6 files changed, 266 insertions, 0 deletions
diff --git a/object/tag/parse.go b/object/tag/parse.go
new file mode 100644
index 00000000..f24d5965
--- /dev/null
+++ b/object/tag/parse.go
@@ -0,0 +1,89 @@
+package tag
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+
+	objectid "codeberg.org/lindenii/furgit/object/id"
+	objectsignature "codeberg.org/lindenii/furgit/object/signature"
+	objecttype "codeberg.org/lindenii/furgit/object/type"
+)
+
+// Parse decodes a tag object body.
+func Parse(body []byte, algo objectid.Algorithm) (*Tag, error) {
+	t := new(Tag)
+	i := 0
+
+	var haveTarget, haveType bool
+
+	for i < len(body) {
+		rel := bytes.IndexByte(body[i:], '\n')
+		if rel < 0 {
+			return nil, errors.New("object: tag: missing newline")
+		}
+
+		line := body[i : i+rel]
+		i += rel + 1
+
+		if len(line) == 0 {
+			break
+		}
+
+		key, value, found := bytes.Cut(line, []byte{' '})
+		if !found {
+			return nil, errors.New("object: tag: malformed header")
+		}
+
+		switch string(key) {
+		case "object":
+			id, err := objectid.ParseHex(algo, string(value))
+			if err != nil {
+				return nil, fmt.Errorf("object: tag: object: %w", err)
+			}
+
+			t.Target = id
+			haveTarget = true
+		case "type":
+			ty, ok := objecttype.ParseName(string(value))
+			if !ok {
+				return nil, errors.New("object: tag: unknown target type")
+			}
+
+			t.TargetType = ty
+			haveType = true
+		case "tag":
+			t.Name = append([]byte(nil), value...)
+		case "tagger":
+			idt, err := objectsignature.Parse(value)
+			if err != nil {
+				return nil, fmt.Errorf("object: tag: tagger: %w", err)
+			}
+
+			t.Tagger = idt
+		case "gpgsig", "gpgsig-sha256":
+			for i < len(body) {
+				nextRel := bytes.IndexByte(body[i:], '\n')
+				if nextRel < 0 {
+					return nil, errors.New("object: tag: unterminated gpgsig")
+				}
+
+				if body[i] != ' ' {
+					break
+				}
+
+				i += nextRel + 1
+			}
+		default:
+			// Ignore unknown headers for now.
+		}
+	}
+
+	if !haveTarget || !haveType {
+		return nil, errors.New("object: tag: missing required headers")
+	}
+
+	t.Message = append([]byte(nil), body[i:]...)
+
+	return t, nil
+}
diff --git a/object/tag/parse_test.go b/object/tag/parse_test.go
new file mode 100644
index 00000000..293350ed
--- /dev/null
+++ b/object/tag/parse_test.go
@@ -0,0 +1,47 @@
+package tag_test
+
+import (
+	"bytes"
+	"testing"
+
+	"codeberg.org/lindenii/furgit/internal/testgit"
+	objectid "codeberg.org/lindenii/furgit/object/id"
+	"codeberg.org/lindenii/furgit/object/tag"
+	objecttype "codeberg.org/lindenii/furgit/object/type"
+)
+
+func TestTagParseFromGit(t *testing.T) {
+	t.Parallel()
+	testgit.ForEachAlgorithm(t, func(t *testing.T, algo objectid.Algorithm) { //nolint:thelper
+		testRepo := testgit.NewRepo(t, testgit.RepoOptions{ObjectFormat: algo, Bare: true})
+		_, _, commitID := testRepo.MakeCommit(t, "subject\n\nbody")
+		tagID := testRepo.TagAnnotated(t, "v1", commitID, "tag message")
+
+		rawBody := testRepo.CatFile(t, "tag", tagID)
+
+		parsed, err := tag.Parse(rawBody, algo)
+		if err != nil {
+			t.Fatalf("ParseTag: %v", err)
+		}
+
+		if parsed.Target != commitID {
+			t.Fatalf("tag target mismatch: got %s want %s", parsed.Target, commitID)
+		}
+
+		if parsed.TargetType != objecttype.TypeCommit {
+			t.Fatalf("tag target type = %v, want %v", parsed.TargetType, objecttype.TypeCommit)
+		}
+
+		if !bytes.Equal(parsed.Name, []byte("v1")) {
+			t.Fatalf("tag name = %q, want %q", parsed.Name, "v1")
+		}
+
+		if parsed.Tagger == nil {
+			t.Fatalf("expected tagger")
+		}
+
+		if !bytes.Contains(parsed.Message, []byte("tag message")) {
+			t.Fatalf("tag message mismatch: %q", parsed.Message)
+		}
+	})
+}
diff --git a/object/tag/serialize.go b/object/tag/serialize.go
new file mode 100644
index 00000000..5f712950
--- /dev/null
+++ b/object/tag/serialize.go
@@ -0,0 +1,68 @@
+package tag
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+
+	objectheader "codeberg.org/lindenii/furgit/object/header"
+	objecttype "codeberg.org/lindenii/furgit/object/type"
+)
+
+// SerializeWithoutHeader renders the raw tag body bytes.
+func (tag *Tag) SerializeWithoutHeader() ([]byte, error) {
+	if tag.Target.Size() == 0 {
+		return nil, errors.New("object: tag: missing target id")
+	}
+
+	var buf bytes.Buffer
+	fmt.Fprintf(&buf, "object %s\n", tag.Target.String())
+
+	tyName, ok := objecttype.Name(tag.TargetType)
+	if !ok {
+		return nil, fmt.Errorf("object: tag: invalid target type %d", tag.TargetType)
+	}
+
+	buf.WriteString("type ")
+	buf.WriteString(tyName)
+	buf.WriteByte('\n')
+
+	buf.WriteString("tag ")
+	buf.Write(tag.Name)
+	buf.WriteByte('\n')
+
+	if tag.Tagger != nil {
+		taggerBytes, err := tag.Tagger.Serialize()
+		if err != nil {
+			return nil, err
+		}
+
+		buf.WriteString("tagger ")
+		buf.Write(taggerBytes)
+		buf.WriteByte('\n')
+	}
+
+	buf.WriteByte('\n')
+	buf.Write(tag.Message)
+
+	return buf.Bytes(), nil
+}
+
+// SerializeWithHeader renders the raw object (header + body).
+func (tag *Tag) SerializeWithHeader() ([]byte, error) {
+	body, err := tag.SerializeWithoutHeader()
+	if err != nil {
+		return nil, err
+	}
+
+	header, ok := objectheader.Encode(objecttype.TypeTag, int64(len(body)))
+	if !ok {
+		return nil, errors.New("object: tag: failed to encode object header")
+	}
+
+	raw := make([]byte, len(header)+len(body))
+	copy(raw, header)
+	copy(raw[len(header):], body)
+
+	return raw, nil
+}
diff --git a/object/tag/serialize_test.go b/object/tag/serialize_test.go
new file mode 100644
index 00000000..a1311c39
--- /dev/null
+++ b/object/tag/serialize_test.go
@@ -0,0 +1,35 @@
+package tag_test
+
+import (
+	"testing"
+
+	"codeberg.org/lindenii/furgit/internal/testgit"
+	objectid "codeberg.org/lindenii/furgit/object/id"
+	"codeberg.org/lindenii/furgit/object/tag"
+)
+
+func TestTagSerialize(t *testing.T) {
+	t.Parallel()
+	testgit.ForEachAlgorithm(t, func(t *testing.T, algo objectid.Algorithm) { //nolint:thelper
+		testRepo := testgit.NewRepo(t, testgit.RepoOptions{ObjectFormat: algo, Bare: true})
+		_, _, commitID := testRepo.MakeCommit(t, "subject\n\nbody")
+		tagID := testRepo.TagAnnotated(t, "v1", commitID, "tag message")
+
+		rawBody := testRepo.CatFile(t, "tag", tagID)
+
+		parsed, err := tag.Parse(rawBody, algo)
+		if err != nil {
+			t.Fatalf("ParseTag: %v", err)
+		}
+
+		rawObj, err := parsed.SerializeWithHeader()
+		if err != nil {
+			t.Fatalf("SerializeWithHeader: %v", err)
+		}
+
+		gotID := algo.Sum(rawObj)
+		if gotID != tagID {
+			t.Fatalf("tag id mismatch: got %s want %s", gotID, tagID)
+		}
+	})
+}
diff --git a/object/tag/tag.go b/object/tag/tag.go
new file mode 100644
index 00000000..4301557e
--- /dev/null
+++ b/object/tag/tag.go
@@ -0,0 +1,17 @@
+// Package tag provides representations, parsers, and serializers for tag objects.
+package tag
+
+import (
+	objectid "codeberg.org/lindenii/furgit/object/id"
+	objectsignature "codeberg.org/lindenii/furgit/object/signature"
+	objecttype "codeberg.org/lindenii/furgit/object/type"
+)
+
+// Tag represents a Git annotated tag object.
+type Tag struct {
+	Target     objectid.ObjectID
+	TargetType objecttype.Type
+	Name       []byte
+	Tagger     *objectsignature.Signature
+	Message    []byte
+}
diff --git a/object/tag/type.go b/object/tag/type.go
new file mode 100644
index 00000000..215103ab
--- /dev/null
+++ b/object/tag/type.go
@@ -0,0 +1,10 @@
+package tag
+
+import objecttype "codeberg.org/lindenii/furgit/object/type"
+
+// ObjectType returns TypeTag.
+func (tag *Tag) ObjectType() objecttype.Type {
+	_ = tag
+
+	return objecttype.TypeTag
+}