Function Convert

tensorflow/core/lib/strings/base64.cc:48–59

Converts a char (8 bits) into a 6-bit value for decoding. If the input char is invalid for base64 encoding, the return value has at least its upper 25 bits set.

// is invalid for base64 encoding, the return value has at least its upper 25
// bits set.
inline uint32 Convert(char x) {
  // If x < 128, then we look up x in the table. If x is valid, then the table
  // will have a value <= 0x3F, otherwise the table will have -1. If x >= 128,
  // we still do some table lookup, but the value is ignored since we explicitly
  // set the high bit of y to 1. Either way, y is negative (high bit set) in
  // case of error.
  const int8 y = kBase64Bytes[x & 0x7F] | (x & 0x80);
  // Casting from int8 to int32 preserves sign by sign extension. If y was
  // negative, at least its 25 high bits of the return value are set.
  const int32 z = static_cast<int32>(y);
  return static_cast<uint32>(z);
}

Status DecodeThreeChars(const char* codes, char* result) {
  const uint32 packed = (Convert(codes[0]) << 18) | (Convert(codes[1]) << 12) |