vigenere with functions p4

vigenere_p4.ts

const key = "password";
const alphabet = "abcdefghijklmnopqrstuvwxyz";
const L = alphabet.length;

type cipher_equation = (Xi:number, Ki:number, L:number) => number;
const encode_char = (Xi:number, Ki:number, L:number) => (Xi + Ki) % L;
const decode_char = (Ci:number, Ki:number, L:number) => (Ci - Ki) >= 0 ? (Ci - Ki) % L : (Ci - Ki + L) % L;

function repeat_key(length:number):string {
  let new_key = key;

  while(new_key.length < length){
    new_key = new_key.repeat(2);
  }

  return new_key.slice(0, length + 1);
}

function vigenere_cipher(str:string, equation:cipher_equation):string {
  const key = repeat_key(str.length);
  return str
    .split("")
    .map((C, i) => {
      const Ci = alphabet.indexOf(C);
      const K = key[i];
      const Ki = alphabet.indexOf(K);

      if(Ci === -1) return C;

      const Ri = equation(Ci, Ki, L);

      return alphabet[Ri];
    })
    .join("");
}

const encode = (str:string) => vigenere_cipher(str, encode_char);
const decode = (str:string) => vigenere_cipher(str, decode_char);