function getLengths(b64: string): [number, number] {
  const len: number = b64.length;

  // if (len % 4 > 0) {
  //   throw new TypeError("Invalid string. Length must be a multiple of 4");
  // }

  // Trim off extra bytes after placeholder bytes are found
  // See: https://github.com/beatgammit/base64-js/issues/42
  let validLen: number = b64.indexOf('=');

  if (validLen === -1) {
    validLen = len;
  }

  const placeHoldersLen: number = validLen === len ? 0 : 4 - (validLen % 4);

  return [validLen, placeHoldersLen];
}

function init(
  lookup: string[],
  revLookup: number[],
  urlsafe: boolean = false,
) {
  function _byteLength(validLen: number, placeHoldersLen: number): number {
    return Math.floor(((validLen + placeHoldersLen) * 3) / 4 - placeHoldersLen);
  }

  function tripletToBase64(num: number): string {
    return (
      lookup[(num >> 18) & 0x3f] +
      lookup[(num >> 12) & 0x3f] +
      lookup[(num >> 6) & 0x3f] +
      lookup[num & 0x3f]
    );
  }

  function encodeChunk(buf: Uint8Array, start: number, end: number): string {
    const out: string[] = new Array((end - start) / 3);

    for (let i: number = start, curTriplet: number = 0; i < end; i += 3) {
      out[curTriplet++] = tripletToBase64(
        (buf[i] << 16) + (buf[i + 1] << 8) + buf[i + 2],
      );
    }

    return out.join('');
  }

  return {
    // base64 is 4/3 + up to two characters of the original data
    byteLength(b64: string): number {
      return _byteLength(...getLengths(b64));
    },
    toUint8Array(b64: string): Uint8Array {
      const [validLen, placeHoldersLen]: number[] = getLengths(b64);

      const buf = new Uint8Array(_byteLength(validLen, placeHoldersLen));

      // If there are placeholders, only get up to the last complete 4 chars
      const len: number = placeHoldersLen ? validLen - 4 : validLen;

      let tmp: number;
      let curByte: number = 0;
      let i: number;

      for (i = 0; i < len; i += 4) {
        tmp = (revLookup[b64.charCodeAt(i)] << 18) |
          (revLookup[b64.charCodeAt(i + 1)] << 12) |
          (revLookup[b64.charCodeAt(i + 2)] << 6) |
          revLookup[b64.charCodeAt(i + 3)];
        buf[curByte++] = (tmp >> 16) & 0xff;
        buf[curByte++] = (tmp >> 8) & 0xff;
        buf[curByte++] = tmp & 0xff;
      }

      if (placeHoldersLen === 2) {
        tmp = (revLookup[b64.charCodeAt(i)] << 2) |
          (revLookup[b64.charCodeAt(i + 1)] >> 4);
        buf[curByte++] = tmp & 0xff;
      } else if (placeHoldersLen === 1) {
        tmp = (revLookup[b64.charCodeAt(i)] << 10) |
          (revLookup[b64.charCodeAt(i + 1)] << 4) |
          (revLookup[b64.charCodeAt(i + 2)] >> 2);
        buf[curByte++] = (tmp >> 8) & 0xff;
        buf[curByte++] = tmp & 0xff;
      }

      return buf;
    },
    fromUint8Array(buf: Uint8Array): string {
      const maxChunkLength: number = 16383; // Must be multiple of 3

      const len: number = buf.length;

      const extraBytes: number = len % 3; // If we have 1 byte left, pad 2 bytes

      const len2: number = len - extraBytes;

      const parts: string[] = new Array(
        Math.ceil(len2 / maxChunkLength) + (extraBytes ? 1 : 0),
      );

      let curChunk: number = 0;
      let chunkEnd: number;

      // Go through the array every three bytes, we'll deal with trailing stuff later
      for (let i: number = 0; i < len2; i += maxChunkLength) {
        chunkEnd = i + maxChunkLength;
        parts[curChunk++] = encodeChunk(
          buf,
          i,
          chunkEnd > len2 ? len2 : chunkEnd,
        );
      }

      let tmp: number;

      // Pad the end with zeros, but make sure to not forget the extra bytes
      if (extraBytes === 1) {
        tmp = buf[len2];
        parts[curChunk] = lookup[tmp >> 2] + lookup[(tmp << 4) & 0x3f];
        if (!urlsafe) parts[curChunk] += '==';
      } else if (extraBytes === 2) {
        tmp = (buf[len2] << 8) | (buf[len2 + 1] & 0xff);
        parts[curChunk] = lookup[tmp >> 10] +
          lookup[(tmp >> 4) & 0x3f] +
          lookup[(tmp << 2) & 0x3f];
        if (!urlsafe) parts[curChunk] += '=';
      }

      return parts.join('');
    },
  };
}

const lookup: string[] = [];
const revLookup: number[] = [];
const code: string =
  'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_';

for (let i: number = 0, l = code.length; i < l; ++i) {
  lookup[i] = code[i];
  revLookup[code.charCodeAt(i)] = i;
}

export const { byteLength, toUint8Array, fromUint8Array } = init(
  lookup,
  revLookup,
  true,
);