object: add Prefix type

CHANGELOG.md

@@ -21,6 +21,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
    interoperable with Git packfile index files.
    ([#12](https://github.com/gg-scm/gg-git/issues/12))
 -  `packfile.ReadHeader` enables random access to a packfile.
+-  `object.Prefix` allows marshaling and unmarshaling the `"blob 42\x00"` prefix
+   used as part of the Git object hash.
 
 ### Changed
 

object/object.go

@@ -22,6 +22,7 @@ https://git-scm.com/book/en/v2/Git-Internals-Git-Objects
 package object
 
 import (
+	"bytes"
 	"crypto/sha1"
 	"fmt"
 	"io"
@@ -64,6 +65,57 @@ func BlobSum(r io.Reader, size int64) (githash.SHA1, error) {
 	return sum, nil
 }
 
+// Prefix is a parsed Git object prefix like "blob 42\x00".
+type Prefix struct {
+	Type Type
+	Size int64
+}
+
+// MarshalBinary returns the result of AppendPrefix.
+func (p Prefix) MarshalBinary() ([]byte, error) {
+	if !p.Type.IsValid() {
+		return nil, fmt.Errorf("marshal git object prefix: unknown type %q", p.Type)
+	}
+	if p.Size < 0 {
+		return nil, fmt.Errorf("marshal git object prefix: negative size")
+	}
+	return AppendPrefix(nil, p.Type, p.Size), nil
+}
+
+// UnmarshalBinary parses an object prefix.
+func (p *Prefix) UnmarshalBinary(data []byte) error {
+	if len(data) == 0 || data[len(data)-1] != 0 {
+		return fmt.Errorf("unmarshal git object prefix: does not end with NUL")
+	}
+	typeEnd := bytes.IndexByte(data, ' ')
+	if typeEnd == -1 {
+		return fmt.Errorf("unmarshal git object prefix: missing space")
+	}
+	typ := Type(data[:typeEnd])
+	if !typ.IsValid() {
+		return fmt.Errorf("unmarshal git object prefix: unknown type %q", typ)
+	}
+	sizeStart := typeEnd + 1
+	sizeEnd := len(data) - 1
+	size, err := strconv.ParseInt(string(data[sizeStart:sizeEnd]), 10, 64)
+	if err != nil {
+		return fmt.Errorf("unmarshal git object prefix: size: %v", err)
+	}
+	if size < 0 {
+		return fmt.Errorf("unmarshal git object prefix: negative size")
+	}
+	p.Type = typ
+	p.Size = size
+	return nil
+}
+
+// String returns the prefix without the trailing NUL byte.
+func (p Prefix) String() string {
+	buf := AppendPrefix(nil, p.Type, p.Size)
+	buf = buf[:len(buf)-1]
+	return string(buf)
+}
+
 // AppendPrefix appends a Git object prefix (e.g. "blob 42\x00")
 // to a byte slice.
 func AppendPrefix(dst []byte, typ Type, n int64) []byte {

object/object_test.go

@@ -17,12 +17,18 @@
 package object
 
 import (
+	"encoding"
 	"strings"
 	"testing"
 
 	"gg-scm.io/pkg/git/githash"
 )
 
+var (
+	_ encoding.BinaryMarshaler   = Prefix{}
+	_ encoding.BinaryUnmarshaler = new(Prefix)
+)
+
 func TestBlobSum(t *testing.T) {
 	tests := []struct {
 		data string
@@ -56,6 +62,56 @@ func TestBlobSum(t *testing.T) {
 	})
 }
 
+func TestPrefixUnmarshalBinary(t *testing.T) {
+	tests := []struct {
+		data      string
+		want      Prefix
+		wantError bool
+	}{
+		{
+			data: "blob 0\x00",
+			want: Prefix{Type: TypeBlob, Size: 0},
+		},
+		{
+			data: "tree 42\x00",
+			want: Prefix{Type: TypeTree, Size: 42},
+		},
+		{
+			data:      "tree abc\x00",
+			wantError: true,
+		},
+		{
+			data:      "tree -42\x00",
+			wantError: true,
+		},
+		{
+			data:      "foo 42\x00",
+			wantError: true,
+		},
+		{
+			data:      "blob 0",
+			wantError: true,
+		},
+	}
+	for _, test := range tests {
+		var got Prefix
+		err := got.UnmarshalBinary([]byte(test.data))
+		if err != nil {
+			if !test.wantError {
+				t.Errorf("new(Prefix).UnmarshalBinary([]byte(%q)) = %v; want <nil>", test.data, err)
+			}
+			continue
+		}
+		if test.wantError {
+			t.Errorf("new(Prefix).UnmarshalBinary([]byte(%q)) = <nil>; want error", test.data)
+			continue
+		}
+		if got != test.want {
+			t.Errorf("new(Prefix).UnarshalBinary([]byte(%q)) yields %+v; want %+v", test.data, got, test.want)
+		}
+	}
+}
+
 func hashLiteral(s string) githash.SHA1 {
 	var h githash.SHA1
 	if err := h.UnmarshalText([]byte(s)); err != nil {