From 3b18a7d8c9608ed0bf13d546018f44e703c62e11 Mon Sep 17 00:00:00 2001
From: Saya Andy <saya.andy@posteo.com>
Date: Mon, 7 Sep 2026 20:26:15 +0700
Subject: diff: add a flip link to the refs panel

The from and to refs of a diff can only be swapped by editing the
url by hand.  Add a flip link to the refs panel that opens the same
diff with the refs swapped, carrying the current head and options.
---
 cgit.css            |  5 +++++
 tests/t0106-diff.sh | 21 +++++++++++++++++++++
 ui-diff.c           |  8 ++++++++
 3 files changed, 34 insertions(+)

diff --git a/cgit.css b/cgit.css
index 7892eca..8563462 100644
--- a/cgit.css
+++ b/cgit.css
@@ -674,6 +674,11 @@ div#cgit a.button {
 	padding: 0em 0.5em;
 }
 
+div#cgit form.diff-ctrls div.cgit-panel a.button {
+	border: solid 1px var(--border-mid);
+	background-color: var(--bg-hl);
+}
+
 div#cgit a.primary {
 	font-size: 100%;
 }
diff --git a/tests/t0106-diff.sh b/tests/t0106-diff.sh
index e4751e9..1100596 100755
--- a/tests/t0106-diff.sh
+++ b/tests/t0106-diff.sh
@@ -45,6 +45,15 @@ test_expect_success 'ref picker: auto-submit on change' '
 	grep "onchange=.this.form.submit()." tmp
 '
 
+test_expect_success 'flip link present' '
+	grep "<a title=.swap from and to. class=.button. href=./foo/diff/?" tmp
+'
+
+test_expect_success 'flip link swaps the default refs' '
+	old=$(git --git-dir="$PWD/repos/foo/.git" rev-parse master~1) &&
+	grep "/foo/diff/?id=$old&amp;id2=master" tmp
+'
+
 test_expect_success 'compare two refs: foo/diff?id=master&id2=HEAD' '
 	cgit_query "url=foo/diff&id=master&id2=HEAD" >tmp
 '
@@ -77,4 +86,16 @@ test_expect_success 'diff with HEAD and root shows content' '
 	grep "diffstat" tmp
 '
 
+test_expect_success 'flip link swaps explicit refs' '
+	cgit_query "url=foo/diff&id=master&id2=HEAD" >tmp &&
+	new=$(git --git-dir="$PWD/repos/foo/.git" rev-parse HEAD) &&
+	grep "/foo/diff/?id=$new&amp;id2=master" tmp
+'
+
+test_expect_success 'flip link omits id for a diff against a root commit' '
+	root=$(git --git-dir="$PWD/repos/foo/.git" rev-list --max-parents=0 HEAD) &&
+	cgit_query "url=foo/diff&id=$root" >tmp &&
+	grep "href=./foo/diff/?id2=$root." tmp
+'
+
 test_done
diff --git a/ui-diff.c b/ui-diff.c
index 5ee3830..13c1e32 100644
--- a/ui-diff.c
+++ b/ui-diff.c
@@ -409,6 +409,14 @@ void cgit_print_diff_ctrls(void)
 		html("<td class='ctrl'>");
 		cgit_print_ref_select("id", new_ref);
 		html("</td></tr>");
+
+		html("<tr><td/><td class='ctrl'>");
+		cgit_diff_link("flip", "swap from and to", "button",
+			       ctx.qry.head,
+			       is_null_oid(old_rev_oid) ? NULL :
+			               oid_to_hex(old_rev_oid),
+			       new_ref, ctx.qry.path);
+		html("</td></tr>");
 	}
 	html("</table>");
 	html("</div>");
-- 
cgit v1.3.1+16