From 73bbca17aa9e050f515c92c6fb92e5786fb4562a Mon Sep 17 00:00:00 2001
From: Amjad Jibon <amjad.jibon@gmail.com>
Date: Tue, 20 Aug 2024 00:53:22 +0800
Subject: [PATCH 1/2] implement undo method to revert last move in game

---
 game.go      | 13 +++++++++++++
 game_test.go | 31 +++++++++++++++++++++++++++++++
 2 files changed, 44 insertions(+)

diff --git a/game.go b/game.go
index 67f7cd2..9f7ec89 100644
--- a/game.go
+++ b/game.go
@@ -195,6 +195,19 @@ func (g *Game) Moves() []*Move {
 	return append([]*Move(nil), g.moves...)
 }
 
+// Undo reverts the last move and restores the previous game state.
+func (g *Game) Undo() (*Move, error) {
+	if len(g.moves) == 0 {
+		return nil, errors.New("chess: no moves to undo")
+	}
+	lastMove := g.moves[len(g.moves)-1]
+	g.moves = g.moves[:len(g.moves)-1]
+	g.positions = g.positions[:len(g.positions)-1]
+	g.pos = g.positions[len(g.positions)-1]
+	g.updatePosition()
+	return lastMove, nil
+}
+
 // Comments returns the comments for the game indexed by moves.
 func (g *Game) Comments() [][]string {
 	return append([][]string(nil), g.comments...)
diff --git a/game_test.go b/game_test.go
index ec60e3e..eb12b08 100644
--- a/game_test.go
+++ b/game_test.go
@@ -110,6 +110,37 @@ func TestThreeFoldRepetition(t *testing.T) {
 	}
 }
 
+func TestUndo(t *testing.T) {
+	g := NewGame()
+	_, err := g.Undo()
+	if err == nil {
+		t.Fatal("should require at least one move before undo")
+	}
+
+	moves := []string{
+		"Nf3", "Nf6", "Ng1", "Ng8",
+	}
+
+	for _, m := range moves {
+		if err = g.MoveStr(m); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	lastMove, err := g.Undo()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if lastMove.String() != "f6g8" {
+		t.Fatal("last move should be f6g8")
+	}
+
+	if g.moves[len(g.moves)-1].String() != "f3g1" {
+		t.Fatal("last move should be f3g1")
+	}
+}
+
 func TestInvalidThreeFoldRepetition(t *testing.T) {
 	g := NewGame()
 	moves := []string{

From eb01031b7780b35f993c55975f01d0d89c15f4fb Mon Sep 17 00:00:00 2001
From: silkrow <si1krow@outlook.com>
Date: Tue, 3 Sep 2024 15:38:39 -0500
Subject: [PATCH 2/2] modified updatePosition function to support Undo feature,
 added test case for TestUndo

---
 game.go      |  4 ++++
 game_test.go | 24 ++++++++++++++++++++++++
 2 files changed, 28 insertions(+)

diff --git a/game.go b/game.go
index 9f7ec89..aefc4b4 100644
--- a/game.go
+++ b/game.go
@@ -391,7 +391,11 @@ func (g *Game) updatePosition() {
 		if g.pos.Turn() == White {
 			g.outcome = BlackWon
 		}
+	} else if method == NoMethod {
+		g.method = NoMethod
+		g.outcome = NoOutcome
 	}
+
 	if g.outcome != NoOutcome {
 		return
 	}
diff --git a/game_test.go b/game_test.go
index eb12b08..51221e2 100644
--- a/game_test.go
+++ b/game_test.go
@@ -139,6 +139,30 @@ func TestUndo(t *testing.T) {
 	if g.moves[len(g.moves)-1].String() != "f3g1" {
 		t.Fatal("last move should be f3g1")
 	}
+
+	g = NewGame()
+	moves = []string{
+		"f4", "e6", "g4", "Qh4",
+	}
+
+	for _, m := range moves {
+		if err = g.MoveStr(m); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	lastMove, err = g.Undo()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if err = g.MoveStr("h6"); err != nil {
+		t.Fatal(err)
+	}
+	if g.Outcome() != NoOutcome {
+		t.Fatal("unexpected game outcome")
+	}
+
 }
 
 func TestInvalidThreeFoldRepetition(t *testing.T) {