function countDecodings(message: string): number {
  if (!message || message[0] === '0') return 0;

  let prev1 = 1;
  let prev2 = 1;

  for (let i = 1; i < message.length; i++) {
    let current = 0;

    let singleDigit = parseInt(message[i]);
    if (singleDigit >= 1 && singleDigit <= 9) {
      current += prev1;
    }

    let doubleDigit = parseInt(message.substring(i -1, i + 1));
    if (doubleDigit >= 10 && doubleDigit <= 26) {
      current += prev2;
    }

    prev2 = prev1;
    prev1 = current;
  }

  return prev1;
}

// No modificar: necesario para evaluar el resultado.
export { countDecodings };