Method ReadSignedNumIncreasing

tensorflow/core/lib/strings/ordered_code.cc:488–529

}

bool OrderedCode::ReadSignedNumIncreasing(StringPiece* src, int64* result) {
  if (src->empty()) return false;
  const uint64 xor_mask = (!((*src)[0] & 0x80)) ? ~0ULL : 0ULL;
  const unsigned char first_byte = (*src)[0] ^ (xor_mask & 0xff);

  // now calculate and test length, and set x to raw (unmasked) result
  int len;
  uint64 x;
  if (first_byte != 0xff) {
    len = 7 - Log2Floor64(first_byte ^ 0xff);
    if (src->size() < static_cast<size_t>(len)) return false;
    x = xor_mask;  // sign extend using xor_mask
    for (int i = 0; i < len; ++i)
      x = (x << 8) | static_cast<unsigned char>((*src)[i]);
  } else {
    len = 8;
    if (src->size() < static_cast<size_t>(len)) return false;
    const unsigned char second_byte = (*src)[1] ^ (xor_mask & 0xff);
    if (second_byte >= 0x80) {
      if (second_byte < 0xc0) {
        len = 9;
      } else {
        const unsigned char third_byte = (*src)[2] ^ (xor_mask & 0xff);
        if (second_byte == 0xc0 && third_byte < 0x80) {
          len = 10;
        } else {
          return false;  // either len > 10 or len == 10 and #bits > 63
        }
      }
      if (src->size() < static_cast<size_t>(len)) return false;
    }
    x = LoadBigEndian64(src->data() + len - 8);
  }

  x ^= kLengthToMask[len];  // remove spurious header bits

  DCHECK_EQ(len, SignedEncodingLength(x)) << "invalid encoding";

  if (result) *result = x;
  src->remove_prefix(len);
  return true;
}

}  // namespace strings
}  // namespace tensorflow