From c6acfa680622d8e75e05510d8cf1858bffe6eceb Mon Sep 17 00:00:00 2001
From: Bruno Albuquerque <bga@gmail.com>
Date: Sat, 28 Mar 2026 06:46:01 -0400
Subject: [PATCH] feat(crdt): add Reverse method to CRDT[T] for undo/redo
 support

---
 CHANGELOG.md      |  2 +-
 README.md         | 15 +++++++++++++++
 crdt/crdt.go      | 14 ++++++++++++++
 crdt/crdt_test.go | 26 ++++++++++++++++++++++++++
 4 files changed, 56 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index d72c87e..8c790ff 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -63,7 +63,7 @@ Public package used directly by generated `*_deep.go` files. Most callers will n
 
 ### CRDTs (`github.com/brunoga/deep/v5/crdt`)
 
-- `CRDT[T]` — Concurrency-safe CRDT wrapper. Create with `NewCRDT(initial, nodeID)`. Key methods: `Edit(fn)`, `ApplyDelta(delta)`, `Merge(other)`, `View()`. JSON-serializable.
+- `CRDT[T]` — Concurrency-safe CRDT wrapper. Create with `NewCRDT(initial, nodeID)`. Key methods: `Edit(fn)`, `ApplyDelta(delta)`, `Merge(other)`, `Reverse(delta)`, `View()`. JSON-serializable. `Reverse` applies the inverse of a delta and returns a new undo delta with a fresh HLC timestamp; calling `Reverse` on that delta produces a redo.
 - `Delta[T]` — A timestamped set of changes produced by `CRDT.Edit`; send to peers and apply with `CRDT.ApplyDelta`.
 - `LWW[T]` — Embeddable Last-Write-Wins register. Update with `Set(v, ts)`; accepts write only if `ts` is strictly newer.
 - `Text` (`[]TextRun`) — Convergent collaborative text. Merge concurrent edits with `MergeTextRuns(a, b)`.
diff --git a/README.md b/README.md
index 803c697..b537dd7 100644
--- a/README.md
+++ b/README.md
@@ -152,6 +152,21 @@ undo := patch.Reverse()
 strictPatch := patch.AsStrict()
 ```
 
+### Undo/Redo with CRDTs
+
+`CRDT[T].Reverse` applies the inverse of a delta to the local node and returns a
+new `Delta[T]` with a fresh HLC timestamp, making it causally after the original
+edit and safe to propagate to peers:
+
+```go
+// Apply an edit, then undo it.
+delta, _ := node.Edit(func(v *MyDoc) { v.Title = "Draft" })
+undo := node.Reverse(delta)   // applies inverse locally, returns undo delta
+
+// Redo: reverse the undo.
+redo := node.Reverse(undo)
+```
+
 ### Standard Interop
 
 Export your Deep patches to standard RFC 6902 JSON Patch format, and parse them back:
diff --git a/crdt/crdt.go b/crdt/crdt.go
index f275f9f..9eea3b2 100644
--- a/crdt/crdt.go
+++ b/crdt/crdt.go
@@ -227,6 +227,20 @@ func textAncestorPath(root reflect.Value, opPath string) (string, bool) {
 	return "", false
 }
 
+// Reverse applies the inverse of delta to this node and returns a new Delta
+// representing the undo operation. The returned Delta carries a fresh HLC
+// timestamp so it is causally after the original edit and will be accepted by
+// ApplyDelta on any peer that has already seen the original.
+//
+// Calling Reverse on the returned Delta produces a redo Delta.
+func (c *CRDT[T]) Reverse(delta Delta[T]) Delta[T] {
+	reversed := delta.patch.Reverse()
+	now := c.clock.Now()
+	undoDelta := Delta[T]{patch: reversed, Timestamp: now}
+	c.ApplyDelta(undoDelta)
+	return undoDelta
+}
+
 // Merge performs a full state-based merge with another CRDT node.
 // For each changed field the node with the strictly newer effective timestamp
 // (max of write clock and tombstone) wins. Text fields are always merged
diff --git a/crdt/crdt_test.go b/crdt/crdt_test.go
index 74303ba..d46239b 100644
--- a/crdt/crdt_test.go
+++ b/crdt/crdt_test.go
@@ -282,6 +282,32 @@ func TestMerge_TextInKeyedSliceElement(t *testing.T) {
 	}
 }
 
+func TestCRDT_Reverse(t *testing.T) {
+	nodeA := NewCRDT(TestUser{ID: 1, Name: "Alice"}, "node-a")
+	nodeB := NewCRDT(TestUser{ID: 1, Name: "Alice"}, "node-b")
+
+	delta := nodeA.Edit(func(u *TestUser) { u.Name = "Bob" })
+
+	// Undo on nodeA
+	undoDelta := nodeA.Reverse(delta)
+	if nodeA.View().Name != "Alice" {
+		t.Errorf("Reverse did not undo: expected Alice, got %s", nodeA.View().Name)
+	}
+
+	// Undo delta propagates to peers
+	nodeB.ApplyDelta(delta)
+	nodeB.ApplyDelta(undoDelta)
+	if nodeB.View().Name != "Alice" {
+		t.Errorf("Reverse delta did not propagate: expected Alice, got %s", nodeB.View().Name)
+	}
+
+	// Redo: reverse the undo
+	nodeA.Reverse(undoDelta)
+	if nodeA.View().Name != "Bob" {
+		t.Errorf("Redo failed: expected Bob, got %s", nodeA.View().Name)
+	}
+}
+
 func TestCRDT_JSON(t *testing.T) {
 	node := NewCRDT(TestUser{ID: 1, Name: "Initial"}, "node1")
 	node.Edit(func(u *TestUser) {