bool Base64Decode()

in app/src/base64.cc [139:198]

• 49 lines of code
• 24 McCabe index (conditional complexity)

bool Base64Decode(const std::string& input, std::string* output) {
  if (!output) {
    return false;
  }
  // All Base64 strings must be padded to 4 bytes, with optionally 1 or 2 of the
  // ending bytes removed.
  if (input.size() % 4 == 1) {
    return false;
  }
  // Workaround for if input and output are the same string.
  bool inplace = (output == &input);
  std::string inplace_buffer;
  std::string* output_ptr = inplace ? &inplace_buffer : output;
  output_ptr->resize(GetBase64DecodedSize(input));

  for (int i = 0, o = 0; i < input.size(); i += 4, o += 3) {
    char input0 = input[i + 0];
    char input1 = input[i + 1];
    // At the end of the string, missing bytes 2 and 3 are considered '='.
    char input2 = i + 2 < input.size() ? input[i + 2] : kBase64NullEnding;
    char input3 = i + 3 < input.size() ? input[i + 3] : kBase64NullEnding;
    // If any unknown characters appear, it's an error.
    if (kBase64TableReverse[input0] < 0 || kBase64TableReverse[input1] < 0 ||
        kBase64TableReverse[input2] < 0 || kBase64TableReverse[input3] < 0) {
      return false;
    }
    // If kBase64NullEnding appears anywhere but the last 1 or 2 characters,
    // or if it appears but input.size() % 4 != 0, it's an error.
    bool at_end = (i + 4 >= input.size());
    if ((input0 == kBase64NullEnding) || (input1 == kBase64NullEnding) ||
        (input2 == kBase64NullEnding && !at_end) ||
        (input2 == kBase64NullEnding && input3 != kBase64NullEnding) ||
        (input3 == kBase64NullEnding && !at_end)) {
      return false;
    }
    uint32_t b0 = kBase64TableReverse[input0] & 0x3f;
    uint32_t b1 = kBase64TableReverse[input1] & 0x3f;
    uint32_t b2 = kBase64TableReverse[input2] & 0x3f;
    uint32_t b3 = kBase64TableReverse[input3] & 0x3f;

    uint32_t stream = (b0 << 18) | (b1 << 12) | (b2 << 6) | b3;
    (*output_ptr)[o + 0] = (stream >> 16) & 0xFF;
    if (input2 != kBase64NullEnding) {
      (*output_ptr)[o + 1] = (stream >> 8) & 0xFF;
    } else if (((stream >> 8) & 0xFF) != 0) {
      // If there are any stale bits in this from input1, the text is malformed.
      return false;
    }
    if (input3 != kBase64NullEnding) {
      (*output_ptr)[o + 2] = (stream >> 0) & 0xFF;
    } else if (((stream >> 0) & 0xFF) != 0) {
      // If there are any stale bits in this from input2, the text is malformed.
      return false;
    }
  }
  if (inplace) {
    *output = inplace_buffer;
  }
  return true;
}