Merge pull request #52 from lichess-org/explosionSquares

Explosion squares

lib/dartchess.dart

@@ -1,8 +1,3 @@
-/// Dart chess library for native platforms.
-///
-/// All classes are immutable except [PgnNode] and [PgnChildNode].
-library dartchess;
-
 export 'src/constants.dart';
 export 'src/models.dart';
 export 'src/square_set.dart';

lib/src/models.dart

@@ -185,7 +185,7 @@ extension type const Rank._(int value) implements int {
 ///
 /// The square is represented with an integer ranging from 0 to 63, using a
 /// little-endian rank-file mapping (LERF):
-/// ```
+/// ```txt
 ///  8 | 56 57 58 59 60 61 62 63
 ///  7 | 48 49 50 51 52 53 54 55
 ///  6 | 40 41 42 43 44 45 46 47

lib/src/position.dart

@@ -1318,6 +1318,24 @@ abstract class Atomic extends Position {
     }
   }
 
+  /// Returns the set of squares that explode when [move] is played.
+  ///
+  /// The explosion center ([move].to) and all surrounding non-pawn occupied
+  /// squares are included. Returns an empty set if the move is not a capture.
+  SquareSet explosionSquares(Move move) {
+    if (move is! NormalMove) return SquareSet.empty;
+    if (_getCastlingSide(move) != null) return SquareSet.empty;
+    final capturedPiece = board.pieceAt(move.to);
+    final isEnPassant =
+        move.to == epSquare && board.pieceAt(move.from)?.role == Role.pawn;
+    if (capturedPiece == null && !isEnPassant) return SquareSet.empty;
+    final surrounding = kingAttacks(move.to)
+        .intersect(board.occupied)
+        .diff(board.pawns)
+        .withoutSquare(move.from);
+    return surrounding.withSquare(move.to);
+  }
+
   /// Plays a move without checking if the move is legal.
   ///
   /// In addition to standard rules, all captures cause an explosion by which

test/position_test.dart

@@ -773,6 +773,31 @@ void main() {
         expect(pos.hasInsufficientMaterial(Side.black), test[2]);
       }
     });
+
+    test('explosionSquares', () {
+      // Giuoco Piano: Bxf7+ explodes f7 (center), f6 (knight), e8 (king), f8 (bishop)
+      // g7 is a pawn and is not included
+      final pos = Atomic.fromSetup(Setup.parseFen(
+          'r1bqkb1r/pppp1ppp/2n2n2/4p3/2B1P3/5N2/PPPP1PPP/RNBQK2R w KQkq - 4 4'));
+      expect(
+        pos.explosionSquares(NormalMove.fromUci('c4f7')),
+        SquareSet.fromSquares([Square.f7, Square.f6, Square.e8, Square.f8]),
+      );
+
+      // Non-capture move returns empty set
+      expect(
+        pos.explosionSquares(NormalMove.fromUci('d2d3')),
+        SquareSet.empty,
+      );
+
+      // En passant: explosion centered on the destination square
+      final epPos = Atomic.fromSetup(Setup.parseFen(
+          'rnbqkbnr/ppp1p1pp/8/3pPp2/8/8/PPPP1PPP/RNBQKBNR w KQkq f6 0 3'));
+      expect(
+        epPos.explosionSquares(NormalMove.fromUci('e5f6')),
+        const SquareSet.fromSquare(Square.f6),
+      );
+    });
   });
 
   group('Antichess', () {