2 files changed, 23 insertions, 0 deletions
diff --git a/src/File.zig b/src/File.zig
index b8368482..929e33bf 100644
--- a/src/File.zig
+++ b/src/File.zig
@@ -53,6 +53,10 @@ pub const State = struct {
     /// Revision last known to match disk, after Save or an external reload.
     /// Equal means the screen matches disk.
     saved_revision: u32 = 0,
+    /// The text at `saved_revision`, hashed as the first edit after it
+    /// replaces it: an edit that brings the text back to it is clean again,
+    /// as an undo to the save point is in acme.
+    saved_hash: ?u64 = null,
     watch_after_save: bool = false,
     /// Seconds of the last content change, for the filesystem's stat; zero until edited.
     mtime: u32 = 0,
@@ -859,9 +863,14 @@ pub fn setContent(p: *Pardes, f: *State, new: []u8) void {
     if (f.mini) |*mini| mini.deinit(p.gpa);
     f.mini = null;
     const starts: []usize = if (f.line_starts.len > 0) editedLineStarts(p.gpa, f.line_starts, f.content, new) catch &.{} else &.{};
+    // ponytail: a whole-text hash per edit; a rolling hash when files grow large.
+    if (f.revision == f.saved_revision or f.saved_hash == null) f.saved_hash = std.hash.Wyhash.hash(0, f.content);
     p.gpa.free(f.content);
     f.content = new;
     f.revision +%= 1;
+    if (f.saved_hash) |h| if (std.hash.Wyhash.hash(0, new) == h) {
+        f.saved_revision = f.revision;
+    };
     f.mtime = pardes.ctlfs.events.now();
     if (f.line_starts.len > 0) p.gpa.free(f.line_starts);
     f.line_starts = starts;
diff --git a/src/ninep/ctl.zig b/src/ninep/ctl.zig
index bb562672..16935f41 100644
--- a/src/ninep/ctl.zig
+++ b/src/ninep/ctl.zig
@@ -744,6 +744,20 @@ test "a second lock is refused until the holder unlocks or closes, and binds nob
     for (p.fs.opens) |o| try testing.expect(o.node == 0);
 }
 
+test "dirty goes back to 0 when the text is the saved text again" {
+    const p = try withFile(testing.allocator, "one\n");
+    defer p.deinit();
+    const serial = serialOf(p);
+    const dirty = Node.of(serial, .dirty);
+    try testing.expectEqualStrings("0\n", rd(p, dirty, 0, 8).bytes);
+    try testing.expectEqual(Status.ok, wr(p, Node.of(serial, .body), "two\n").reply.status);
+    try testing.expectEqualStrings("1\n", rd(p, dirty, 0, 8).bytes);
+    try testing.expectEqual(Status.ok, wr(p, Node.of(serial, .ctl), "Undo").reply.status);
+    try testing.expectEqualStrings("0\n", rd(p, dirty, 0, 8).bytes);
+    try testing.expectEqual(Status.ok, wr(p, Node.of(serial, .ctl), "Redo").reply.status);
+    try testing.expectEqualStrings("1\n", rd(p, dirty, 0, 8).bytes);
+}
+
 test "a message from a root ctl write is the session's in the log" {
     const p = try withFile(testing.allocator, "x\n");
     defer p.deinit();