/** EAN-13 check digit: weights alternate 1,3 over the first 12 digits. */
export function ean13CheckDigit(first12: string): number {
  if (!/^\d{12}$/.test(first12)) throw new Error('EAN-13 body must be 12 digits')
  let sum = 0
  for (let i = 0; i < 12; i++) {
    sum += Number(first12[i]) * (i % 2 === 0 ? 1 : 3)
  }
  return (10 - (sum % 10)) % 10
}

export function isValidEan13(code: string): boolean {
  return /^\d{13}$/.test(code) && ean13CheckDigit(code.slice(0, 12)) === Number(code[12])
}