base128.js

const encoder = new TextEncoder();
const decoder = new TextDecoder('utf-8', { ignoreBOM: true });

const U8 = {
  /** @param {string} data */
  encode: (data) => {
    return encoder.encode(data);
  },

  /** @param {Uint8Array} data */
  decode: (data) => {
    return decoder.decode(data);
  },
};

const Uint1Array = {
  /* API */

  encode: (bytes) => {
    const bits = new Uint8Array(bytes.length * 8);

    for (let cursor = 0, i = 0, l = bytes.length; i < l; i++) {
      const byte = bytes[i];

      bits[cursor++] = (byte >> 7) & 1;
      bits[cursor++] = (byte >> 6) & 1;
      bits[cursor++] = (byte >> 5) & 1;
      bits[cursor++] = (byte >> 4) & 1;
      bits[cursor++] = (byte >> 3) & 1;
      bits[cursor++] = (byte >> 2) & 1;
      bits[cursor++] = (byte >> 1) & 1;
      bits[cursor++] = byte & 1;
    }

    return bits;
  },

  decode: (bits) => {
    const bytes = new Uint8Array(Math.ceil(bits.length / 8));

    for (let cursor = 0, i = 0, l = bits.length; i < l; i += 8) {
      const b1 = bits[i];
      const b2 = bits[i + 1] | 0;
      const b3 = bits[i + 2] | 0;
      const b4 = bits[i + 3] | 0;
      const b5 = bits[i + 4] | 0;
      const b6 = bits[i + 5] | 0;
      const b7 = bits[i + 6] | 0;
      const b8 = bits[i + 7] | 0;

      bytes[cursor++] =
        (b1 << 7) |
        (b2 << 6) |
        (b3 << 5) |
        (b4 << 4) |
        (b5 << 3) |
        (b6 << 2) |
        (b7 << 1) |
        b8;
    }

    return bytes;
  },
};

const Uint7Array = {
  /* API */

  encode: (bits) => {
    const uint7 = new Uint8Array(Math.ceil(bits.length / 7) + 1);

    for (let cursor = 0, i = 0, l = bits.length; i < l; i += 7) {
      const b1 = bits[i];
      const b2 = bits[i + 1] | 0;
      const b3 = bits[i + 2] | 0;
      const b4 = bits[i + 3] | 0;
      const b5 = bits[i + 4] | 0;
      const b6 = bits[i + 5] | 0;
      const b7 = bits[i + 6] | 0;

      uint7[cursor++] =
        (b1 << 6) | (b2 << 5) | (b3 << 4) | (b4 << 3) | (b5 << 2) | (b6 << 1) | b7;
    }

    uint7[uint7.length - 1] = 7 - (bits.length % 7 || 7); // The last item contains the number of extra bits used for padding, that should be trimmed when decoding

    return uint7;
  },

  decode: (uint7) => {
    const bits = new Uint8Array(
      Math.max(0, (uint7.length - 1) * 7 - (uint7[uint7.length - 1] || 0))
    );

    for (let cursor = 0, i = 0, l = uint7.length - 1; i < l; i++) {
      const byte = uint7[i];

      bits[cursor++] = (byte >> 6) & 1;
      bits[cursor++] = (byte >> 5) & 1;
      bits[cursor++] = (byte >> 4) & 1;
      bits[cursor++] = (byte >> 3) & 1;
      bits[cursor++] = (byte >> 2) & 1;
      bits[cursor++] = (byte >> 1) & 1;
      bits[cursor++] = byte & 1;
    }

    return bits;
  },
};

const MAX_ARGUMENTS_LENGTH = 8192; // Above this threshold we risk a "call stack size exceeded" error
/** @param {ArrayLike<number> | Uint8Array | Uint16Array | Uint32Array} charCodes @returns {string} */
const fromCharCodes = (charCodes) => {
  if (!Array.isArray(charCodes)) return fromCharCodes(Array.from(charCodes));

  const { length } = charCodes;

  if (length <= MAX_ARGUMENTS_LENGTH) {
    // Small-enough length, no need to use chunks

    return String.fromCharCode.apply(String, charCodes);
  } else {
    // Large-enough length, encoding in chunks

    const chunks = [];

    for (let ci = 0, i = 0; i < length; ci++) {
      const slice = charCodes.slice(i, (i += MAX_ARGUMENTS_LENGTH));

      chunks[ci] = String.fromCharCode.apply(String, slice);
    }

    return chunks.join('');
  }
};

const Base128 = {
  /* API */

  encode: (data) => {
    return fromCharCodes(Uint7Array.encode(Uint1Array.encode(data)));
  },

  encodeStr: (data) => {
    return Base128.encode(U8.encode(data));
  },

  decode: (data) => {
    const uint7 = new Uint8Array(data.length);

    for (let i = 0, l = data.length; i < l; i++) {
      uint7[i] = data.charCodeAt(i);
    }

    return Uint1Array.decode(Uint7Array.decode(uint7));
  },

  decodeStr: (data) => {
    return U8.decode(Base128.decode(data));
  },
};

const message = '201873|720489|123qweas|123qweas';
const encoded = Base128.encodeStr(message);
const decoded = Base128.decodeStr(encoded);
console.log(encoded);
console.log(decoded);