1 files changed, 56 insertions, 0 deletions
diff --git a/internal/oldgit/fmtpatch.go b/internal/oldgit/fmtpatch.go
new file mode 100644
index 0000000..79be5d8
--- /dev/null
+++ b/internal/oldgit/fmtpatch.go
@@ -0,0 +1,56 @@
+// SPDX-License-Identifier: AGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (c) 2025 Runxi Yu <https://runxiyu.org>
+
+package oldgit
+
+import (
+	"bytes"
+	"fmt"
+	"strings"
+	"time"
+
+	"github.com/go-git/go-git/v5/plumbing/object"
+)
+
+// FmtCommitPatch formats a commit object as if it was returned by
+// git-format-patch.
+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 {
+		return "", err
+	}
+
+	author = commit.Author
+	date = author.When.Format(time.RFC1123Z)
+
+	commitTitle, commitDetails, _ = strings.Cut(commit.Message, "\n")
+
+	// This date is hardcoded in Git.
+	fmt.Fprintf(&buf, "From %s Mon Sep 17 00:00:00 2001\n", commit.Hash)
+	fmt.Fprintf(&buf, "From: %s <%s>\n", author.Name, author.Email)
+	fmt.Fprintf(&buf, "Date: %s\n", date)
+	fmt.Fprintf(&buf, "Subject: [PATCH] %s\n\n", commitTitle)
+
+	if commitDetails != "" {
+		commitDetails1, commitDetails2, _ := strings.Cut(commitDetails, "\n")
+		if strings.TrimSpace(commitDetails1) == "" {
+			commitDetails = commitDetails2
+		}
+		buf.WriteString(commitDetails)
+		buf.WriteString("\n")
+	}
+	buf.WriteString("---\n")
+	fmt.Fprint(&buf, patch.Stats().String())
+	fmt.Fprintln(&buf)
+
+	buf.WriteString(patch.String())
+
+	fmt.Fprintf(&buf, "\n-- \n2.48.1\n")
+
+	return buf.String(), nil
+}