implement undo method to revert last move in game

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...)
@@ -378,7 +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
 	}

game_test.go

@@ -110,6 +110,61 @@ 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")
+	}
+
+	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) {
 	g := NewGame()
 	moves := []string{