Undo and Redo are builtins, as in acme's tags, so a pane's ctl and exec can step its body

Over 9P there was no way to take back an edit: acme's Undo and Redo were keys only. They are now pane builtins, reached from a tag, a pane ctl and exec, and listed in /commands. They are not added to the default tags, which would change every tag.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

3 files changed, 30 insertions, 1 deletions

diff --git a/src/builtins.zig b/src/builtins.zig
index 23d853c8..a755969b 100644
--- a/src/builtins.zig
+++ b/src/builtins.zig
@@ -850,6 +850,20 @@ pub const Togglettymode = struct {
     }
 };
 
+/// acme's Undo and Redo (exec.c `undo`): step the pane's body back or
+/// forward through its edits, from a tag, a pane ctl or exec.
+pub const Undo = struct {
+    pub fn run(c: Ctx) void {
+        edit.doUndo(c.p, &c.pane.body);
+    }
+};
+
+pub const Redo = struct {
+    pub fn run(c: Ctx) void {
+        edit.doRedo(c.p, &c.pane.body);
+    }
+};
+
 /// Fold a pane to its tag, or restore its share of the column.
 pub const Collapse = struct {
     pub fn run(c: Ctx) void {
diff --git a/src/config.zig b/src/config.zig
index 186b29a1..416754a6 100644
--- a/src/config.zig
+++ b/src/config.zig
@@ -49,6 +49,8 @@ pub const leader_path = paths: {
         .Mode = null,
         .Togglettymode = null,
         .Collapse = null,
+        .Undo = null,
+        .Redo = null,
         .Repl = null,
         .Kill = null,
         .Exit = null,
diff --git a/src/ninep/ctl.zig b/src/ninep/ctl.zig
index a64521e7..e4e983a8 100644
--- a/src/ninep/ctl.zig
+++ b/src/ninep/ctl.zig
@@ -542,10 +542,23 @@ test "commands lists every builtin with its argument and the ctl that takes it"
         try testing.expect(words.next() == null);
     }
     try testing.expectEqual(std.enums.values(Builtin).len, lines);
-    for ([_][]const u8{ "\nDel arg pane\n", "\nNewcol root\n", "\nSave arg pane\n", "\nVerbose arg root\n" }) |row|
+    for ([_][]const u8{ "\nDel arg pane\n", "\nNewcol root\n", "\nSave arg pane\n", "\nVerbose arg root\n", "\nUndo pane\n", "\nRedo pane\n" }) |row|
         try testing.expect(std.mem.indexOf(u8, listed.bytes, row) != null);
 }
 
+test "Undo and Redo on a pane's ctl step its body through its edits" {
+    const p = try withFile(testing.allocator, "one\n");
+    defer p.deinit();
+    const serial = serialOf(p);
+    const body = Node.of(serial, .body);
+    try testing.expectEqual(Status.ok, wr(p, body, "two\n").reply.status);
+    try testing.expectEqualStrings("one\ntwo\n", rd(p, body, 0, 64).bytes);
+    try testing.expectEqual(Status.ok, wr(p, Node.of(serial, .ctl), "Undo").reply.status);
+    try testing.expectEqualStrings("one\n", rd(p, body, 0, 64).bytes);
+    try testing.expectEqual(Status.ok, wr(p, Node.of(serial, .ctl), "Redo").reply.status);
+    try testing.expectEqualStrings("one\ntwo\n", rd(p, body, 0, 64).bytes);
+}
+
 test "focus reads the pane with the keyboard, and a serial written to it moves the keyboard there" {
     const p = try withFile(testing.allocator, "x\n");
     defer p.deinit();