/**
 * The most commonly used EAN standard is
 * the thirteen-digit EAN-13, while the
 * less commonly used 8-digit EAN-8 barcode was
 * introduced for use on small packages.
 * Also EAN/UCC-14 is used for Grouping of individual
 * trade items above unit level(Intermediate, Carton or Pallet).
 * For more info about EAN-14 checkout: https://www.gtin.info/itf-14-barcodes/
 * EAN consists of:
 * GS1 prefix, manufacturer code, product code and check digit
 * Reference: https://en.wikipedia.org/wiki/International_Article_Number
 * Reference: https://www.gtin.info/
 */

import assertString from './util/assertString';

/**
 * Define EAN Lengths; 8 for EAN-8; 13 for EAN-13; 14 for EAN-14
 * and Regular Expression for valid EANs (EAN-8, EAN-13, EAN-14),
 * with exact numeric matching of 8 or 13 or 14 digits [0-9]
 */
var LENGTH_EAN_8 = 8;
var LENGTH_EAN_14 = 14;
var validEanRegex = /^(\d{8}|\d{13}|\d{14})$/;

/**
 * Get position weight given:
 * EAN length and digit index/position
 *
 * @param {number} length
 * @param {number} index
 * @return {number}
 */
function getPositionWeightThroughLengthAndIndex(length, index) {
  if (length === LENGTH_EAN_8 || length === LENGTH_EAN_14) {
    return index % 2 === 0 ? 3 : 1;
  }
  return index % 2 === 0 ? 1 : 3;
}

/**
 * Calculate EAN Check Digit
 * Reference: https://en.wikipedia.org/wiki/International_Article_Number#Calculation_of_checksum_digit
 *
 * @param {string} ean
 * @return {number}
 */
function calculateCheckDigit(ean) {
  var checksum = ean.slice(0, -1).split('').map(function (_char, index) {
    return Number(_char) * getPositionWeightThroughLengthAndIndex(ean.length, index);
  }).reduce(function (acc, partialSum) {
    return acc + partialSum;
  }, 0);
  var remainder = 10 - checksum % 10;
  return remainder < 10 ? remainder : 0;
}

/**
 * Check if string is valid EAN:
 * Matches EAN-8/EAN-13/EAN-14 regex
 * Has valid check digit.
 *
 * @param {string} str
 * @return {boolean}
 */
export default function isEAN(str) {
  assertString(str);
  var actualCheckDigit = Number(str.slice(-1));
  return validEanRegex.test(str) && actualCheckDigit === calculateCheckDigit(str);
}