chainhash: Add DoubleHashRaw

DoubleHashRaw provides a simple function for doing double hashes.  Since
it uses the digest instead of making the caller allocate a byte slice, it
can be more memory efficient vs other double hash functions.

chaincfg/chainhash/hashfuncs.go

@@ -5,7 +5,10 @@
 
 package chainhash
 
-import "crypto/sha256"
+import (
+	"crypto/sha256"
+	"io"
+)
 
 // HashB calculates hash(b) and returns the resulting bytes.
 func HashB(b []byte) []byte {
@@ -31,3 +34,24 @@ func DoubleHashH(b []byte) Hash {
 	first := sha256.Sum256(b)
 	return Hash(sha256.Sum256(first[:]))
 }
+
+// DoubleHashRaw calculates hash(hash(w)) where w is the resulting bytes from
+// the given serialize function and returns the resulting bytes as a Hash.
+func DoubleHashRaw(serialize func(w io.Writer) error) Hash {
+	// Encode the transaction into the hash.  Ignore the error returns
+	// since the only way the encode could fail is being out of memory
+	// or due to nil pointers, both of which would cause a run-time panic.
+	h := sha256.New()
+	_ = serialize(h)
+
+	// This buf is here because Sum() will append the result to the passed
+	// in byte slice.  Pre-allocating here saves an allocation on the second
+	// hash as we can reuse it.  This allocation also does not escape to the
+	// heap, saving an allocation.
+	buf := make([]byte, 0, HashSize)
+	first := h.Sum(buf)
+	h.Reset()
+	h.Write(first)
+	res := h.Sum(buf)
+	return *(*Hash)(res)
+}

chaincfg/chainhash/hashfuncs_test.go

@@ -6,6 +6,7 @@ package chainhash
 
 import (
 	"fmt"
+	"io"
 	"testing"
 )
 
@@ -133,4 +134,20 @@ func TestDoubleHashFuncs(t *testing.T) {
 			continue
 		}
 	}
+
+	// Ensure the hash function which accepts a hash.Hash returns the expected
+	// result when given a hash.Hash that is of type SHA256.
+	for _, test := range tests {
+		serialize := func(w io.Writer) error {
+			w.Write([]byte(test.in))
+			return nil
+		}
+		hash := DoubleHashRaw(serialize)
+		h := fmt.Sprintf("%x", hash[:])
+		if h != test.out {
+			t.Errorf("DoubleHashRaw(%q) = %s, want %s", test.in, h,
+				test.out)
+			continue
+		}
+	}
 }