index.js

'use strict';

/* 
    Code based on https://en.wikipedia.org/wiki/Luhn_mod_N_algorithm
    with minimal modifications.
*/

module.exports = function calculateLuhnModN(codePointFromCharacter, characterFromCodePoint, n, input) {
  var factor = 2;
  var sum = 0;

  // Starting from the right and working leftwards is easier since 
  // the initial "factor" will always be "2" 
  for (var i = input.length - 1; i >= 0; i--) {
    var codePoint = codePointFromCharacter(input[i]);
    var addend = factor * codePoint;

    // Alternate the "factor" that each "codePoint" is multiplied by
    factor = (factor == 2) ? 1 : 2;

    // Sum the digits of the "addend" as expressed in base "n"
    addend = Math.floor(addend / n) + (addend % n);
    sum += addend;
  }

  // Calculate the number that must be added to the "sum" 
  // to make it divisible by "n"
  var remainder = sum % n;
  var checkCodePoint = (n - remainder) % n;

  return characterFromCodePoint(checkCodePoint);
}