oldgit: Separate some go-git stuff into here

2 files changed, 43 insertions, 4 deletions

diff --git a/git_format_patch.go b/internal/oldgit/fmtpatch.go
index 5628ce1..79be5d8 100644
--- a/git_format_patch.go
+++ b/internal/oldgit/fmtpatch.go
@@ -1,7 +1,7 @@
 // SPDX-License-Identifier: AGPL-3.0-only
 // SPDX-FileCopyrightText: Copyright (c) 2025 Runxi Yu <https://runxiyu.org>
 
-package forge
+package oldgit
 
 import (
 	"bytes"
@@ -12,16 +12,16 @@ import (
 	"github.com/go-git/go-git/v5/plumbing/object"
 )
 
-// fmtCommitPatch formats a commit object as if it was returned by
+// FmtCommitPatch formats a commit object as if it was returned by
 // git-format-patch.
-func fmtCommitPatch(commit *object.Commit) (final string, err error) {
+func FmtCommitPatch(commit *object.Commit) (final string, err error) {
 	var patch *object.Patch
 	var buf bytes.Buffer
 	var author object.Signature
 	var date string
 	var commitTitle, commitDetails string
 
-	if _, patch, err = commitToPatch(commit); err != nil {
+	if _, patch, err = CommitToPatch(commit); err != nil {
 		return "", err
 	}
 
diff --git a/internal/oldgit/patch.go b/internal/oldgit/patch.go
new file mode 100644
index 0000000..329bdfb
--- /dev/null
+++ b/internal/oldgit/patch.go
@@ -0,0 +1,39 @@
+package oldgit
+
+import (
+	"errors"
+
+	"github.com/go-git/go-git/v5/plumbing"
+	"github.com/go-git/go-git/v5/plumbing/object"
+)
+
+// CommitToPatch creates an [object.Patch] from the first parent of a given
+// [object.Commit].
+//
+// TODO: This function should be deprecated as it only diffs with the first
+// parent and does not correctly handle merge commits.
+func CommitToPatch(commit *object.Commit) (parentCommitHash plumbing.Hash, patch *object.Patch, err error) {
+	var parentCommit *object.Commit
+	var commitTree *object.Tree
+
+	parentCommit, err = commit.Parent(0)
+	switch {
+	case errors.Is(err, object.ErrParentNotFound):
+		if commitTree, err = commit.Tree(); err != nil {
+			return
+		}
+		if patch, err = NullTree.Patch(commitTree); err != nil {
+			return
+		}
+	case err != nil:
+		return
+	default:
+		parentCommitHash = parentCommit.Hash
+		if patch, err = parentCommit.Patch(commit); err != nil {
+			return
+		}
+	}
+	return
+}
+
+var NullTree object.Tree //nolint:gochecknoglobals