func bitrotWriterSum(w io.Writer) []byte {
	if bw, ok := w.(*wholeBitrotWriter); ok {
		return bw.Sum(nil)
	}
	return nil
}

// Returns the size of the file with bitrot protection
func bitrotShardFileSize(size int64, shardSize int64, algo BitrotAlgorithm) int64 {
	if algo != HighwayHash256S {
		return size
	}
	return ceilFrac(size, shardSize)*int64(algo.New().Size()) + size
}