7 files changed, 338 insertions, 0 deletions
diff --git a/object/commit/commit.go b/object/commit/commit.go
new file mode 100644
index 00000000..e2e087f5
--- /dev/null
+++ b/object/commit/commit.go
@@ -0,0 +1,18 @@
+// Package commit provides representations, parsers, and serializers for commit objects.
+package commit
+
+import (
+	objectid "codeberg.org/lindenii/furgit/object/id"
+	objectsignature "codeberg.org/lindenii/furgit/object/signature"
+)
+
+// Commit represents a Git commit object.
+type Commit struct {
+	Tree         objectid.ObjectID
+	Parents      []objectid.ObjectID
+	Author       objectsignature.Signature
+	Committer    objectsignature.Signature
+	Message      []byte
+	ChangeID     string
+	ExtraHeaders []ExtraHeader
+}
diff --git a/object/commit/extraheader.go b/object/commit/extraheader.go
new file mode 100644
index 00000000..79d4f9cc
--- /dev/null
+++ b/object/commit/extraheader.go
@@ -0,0 +1,7 @@
+package commit
+
+// ExtraHeader represents an extra header in a Git object.
+type ExtraHeader struct {
+	Key   string
+	Value []byte
+}
diff --git a/object/commit/parse.go b/object/commit/parse.go
new file mode 100644
index 00000000..9dcc930d
--- /dev/null
+++ b/object/commit/parse.go
@@ -0,0 +1,94 @@
+package commit
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+
+	objectid "codeberg.org/lindenii/furgit/object/id"
+	objectsignature "codeberg.org/lindenii/furgit/object/signature"
+)
+
+// Parse decodes a commit object body.
+func Parse(body []byte, algo objectid.Algorithm) (*Commit, error) {
+	c := new(Commit)
+
+	i := 0
+	for i < len(body) {
+		rel := bytes.IndexByte(body[i:], '\n')
+		if rel < 0 {
+			return nil, errors.New("object: commit: 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: commit: malformed header")
+		}
+
+		switch string(key) {
+		case "tree":
+			id, err := objectid.ParseHex(algo, string(value))
+			if err != nil {
+				return nil, fmt.Errorf("object: commit: tree: %w", err)
+			}
+
+			c.Tree = id
+		case "parent":
+			id, err := objectid.ParseHex(algo, string(value))
+			if err != nil {
+				return nil, fmt.Errorf("object: commit: parent: %w", err)
+			}
+
+			c.Parents = append(c.Parents, id)
+		case "author":
+			idt, err := objectsignature.Parse(value)
+			if err != nil {
+				return nil, fmt.Errorf("object: commit: author: %w", err)
+			}
+
+			c.Author = *idt
+		case "committer":
+			idt, err := objectsignature.Parse(value)
+			if err != nil {
+				return nil, fmt.Errorf("object: commit: committer: %w", err)
+			}
+
+			c.Committer = *idt
+		case "change-id":
+			c.ChangeID = string(value)
+		case "gpgsig", "gpgsig-sha256":
+			for i < len(body) {
+				nextRel := bytes.IndexByte(body[i:], '\n')
+				if nextRel < 0 {
+					return nil, errors.New("object: commit: unterminated gpgsig")
+				}
+
+				if body[i] != ' ' {
+					break
+				}
+
+				i += nextRel + 1
+			}
+		default:
+			c.ExtraHeaders = append(c.ExtraHeaders, ExtraHeader{
+				Key:   string(key),
+				Value: append([]byte(nil), value...),
+			})
+		}
+	}
+
+	if i > len(body) {
+		return nil, errors.New("object: commit: parser position out of bounds")
+	}
+
+	c.Message = append([]byte(nil), body[i:]...)
+
+	return c, nil
+}
diff --git a/object/commit/parse_test.go b/object/commit/parse_test.go
new file mode 100644
index 00000000..ad2c7aed
--- /dev/null
+++ b/object/commit/parse_test.go
@@ -0,0 +1,91 @@
+package commit_test
+
+import (
+	"bytes"
+	"fmt"
+	"testing"
+
+	"codeberg.org/lindenii/furgit/internal/testgit"
+	"codeberg.org/lindenii/furgit/object/commit"
+	objectid "codeberg.org/lindenii/furgit/object/id"
+)
+
+func TestCommitParseFromGit(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})
+		_, treeID, commitID := testRepo.MakeCommit(t, "subject\n\nbody")
+
+		rawBody := testRepo.CatFile(t, "commit", commitID)
+
+		parsed, err := commit.Parse(rawBody, algo)
+		if err != nil {
+			t.Fatalf("ParseCommit: %v", err)
+		}
+
+		if parsed.Tree != treeID {
+			t.Fatalf("tree id mismatch: got %s want %s", parsed.Tree, treeID)
+		}
+
+		if len(parsed.Parents) != 0 {
+			t.Fatalf("parent count = %d, want 0", len(parsed.Parents))
+		}
+
+		if !bytes.Equal(parsed.Author.Name, []byte("Test Author")) {
+			t.Fatalf("author name = %q, want %q", parsed.Author.Name, "Test Author")
+		}
+
+		if !bytes.Equal(parsed.Committer.Name, []byte("Test Committer")) {
+			t.Fatalf("committer name = %q, want %q", parsed.Committer.Name, "Test Committer")
+		}
+
+		if !bytes.Contains(parsed.Message, []byte("subject")) {
+			t.Fatalf("commit message missing subject: %q", parsed.Message)
+		}
+	})
+}
+
+func TestCommitParseMultipleParents(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})
+
+		_, treeID := testRepo.MakeSingleFileTree(t, "file.txt", []byte("merge-content\n"))
+		parent1 := testRepo.CommitTree(t, treeID, "parent-one")
+		parent2 := testRepo.CommitTree(t, treeID, "parent-two", parent1)
+
+		rawCommit := fmt.Sprintf(
+			"tree %s\nparent %s\nparent %s\nauthor Test Author <test@example.org> 1234567890 +0000\ncommitter Test Committer <committer@example.org> 1234567890 +0000\n\nMerge commit\n",
+			treeID,
+			parent1,
+			parent2,
+		)
+		mergeID := testRepo.HashObject(t, "commit", []byte(rawCommit))
+		rawBody := testRepo.CatFile(t, "commit", mergeID)
+
+		parsed, err := commit.Parse(rawBody, algo)
+		if err != nil {
+			t.Fatalf("ParseCommit(merge): %v", err)
+		}
+
+		if parsed.Tree != treeID {
+			t.Fatalf("merge tree = %s, want %s", parsed.Tree, treeID)
+		}
+
+		if len(parsed.Parents) != 2 {
+			t.Fatalf("merge parent count = %d, want 2", len(parsed.Parents))
+		}
+
+		if parsed.Parents[0] != parent1 {
+			t.Fatalf("merge parent[0] = %s, want %s", parsed.Parents[0], parent1)
+		}
+
+		if parsed.Parents[1] != parent2 {
+			t.Fatalf("merge parent[1] = %s, want %s", parsed.Parents[1], parent2)
+		}
+
+		if !bytes.Equal(parsed.Message, []byte("Merge commit\n")) {
+			t.Fatalf("merge message = %q, want %q", parsed.Message, "Merge commit\n")
+		}
+	})
+}
diff --git a/object/commit/serialize.go b/object/commit/serialize.go
new file mode 100644
index 00000000..721cacf6
--- /dev/null
+++ b/object/commit/serialize.go
@@ -0,0 +1,84 @@
+package commit
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+
+	objectheader "codeberg.org/lindenii/furgit/object/header"
+	objecttype "codeberg.org/lindenii/furgit/object/type"
+)
+
+// SerializeWithoutHeader renders the raw commit body bytes.
+func (commit *Commit) SerializeWithoutHeader() ([]byte, error) {
+	var buf bytes.Buffer
+
+	if commit.Tree.Size() == 0 {
+		return nil, errors.New("object: commit: missing tree id")
+	}
+
+	fmt.Fprintf(&buf, "tree %s\n", commit.Tree.String())
+
+	for _, parent := range commit.Parents {
+		fmt.Fprintf(&buf, "parent %s\n", parent.String())
+	}
+
+	authorBytes, err := commit.Author.Serialize()
+	if err != nil {
+		return nil, err
+	}
+
+	buf.WriteString("author ")
+	buf.Write(authorBytes)
+	buf.WriteByte('\n')
+
+	committerBytes, err := commit.Committer.Serialize()
+	if err != nil {
+		return nil, err
+	}
+
+	buf.WriteString("committer ")
+	buf.Write(committerBytes)
+	buf.WriteByte('\n')
+
+	if commit.ChangeID != "" {
+		buf.WriteString("change-id ")
+		buf.WriteString(commit.ChangeID)
+		buf.WriteByte('\n')
+	}
+
+	for _, h := range commit.ExtraHeaders {
+		if h.Key == "" {
+			return nil, errors.New("object: commit: extra header has empty key")
+		}
+
+		buf.WriteString(h.Key)
+		buf.WriteByte(' ')
+		buf.Write(h.Value)
+		buf.WriteByte('\n')
+	}
+
+	buf.WriteByte('\n')
+	buf.Write(commit.Message)
+
+	return buf.Bytes(), nil
+}
+
+// SerializeWithHeader renders the raw object (header + body).
+func (commit *Commit) SerializeWithHeader() ([]byte, error) {
+	body, err := commit.SerializeWithoutHeader()
+	if err != nil {
+		return nil, err
+	}
+
+	header, ok := objectheader.Encode(objecttype.TypeCommit, int64(len(body)))
+	if !ok {
+		return nil, errors.New("object: commit: 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/commit/serialize_test.go b/object/commit/serialize_test.go
new file mode 100644
index 00000000..e58a8078
--- /dev/null
+++ b/object/commit/serialize_test.go
@@ -0,0 +1,34 @@
+package commit_test
+
+import (
+	"testing"
+
+	"codeberg.org/lindenii/furgit/internal/testgit"
+	"codeberg.org/lindenii/furgit/object/commit"
+	objectid "codeberg.org/lindenii/furgit/object/id"
+)
+
+func TestCommitSerialize(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")
+
+		rawBody := testRepo.CatFile(t, "commit", commitID)
+
+		parsed, err := commit.Parse(rawBody, algo)
+		if err != nil {
+			t.Fatalf("ParseCommit: %v", err)
+		}
+
+		rawObj, err := parsed.SerializeWithHeader()
+		if err != nil {
+			t.Fatalf("SerializeWithHeader: %v", err)
+		}
+
+		gotID := algo.Sum(rawObj)
+		if gotID != commitID {
+			t.Fatalf("commit id mismatch: got %s want %s", gotID, commitID)
+		}
+	})
+}
diff --git a/object/commit/type.go b/object/commit/type.go
new file mode 100644
index 00000000..b8aa11e8
--- /dev/null
+++ b/object/commit/type.go
@@ -0,0 +1,10 @@
+package commit
+
+import objecttype "codeberg.org/lindenii/furgit/object/type"
+
+// ObjectType returns TypeCommit.
+func (commit *Commit) ObjectType() objecttype.Type {
+	_ = commit
+
+	return objecttype.TypeCommit
+}