feat: Add dead positions to game system

src/Game/Game.php

@@ -33,7 +33,7 @@ class Game {
     }
 
     public function getPieces(Side $side): array {
-        return array_filter($this->pieces, fn($p) => $p->getSide() == $side);
+        return array_values(array_filter($this->pieces, fn($p) => $p->getSide() == $side));
     }
 
     private function &findPiece(Piece $needle): Piece {
@@ -312,6 +312,46 @@ class Game {
         $this->history->add($this);
     }
 
+    private function isDeadPosition(): bool {
+            $ownPieces = $this->getPieces($this->current);
+            $opponentPieces = $this->getPieces($this->current->getNext());
+
+            $getRemaining = function (array $pieces): Piece {
+                return ($pieces[0] instanceof King ? $pieces[1] : $pieces[0]);
+            };
+
+            if (count($ownPieces) > 2) {
+                return false;
+            } else if (count($opponentPieces) > 2) {
+                return false;
+            } else if (count($ownPieces) == 1 && count($opponentPieces) == 1) {
+                return true;
+            } else if (count($ownPieces) == 1) {
+                $opponentRemaining = $getRemaining($opponentPieces);
+                return !(
+                    $opponentRemaining instanceof Queen ||
+                    $opponentRemaining instanceof Rook ||
+                    $opponentRemaining instanceof Pawn
+                );
+            } else if (count($opponentPieces) == 1) {
+                $ownRemaining = $getRemaining($ownPieces);
+                return !(
+                    $ownRemaining instanceof Queen ||
+                    $ownRemaining instanceof Rook ||
+                    $ownRemaining instanceof Pawn
+                );
+            } else { // both have 1 piece left besides king
+                $ownRemaining = $getRemaining($ownPieces);
+                $opponentRemaining = $getRemaining($opponentPieces);
+
+                return (
+                    $ownRemaining instanceof Bishop &&
+                    $opponentRemaining instanceof Bishop &&
+                    $ownRemaining->getPosition()->getSquareColor() == $opponentRemaining->getPosition()->getSquareColor()
+                );
+            }
+    }
+
     public function getGameState(bool $onlyIsLegal = false): GameState {
         $allOccupied = $this->getAllOccupied();
 
@@ -327,6 +367,10 @@ class Game {
             return GameState::THREEFOLD_REPETITION;
         }
 
+        if ($this->isDeadPosition()) {
+            return GameState::DEAD_POSITION;
+        }
+
         $legalMoves = $this->getLegalMoves();
 
         if ($this->isCheck($allOccupied)) {

src/Game/Position.php

@@ -17,6 +17,10 @@ class Position {
             $this->rank >= 0 && $this->rank < 8;
     }
 
+    public function getSquareColor(): Side {
+        return ($this->rank % 2) ^ ($this->file % 2) ? Side::WHITE : Side::BLACK;
+    }
+
     public function __toString(): string {
         return ["a", "b", "c", "d", "e", "f", "g", "h"][$this->file] . ($this->rank + 1);
     }

tests/Game/GameTest.php

@@ -140,6 +140,7 @@ final class GameTest extends TestCase {
             [
                 new King(new Position(1, 1), Side::BLACK, true),
                 new King(new Position(7, 6), Side::WHITE, true),
+                new Pawn(new Position(3, 6), Side::WHITE, true), // to avoid dead position
             ],
             Side::BLACK
         );
@@ -257,6 +258,99 @@ final class GameTest extends TestCase {
         $this->assertEquals(GameState::DEFAULT, $subject->getGameState());
     }
 
+    public function testGameState_deadPosition_onlyKings() {
+        $subject = new Game(
+            [
+                new King(new Position(0, 7), Side::BLACK),
+                new King(new Position(0, 5), Side::WHITE),
+            ],
+            Side::BLACK
+        );
+
+        $this->assertEquals(GameState::DEAD_POSITION, $subject->getGameState());
+    }
+
+    public function testGameState_deadPosition_kingVsKingAndBishop() {
+        $subject = new Game(
+            [
+                new King(new Position(0, 7), Side::BLACK),
+                new King(new Position(0, 5), Side::WHITE),
+                new Bishop(new Position(1, 0), Side::WHITE),
+            ],
+            Side::BLACK
+        );
+
+        $this->assertEquals(GameState::DEAD_POSITION, $subject->getGameState());
+    }
+
+    public function testGameState_deadPosition_kingVsKingAndKnight() {
+        $subject = new Game(
+            [
+                new King(new Position(0, 7), Side::BLACK),
+                new King(new Position(0, 5), Side::WHITE),
+                new Knight(new Position(1, 0), Side::WHITE),
+            ],
+            Side::BLACK
+        );
+
+        $this->assertEquals(GameState::DEAD_POSITION, $subject->getGameState());
+    }
+
+    public function testGameState_deadPosition_kingAndBishopVsKingAndBishopSameColor() {
+        $subject = new Game(
+            [
+                new King(new Position(0, 7), Side::BLACK),
+                new King(new Position(0, 5), Side::WHITE),
+                new Bishop(new Position(1, 0), Side::WHITE),
+                new Bishop(new Position(2, 1), Side::BLACK),
+            ],
+            Side::BLACK
+        );
+
+        $this->assertEquals(GameState::DEAD_POSITION, $subject->getGameState());
+    }
+
+    public function testGameState_deadPosition_kingAndBishopVsKingAndBishopDifferentColor_notDead() {
+        $subject = new Game(
+            [
+                new King(new Position(0, 7), Side::BLACK),
+                new King(new Position(0, 5), Side::WHITE),
+                new Bishop(new Position(1, 0), Side::WHITE),
+                new Bishop(new Position(1, 1), Side::BLACK),
+            ],
+            Side::BLACK
+        );
+
+        $this->assertEquals(GameState::DEFAULT, $subject->getGameState());
+    }
+
+    public function testGameState_deadPosition_kingAndKnightVsKing_notDead() {
+        $subject = new Game(
+            [
+                new King(new Position(0, 7), Side::BLACK),
+                new King(new Position(0, 5), Side::WHITE),
+                new Bishop(new Position(1, 0), Side::WHITE),
+                new Knight(new Position(1, 1), Side::BLACK),
+            ],
+            Side::BLACK
+        );
+
+        $this->assertEquals(GameState::DEFAULT, $subject->getGameState());
+    }
+
+    public function testGameState_deadPosition_kingAndPawnVsKing_notDead() {
+        $subject = new Game(
+            [
+                new King(new Position(0, 7), Side::BLACK),
+                new King(new Position(0, 5), Side::WHITE),
+                new Pawn(new Position(1, 1), Side::BLACK, true),
+            ],
+            Side::BLACK
+        );
+
+        $this->assertEquals(GameState::DEFAULT, $subject->getGameState());
+    }
+
     public function testLegalMoves_pawnPinnedBecauseOfCheckKingRestrictedByQueenAndPawn() {
         $subject = new Game(
             [