Function MaxCompressedLength

snappy.cc:197–225

}  // namespace

size_t MaxCompressedLength(size_t source_bytes) {
  // Avoid integer overflow that could cause undersized buffer allocations.
  // Return std::numeric_limits<size_t>::max() to force a controlled allocation
  // failure.
  if (source_bytes > (std::numeric_limits<size_t>::max() - 32) / 7 * 6) {
    return std::numeric_limits<size_t>::max();
  }
  // Compressed data can be defined as:
  //    compressed := item* literal*
  //    item       := literal* copy
  //
  // The trailing literal sequence has a space blowup of at most 62/60
  // since a literal of length 60 needs one tag byte + one extra byte
  // for length information.
  //
  // Item blowup is trickier to measure.  Suppose the "copy" op copies
  // 4 bytes of data.  Because of a special check in the encoding code,
  // we produce a 4-byte copy only if the offset is < 65536.  Therefore
  // the copy op takes 3 bytes to encode, and this type of item leads
  // to at most the 62/60 blowup for representing literals.
  //
  // Suppose the "copy" op copies 5 bytes of data.  If the offset is big
  // enough, it will take 5 bytes to encode the copy op.  Therefore the
  // worst case here is a one-byte literal followed by a five-byte copy.
  // I.e., 6 bytes of input turn into 7 bytes of "compressed" data.
  //
  // This last factor dominates the blowup, so the final estimate is:
  return 32 + source_bytes + source_bytes / 6;
}

namespace {