[#230] pkg: Implement string encode/decode methods on Checksum

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/checksum.go

@@ -3,8 +3,10 @@ package pkg
 import (
 	"bytes"
 	"crypto/sha256"
+	"encoding/hex"
 
 	"github.com/nspcc-dev/neofs-api-go/v2/refs"
+	"github.com/pkg/errors"
 )
 
 // Checksum represents v2-compatible checksum.
@@ -109,3 +111,35 @@ func (c *Checksum) UnmarshalJSON(data []byte) error {
 	return (*refs.Checksum)(c).
 		UnmarshalJSON(data)
 }
+
+func (c *Checksum) String() string {
+	return hex.EncodeToString(
+		(*refs.Checksum)(c).
+			GetSum(),
+	)
+}
+
+// Parse parses Checksum from its string representation.
+func (c *Checksum) Parse(s string) error {
+	data, err := hex.DecodeString(s)
+	if err != nil {
+		return err
+	}
+
+	var typ refs.ChecksumType
+
+	switch ln := len(data); ln {
+	default:
+		return errors.Errorf("unsupported checksum length %d", ln)
+	case sha256.Size:
+		typ = refs.SHA256
+	case 64:
+		typ = refs.TillichZemor
+	}
+
+	cV2 := (*refs.Checksum)(c)
+	cV2.SetType(typ)
+	cV2.SetSum(data)
+
+	return nil
+}

pkg/checksum_test.go

@@ -90,4 +90,12 @@ func TestChecksumEncoding(t *testing.T) {
 
 		require.Equal(t, cs, cs2)
 	})
+
+	t.Run("string", func(t *testing.T) {
+		cs2 := NewChecksum()
+
+		require.NoError(t, cs2.Parse(cs.String()))
+
+		require.Equal(t, cs, cs2)
+	})
 }