test: add Game current position invariant coverage

game.go

@@ -828,16 +828,37 @@ func (g *Game) Clone() *Game {
 	// clone do not impact the parent
 	ret.rootMove = g.rootMove.Clone()
 	ret.rootMove.cloneChildren(g.rootMove.children)
-	mlen := len(ret.Moves())
-	if mlen == 0 {
+	if g.currentMove == nil {
 		ret.currentMove = ret.rootMove
 	} else {
-		ret.currentMove = ret.Moves()[mlen-1]
+		ret.currentMove = findClonedMove(g.rootMove, ret.rootMove, g.currentMove)
+		if ret.currentMove == nil {
+			ret.currentMove = ret.rootMove
+		}
 	}
+	ret.pos = ret.currentMove.position
 
 	return ret
 }
 
+func findClonedMove(original, clone, target *Move) *Move {
+	if original == nil || clone == nil || target == nil {
+		return nil
+	}
+	if original == target {
+		return clone
+	}
+	for i, child := range original.children {
+		if i >= len(clone.children) {
+			return nil
+		}
+		if found := findClonedMove(child, clone.children[i], target); found != nil {
+			return found
+		}
+	}
+	return nil
+}
+
 // Positions returns all positions in the game in the main line.
 // This includes the starting position and all positions after each move.
 func (g *Game) Positions() []*Position {

game_invariants_test.go

@@ -0,0 +1,113 @@
+package chess
+
+import (
+	"strings"
+	"testing"
+)
+
+func assertGameCurrentPositionInvariant(t *testing.T, g *Game) {
+	t.Helper()
+
+	if g == nil {
+		t.Fatal("game is nil")
+	}
+	if g.pos == nil {
+		t.Fatal("game current position is nil")
+	}
+	if g.Position() == nil {
+		t.Fatal("Position() is nil")
+	}
+	if g.CurrentPosition() == nil {
+		t.Fatal("CurrentPosition() is nil")
+	}
+	if g.Position().String() != g.pos.String() {
+		t.Fatalf("Position() = %q, want game current position %q", g.Position(), g.pos)
+	}
+	if g.CurrentPosition().String() != g.pos.String() {
+		t.Fatalf("CurrentPosition() = %q, want game current position %q", g.CurrentPosition(), g.pos)
+	}
+	if g.currentMove != nil && g.currentMove.position != nil && g.currentMove.position.String() != g.pos.String() {
+		t.Fatalf("current move position = %q, want game current position %q", g.currentMove.position, g.pos)
+	}
+}
+
+func TestGameCurrentPositionInvariantAfterClonePreservesCursor(t *testing.T) {
+	g := NewGame()
+	for _, move := range []string{"e4", "e5", "Nf3"} {
+		if err := g.PushMove(move, nil); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	if !g.GoBack() {
+		t.Fatal("expected to navigate back")
+	}
+	want := g.CurrentPosition().String()
+
+	clone := g.Clone()
+
+	assertGameCurrentPositionInvariant(t, clone)
+	if got := clone.CurrentPosition().String(); got != want {
+		t.Fatalf("clone current position = %q, want %q", got, want)
+	}
+}
+
+func TestGameCurrentPositionInvariantAfterDirectGameOperations(t *testing.T) {
+	g := NewGame()
+	assertGameCurrentPositionInvariant(t, g)
+
+	for _, move := range []string{"e4", "e5", "Nf3"} {
+		if err := g.PushMove(move, nil); err != nil {
+			t.Fatal(err)
+		}
+		assertGameCurrentPositionInvariant(t, g)
+	}
+
+	if !g.GoBack() {
+		t.Fatal("expected to navigate back")
+	}
+	assertGameCurrentPositionInvariant(t, g)
+
+	if !g.GoForward() {
+		t.Fatal("expected to navigate forward")
+	}
+	assertGameCurrentPositionInvariant(t, g)
+}
+
+func TestGameCurrentPositionInvariantAfterPGNParse(t *testing.T) {
+	opt, err := PGN(strings.NewReader("1. e4 e5 2. Nf3 *"))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	g := NewGame(opt)
+
+	assertGameCurrentPositionInvariant(t, g)
+}
+
+func TestGameCurrentPositionInvariantAfterSplitUsesLineLeaf(t *testing.T) {
+	g := NewGame()
+	for _, move := range []string{"e4", "e5", "Nf3"} {
+		if err := g.PushMove(move, nil); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	if !g.GoBack() {
+		t.Fatal("expected to navigate back before adding a variation")
+	}
+	if err := g.PushMove("Nc3", nil); err != nil {
+		t.Fatal(err)
+	}
+
+	splitGames := g.Split()
+	if len(splitGames) != 2 {
+		t.Fatalf("split game count = %d, want 2", len(splitGames))
+	}
+	for _, splitGame := range splitGames {
+		assertGameCurrentPositionInvariant(t, splitGame)
+		if !splitGame.IsAtEnd() {
+			t.Fatalf("split game current position = %q, want leaf position", splitGame.CurrentPosition())
+		}
+	}
+}