cmdo/valkyr

const chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_";

// Use a lookup table to find the index.
const lookup = new Uint8Array(256);
for (let i = 0; i < chars.length; i++) {
  lookup[chars.charCodeAt(i)] = i;
}

function encode(arraybuffer: ArrayBuffer) {
  const bytes = new Uint8Array(arraybuffer);
  const len = bytes.length;
  let i,
    base64url = "";

  for (i = 0; i < len; i += 3) {
    base64url += chars[bytes[i] >> 2];
    base64url += chars[((bytes[i] & 3) << 4) | (bytes[i + 1] >> 4)];
    base64url += chars[((bytes[i + 1] & 15) << 2) | (bytes[i + 2] >> 6)];
    base64url += chars[bytes[i + 2] & 63];
  }

  if (len % 3 === 2) {
    base64url = base64url.substring(0, base64url.length - 1);
  } else if (len % 3 === 1) {
    base64url = base64url.substring(0, base64url.length - 2);
  }

  return base64url;
}

function decode(base64string: string) {
  const bufferLength = base64string.length * 0.75;
  const len = base64string.length;
  let i,
    p = 0,
    encoded1,
    encoded2,
    encoded3,
    encoded4;

  const bytes = new Uint8Array(bufferLength);

  for (i = 0; i < len; i += 4) {
    encoded1 = lookup[base64string.charCodeAt(i)];
    encoded2 = lookup[base64string.charCodeAt(i + 1)];
    encoded3 = lookup[base64string.charCodeAt(i + 2)];
    encoded4 = lookup[base64string.charCodeAt(i + 3)];

    bytes[p++] = (encoded1 << 2) | (encoded2 >> 4);
    bytes[p++] = ((encoded2 & 15) << 4) | (encoded3 >> 2);
    bytes[p++] = ((encoded3 & 3) << 6) | (encoded4 & 63);
  }

  return bytes.buffer;
}

export const base64url = {
  encode,
  decode
};