Function decodeBase64

src/preview.cpp:856–888

const char encodeBase64Table[64 + 1] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

DataBuf decodeBase64(const std::string& src) {
  // create decoding table
  unsigned long invalid = 64;
  auto decodeBase64Table = std::vector<unsigned long>(256, invalid);
  for (unsigned long i = 0; i < 64; i++)
    decodeBase64Table[static_cast<unsigned char>(encodeBase64Table[i])] = i;

  // calculate dest size
  auto validSrcSize = static_cast<unsigned long>(
      std::count_if(src.begin(), src.end(), [&](unsigned char c) { return decodeBase64Table.at(c) != invalid; }));
  if (validSrcSize > ULONG_MAX / 3)
    return {};  // avoid integer overflow
  const unsigned long destSize = (validSrcSize * 3) / 4;

  // allocate dest buffer
  DataBuf dest(destSize);

  // decode
  for (unsigned long srcPos = 0, destPos = 0; destPos < destSize;) {
    unsigned long buffer = 0;
    for (int bufferPos = 3; bufferPos >= 0 && srcPos < src.size(); srcPos++) {
      unsigned long srcValue = decodeBase64Table[static_cast<unsigned char>(src[srcPos])];
      if (srcValue == invalid)
        continue;
      buffer |= srcValue << (bufferPos * 6);
      bufferPos--;
    }
    for (int bufferPos = 2; bufferPos >= 0 && destPos < destSize; bufferPos--, destPos++) {
      dest.write_uint8(destPos, static_cast<byte>((buffer >> (bufferPos * 8)) & 0xFF));
    }
  }
  return dest;
}

DataBuf decodeAi7Thumbnail(const DataBuf& src) {
  const byte* colorTable = src.c_data();