s) => parseInt(s) + r, 0);

          num = nextNum;
        }
      }

      result.push(num);
      return result;
    }, [])
    .reduce((result, num) => num + result, 0);

  const divByTen = payloadResult % 10;

  const subByTen = 10 - divByTen;

  return subByTen === checkDigit;
};

/** Test cases */

const testCases: Array<[number, boolean]> = [
  [1111111111111111, false],
  [2222222222222222, false],
  [3333333333333333, false],
  [4444444444444444, false],
  [5555555555555555, false],
  [6666666666666666, false],
  [7777777777777777, false],
  [1234123412341234, false],
  [4321432143214321, false],
  [1111111111111117, true],
  [2222222222222224, true],
  [3333333333333331, true],
  [4444444444444448, true],
  [5555555555555557, true],
  [6666666666666664, true],
  [7777777777777771, true],
  [1234123412341238, true],
  [4321432143214327, true],
];

testCases.forEach(([input, expected], index) => {
  const received = luhnAlgorithm(input);

  if (received !== expected) {
    throw new Error(
      `Test case ${
        index + 1
      } failed: for input of ${input}, received ${received} instead of ${expected}`,
    );
  }
});