tz: Add BinaryMarshaler/BinaryUnmarshaler implementation to hash

Closes #47.

Signed-off-by: Evgenii Baidakov <[email protected]>

tz/digest.go

@@ -1,6 +1,8 @@
 package tz
 
 import (
+	"errors"
+	"fmt"
 	"hash"
 
 	"github.com/nspcc-dev/tzhash/gf127"
@@ -22,6 +24,8 @@ type digest struct {
 }
 
 // New returns a new [hash.Hash] computing the Tillich-Zémor checksum.
+// The Hash also implements encoding.BinaryMarshaler and encoding.BinaryUnmarshaler
+// to marshal and unmarshal the internal state of the hash.
 func New() hash.Hash {
 	d := new(digest)
 	d.Reset()
@@ -122,3 +126,50 @@ func mulBitRightGeneric(c00, c10, c01, c11 *GF127, bit bool, tmp *GF127) {
 		*c11 = *tmp
 	}
 }
+
+// MarshalBinary implements [encoding.BinaryMarshaler].
+func (d *digest) MarshalBinary() ([]byte, error) {
+	var (
+		b          = make([]byte, Size)
+		start, end int
+	)
+
+	for i, a := range d.x {
+		state, err := a.MarshalBinary()
+		if err != nil {
+			return nil, err
+		}
+
+		start = gf127.ByteSize * i
+		end = start + gf127.ByteSize
+
+		copy(b[start:end], state)
+	}
+
+	return b, nil
+}
+
+// UnmarshalBinary implements [encoding.BinaryUnmarshaler].
+func (d *digest) UnmarshalBinary(b []byte) error {
+	if len(b) != Size {
+		return errors.New("tz: invalid hash state size")
+	}
+
+	var (
+		start, end int
+	)
+
+	for i := 0; i < 4; i++ {
+		var g GF127
+		start = gf127.ByteSize * i
+		end = start + gf127.ByteSize
+
+		if err := g.UnmarshalBinary(b[start:end]); err != nil {
+			return fmt.Errorf("gf127 unmarshal: %w", err)
+		}
+
+		d.x[i] = g
+	}
+
+	return nil
+}

tz/digest_test.go

@@ -0,0 +1,56 @@
+package tz
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func newDigest() *digest {
+	d := &digest{}
+	d.Reset()
+	return d
+}
+
+func Test_digest_marshaling(t *testing.T) {
+	var (
+		d              = newDigest()
+		marshaledState []byte
+		hashSum        []byte
+	)
+
+	for i := byte(0); i < 10; i++ {
+		n, err := d.Write([]byte{i})
+		require.NoError(t, err)
+		require.Equal(t, 1, n)
+
+		t.Run("marshal", func(t *testing.T) {
+			marshaledState, err = d.MarshalBinary()
+
+			require.NoError(t, err)
+			require.Len(t, marshaledState, Size)
+
+			hashSum = d.Sum(nil)
+			require.Len(t, hashSum, Size)
+		})
+
+		t.Run("unmarshal", func(t *testing.T) {
+			unmarshalDigest := newDigest()
+			err = unmarshalDigest.UnmarshalBinary(marshaledState)
+			require.NoError(t, err)
+
+			unmarshalDigestHash := unmarshalDigest.Sum(nil)
+			require.Len(t, unmarshalDigestHash, Size)
+
+			require.Equal(t, hashSum, unmarshalDigestHash)
+		})
+	}
+
+	t.Run("invalid unmarshal", func(t *testing.T) {
+		unmarshalDigest := newDigest()
+		state := []byte{1, 2, 3}
+
+		err := unmarshalDigest.UnmarshalBinary(state)
+		require.Error(t, err)
+	})
+}