Add bear off test case

board.go

@@ -370,6 +370,7 @@ func (b Board) _available(player int8) [][2]int8 {
 
 	var moves [][2]int8
 
+	// Enter board from home space.
 	if b[SpaceVariant] != VariantBackgammon && ((player == 1 && b[SpaceEnteredPlayer] == 0) || (player == 2 && b[SpaceEnteredOpponent] == 0)) && b[homeSpace] != 0 {
 		for space := int8(1); space < 25; space++ {
 			v := b[space]
@@ -378,10 +379,14 @@ func (b Board) _available(player int8) [][2]int8 {
 			}
 		}
 	}
+
 	for from := int8(0); from < 28; from++ {
+		// Skip invalid spaces, spaces without player checkers and non-bar spaces when a checker is on the bar.
 		if from == SpaceHomePlayer || from == SpaceHomeOpponent || from == opponentBarSpace || checkers(player, b[from]) == 0 || (onBar && from != barSpace) {
 			continue
 		}
+
+		// Iterate over destination spaces to determine available moves.
 		if player == 1 && b[SpaceVariant] != VariantTabula {
 			for to := int8(0); to < from; to++ {
 				if to == SpaceBarPlayer || to == SpaceBarOpponent || to == SpaceHomeOpponent || (to == SpaceHomePlayer && !mayBearOff()) {

board_test.go

@@ -63,6 +63,42 @@ func TestMoveBackgammon(t *testing.T) {
 	}
 }
 
+func TestBearOffBackgammon(t *testing.T) {
+	b := Board{0, 0, 2, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, -1, 0, 0, -3, 0, 0, -3, -6, -2, 0, 0, 0, 3, 3, 0, 0, 1, 1, 0}
+	available, _ := b.Available(1)
+	got, expected := len(available), 1
+	if got != expected {
+		t.Errorf("unexpected number of move combinations: expected %d: got %d", expected, got)
+	}
+	var foundMoves [][2]int8
+	for i := 0; i < 4; i++ {
+		move := available[0][i]
+		if move[0] == 0 && move[1] == 0 {
+			break
+		}
+		foundMoves = append(foundMoves, move)
+	}
+	got, expected = len(foundMoves), 2
+	if got != expected {
+		t.Errorf("unexpected number of legal moves: expected %d: got %d", expected, got)
+	}
+	var found30 bool
+	var found41 bool
+	for _, move := range foundMoves {
+		if move[0] == 3 && move[1] == 0 {
+			found30 = true
+		} else if move[0] == 4 && move[1] == 1 {
+			found41 = true
+		}
+	}
+	if !found30 {
+		t.Errorf("expected move 3/0 was not found")
+	}
+	if !found41 {
+		t.Errorf("expected move 4/1 was not found")
+	}
+}
+
 func TestMoveTabula(t *testing.T) {
 	{
 		b := NewBoard(VariantTabula)