fast browser-friendly base32 codec

base32.ts

// base32 implementation for fun.

const alphabet = "0123456789abcdefghjkmnpqrtuvwxyz";
if (alphabet.length != 32) throw new Error("Base 32 requires a 32-character alphabet");

// chars that look similar (dwim):
const alias = { o: 0, i: 1, l: 1, s: 5 };

const reverseLookup = buildReverseTable();

function buildReverseTable() {
  const table = new Uint8Array(128);
  for (let i = 0; i < 128; i++) table[i] = 0;
  for (let i = 0; i < alphabet.length; i++) table[alphabet.charCodeAt(i)] = i;
  Object.keys(alias).forEach(k => {
    table[k.charCodeAt(0)] = alias[k];
  });
  // capitals should alias to lowercase:
  for (let i = 0x40; i < 0x60; i++) table[i] = table[i + 0x20];
  return table;
}

function encode(buffer: Uint8Array): string {
  // can't use Uint8Array here, because there's no way to map it into a string. :(
  const outBuffer: number[] = new Array(Math.ceil(1.6 * buffer.length));
  let acc = 0, bits = 0, j = 0;
  for (let i = 0; i < buffer.length; i++) {
    acc = (acc << 8) | buffer[i];
    bits += 8;
    while (bits >= 5) {
      outBuffer[j++] = (acc >> (bits - 5)) & 0x1f;
      acc = (acc & 0xfff);
      bits -= 5;
    }
  }
  if (bits > 0) outBuffer[j++] = (acc << (5 - bits)) & 0x1f;
  return outBuffer.map(c => alphabet[c]).join("");
}

function decode(buffer: string): Uint8Array {
  const outBuffer = new Uint8Array(Math.ceil(buffer.length * 0.625));
  let acc = 0, bits = 0, j = 0;
  for (let i = 0; i < buffer.length; i++) {
    acc = (acc << 5) | reverseLookup[buffer.charCodeAt(i) & 0x7f];
    bits += 5;
    while (bits >= 8) {
      outBuffer[j++] = (acc >> (bits - 8)) & 0xff;
      acc = (acc & 0xffff);
      bits -= 8;
    }
  }
  if (bits > 0) outBuffer[j++] = (acc << (8 - bits)) & 0xff;
  return outBuffer;
}

// quick demo/test, you can erase this part.
const data = new Uint8Array([
  0x17, 0x05, 0x76, 0x84, 0xbe, 0xa1, 0xf9, 0x33, 0x14, 0x18,
  0xb6, 0x33, 0xa8, 0xf3, 0x73, 0x11, 0x9d, 0x76, 0x5f, 0xd4
]);
const encoded = "2w2qd15ym7wk650rprtuhwvk26eqcqym";
console.log(data, encode(data));
console.log(encoded, decode(encoded));