A very simple mulberry cipher

mulberry.ts

const chars = [...'0123456789abcdefghijklmnopqrstuvwxyz'];
const charsMap = Object.fromEntries(chars.map((letter, i) => [letter, i]));

/**
 * Function that encodes a string using mulberry32.
 * @param what What do you want to encode?
 * @param seed Pick a seed, any seed.
 * @returns Encoded string.
 */
export const encode = (what: string, seed: number) => {
  const shuffledChars = chars.slice();
  shuffle(mulberry32(seed), shuffledChars);

  return [...what].map((letter, i) => shuffledChars[(charsMap[letter] + i) % shuffledChars.length]).join('');
};

/**
 * Function that decodes a string that was encoded using mulberry32.
 * @param seed Seed that was used for encoding the original string.
 * @param encoded THe original string.
 * @returns Decoded string.
 */
export const decode = (encoded: string, seed: number) => {
  const shuffledChars = chars.slice();
  shuffle(mulberry32(seed), shuffledChars);

  const shuffledMap = Object.fromEntries(shuffledChars.map((letter, i) => [letter, i]));
  const max = Math.ceil(encoded.length / shuffledChars.length) * shuffledChars.length;

  return [...encoded].map((letter, i) => chars[(shuffledMap[letter] - i + max) % shuffledChars.length]).join('');
};

const mulberry32 = (seed: number) => {
  return () => {
    seed |= 0;
    seed = (seed + 0x6d2b79f5) | 0;
    let t = Math.imul(seed ^ (seed >>> 15), 1 | seed);
    t = (t + Math.imul(t ^ (t >>> 7), 61 | t)) ^ t;
    return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
  };
};

const shuffle = (rand: () => number, array: unknown[]) => {
  for (let currentIndex = array.length - 1; currentIndex >= 0; currentIndex--) {
    const randomIndex = Math.floor(rand() * currentIndex);
    [array[currentIndex], array[randomIndex]] = [array[randomIndex], array[currentIndex]];
  }
};