[board] Implement Position::is_king_in_check()

Returns true if the king of the current player is in check.

board/src/position/position.rs

@@ -4,8 +4,10 @@ use super::{flags::Flags, Pieces};
 use crate::{
     moves::Moves,
     piece::{Color, Piece, PiecePlacementError, PlacedPiece, Shape},
+    sight::Sight,
     BitBoard, Square,
 };
+use std::cell::OnceCell;
 use std::fmt;
 use std::fmt::Write;
 
@@ -26,7 +28,7 @@ macro_rules! position {
     };
 }
 
-#[derive(Clone, Eq, Hash, PartialEq)]
+#[derive(Clone, Eq, PartialEq)]
 pub struct Position {
     color_to_move: Color,
 
@@ -46,6 +48,8 @@ pub struct Position {
     pieces_per_type: [[BitBoard; 6]; 2],
 
     en_passant_square: Option<Square>,
+
+    sight: [OnceCell<BitBoard>; 2],
 }
 
 impl Position {
@@ -73,6 +77,7 @@ impl Position {
                 ],
             ],
             en_passant_square: None,
+            sight: [OnceCell::new(), OnceCell::new()],
         }
     }
 
@@ -105,6 +110,7 @@ impl Position {
             ],
             pieces_per_type: [white_pieces, black_pieces],
             en_passant_square: None,
+            sight: [OnceCell::new(), OnceCell::new()],
         }
     }
 
@@ -213,6 +219,25 @@ impl Position {
     pub fn en_passant_square(&self) -> Option<Square> {
         self.en_passant_square
     }
+
+    pub(crate) fn sight_of_player(&self, color: Color) -> BitBoard {
+        *self.sight[color as usize].get_or_init(|| {
+            self.pieces(color).fold(BitBoard::empty(), |acc, pp| {
+                acc | pp.sight_in_position(&self)
+            })
+        })
+    }
+
+    pub(crate) fn is_king_in_check(&self) -> bool {
+        let sight_of_opposing_player = self.sight_of_player(self.color_to_move.other());
+        let king_square = self
+            .bitboard_for_piece(Piece::king(self.color_to_move))
+            .occupied_squares()
+            .next()
+            .unwrap();
+        sight_of_opposing_player.is_set(king_square)
+    }
+}
 }
 
 impl Default for Position {
@@ -288,4 +313,22 @@ mod tests {
             .place_piece(piece, square)
             .expect_err("Placed white queen on e4 a second time?!");
     }
+
+    #[test]
+    fn king_is_in_check() {
+        let pos = position![
+            White King on E1,
+            Black Rook on E8,
+        ];
+        assert!(pos.is_king_in_check());
+    }
+
+    #[test]
+    fn king_is_not_in_check() {
+        let pos = position![
+            White King on F1,
+            Black Rook on E8,
+        ];
+        assert!(!pos.is_king_in_check());
+    }
 }