src/base64Decode.js

/**
 * @license
 * Copyright 2020 The Emscripten Authors
 * SPDX-License-Identifier: MIT
 */

#if WASM2JS && 0 // TODO: Figure out a way to enable this kind of sharing.

// Binaryen defines the following function if Wasm2JS is being used:
//   function base64DecodeToExistingUint8Array(uint8Array, offset, b64);
// so should reuse that when available. However that lives inside the asm module
// for the time being, so cannot access it directly from here. Hence this block
// is disabled atm.

function base64Decode(b64) {
#if ASSERTIONS
  assert(b64.length % 4 == 0);
#endif
  return base64DecodeToExistingUint8Array(new Uint8Array(b64.length*3>>2), 0, b64);
}

#else

// Precreate a reverse lookup table from chars "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/" back to bytes to make decoding fast.
for (var base64ReverseLookup = new Uint8Array(123/*'z'+1*/), i = 25; i >= 0; --i) {
  base64ReverseLookup[48+i] = 52+i; // '0-9'
  base64ReverseLookup[65+i] = i; // 'A-Z'
  base64ReverseLookup[97+i] = 26+i; // 'a-z'
}
base64ReverseLookup[43] = 62; // '+'
base64ReverseLookup[47] = 63; // '/'


// Decodes a _known valid_ base64 string (without validation) and returns it as a new Uint8Array.
// Benchmarked to be around 5x faster compared to a simple
// "Uint8Array.from(atob(b64), c => c.charCodeAt(0))" (TODO: perhaps use this form in -Oz builds?)
/** @noinline */
function base64Decode(b64) {
#if ENVIRONMENT_MAY_BE_NODE
  if (typeof ENVIRONMENT_IS_NODE !== 'undefined' && ENVIRONMENT_IS_NODE) {
    var buf = Buffer.from(b64, 'base64');
    return new Uint8Array(buf.buffer, buf.byteOffset, buf.byteLength);
  }
#endif

#if ASSERTIONS
  assert(b64.length % 4 == 0);
#endif
  var b1, b2, i = 0, j = 0, bLength = b64.length, output = new Uint8Array((bLength*3>>2) - (b64[bLength-2] == '=') - (b64[bLength-1] == '='));
  for (; i < bLength; i += 4, j += 3) {
    b1 = base64ReverseLookup[b64.charCodeAt(i+1)];
    b2 = base64ReverseLookup[b64.charCodeAt(i+2)];
    output[j] = base64ReverseLookup[b64.charCodeAt(i)] << 2 | b1 >> 4;
    output[j+1] = b1 << 4 | b2 >> 2;
    output[j+2] = b2 << 6 | base64ReverseLookup[b64.charCodeAt(i+3)];
  }
  return output;
}

#endif // ~WASM2JS