type Hash interface {
// Write (via the embedded io.Writer interface) adds more data to the running hash.
// It never returns an error.
io.Writer
// Sum appends the current hash to b and returns the resulting slice.
// It does not change the underlying hash state.
Sum(b []byte) []byte
// Reset resets the Hash to its initial state.
Reset()
// Size returns the number of bytes Sum will return.
Size() int
// BlockSize returns the hash's underlying block size.
// The Write method must be able to accept any amount
// of data, but it may operate more efficiently if all writes
// are a multiple of the block size.
BlockSize() int
}