[position] Implement a SliderRayToSquare trait

This trait declares ray_to_square() which should return a BitBoard representing
a ray from a Square to another Square. The ray should include the target Square.

PlacedPiece implements this trait.

bitboard/src/bitboard.rs

@@ -39,7 +39,7 @@ impl BitBoard {
         library::FILES[*file as usize]
     }
 
-    pub fn ray(sq: Square, dir: Direction) -> BitBoard {
+    pub fn ray(sq: Square, dir: Direction) -> &'static BitBoard {
         library::library().ray(sq, dir)
     }
 
@@ -107,6 +107,24 @@ impl BitBoard {
     pub fn occupied_squares_trailing(&self) -> impl Iterator<Item = Square> {
         LeadingBitScanner::new(self.0).map(|idx| unsafe { Square::from_index(idx as u8) })
     }
+
+    pub fn first_occupied_square(&self) -> Option<Square> {
+        let leading_zeros = self.0.leading_zeros() as u8;
+        if leading_zeros < Square::NUM as u8 {
+            unsafe { Some(Square::from_index(Square::NUM as u8 - leading_zeros - 1)) }
+        } else {
+            None
+        }
+    }
+
+    pub fn first_occupied_square_trailing(&self) -> Option<Square> {
+        let trailing_zeros = self.0.trailing_zeros() as u8;
+        if trailing_zeros < Square::NUM as u8 {
+            unsafe { Some(Square::from_index(trailing_zeros)) }
+        } else {
+            None
+        }
+    }
 }
 
 impl Default for BitBoard {
@@ -377,4 +395,14 @@ mod tests {
         assert_eq!(BitBoard::from(Square::A1), BitBoard(0b1));
         assert_eq!(BitBoard::from(Square::H8), BitBoard(1 << 63));
     }
+
+    #[test]
+    fn first_occupied_squares() {
+        let bb = bitboard![A8, E1];
+        assert_eq!(bb.first_occupied_square(), Some(Square::A8));
+        assert_eq!(bb.first_occupied_square_trailing(), Some(Square::E1));
+
+        let bb = bitboard![D6, E7, F8];
+        assert_eq!(bb.first_occupied_square_trailing(), Some(Square::D6));
+    }
 }

bitboard/src/library.rs

@@ -220,8 +220,8 @@ impl MoveLibrary {
         ray
     }
 
-    pub(super) fn ray(&self, sq: Square, dir: Direction) -> BitBoard {
-        self.rays[sq as usize][dir as usize]
+    pub(super) fn ray(&self, sq: Square, dir: Direction) -> &BitBoard {
+        &self.rays[sq as usize][dir as usize]
     }
 
     pub(super) fn pawn_pushes(&self, sq: Square, color: Color) -> BitBoard {

position/src/sight.rs

@@ -26,6 +26,10 @@ pub(crate) trait SightExt {
     fn king_sight(&self, pieces: &PieceBitBoards) -> BitBoard;
 }
 
+pub(crate) trait SliderRayToSquareExt {
+    fn ray_to_square(&self, square: Square) -> Option<BitBoard>;
+}
+
 impl SightExt for PlacedPiece {
     fn sight(&self, pieces: &PieceBitBoards, en_passant_square: Option<Square>) -> BitBoard {
         match self.shape() {
@@ -183,8 +187,80 @@ impl SightExt for PlacedPiece {
     }
 }
 
+impl SliderRayToSquareExt for PlacedPiece {
+    fn ray_to_square(&self, target: Square) -> Option<BitBoard> {
+        macro_rules! ray {
+            ($square:expr, $direction:ident) => {
+                (
+                    BitBoard::ray($square, Direction::$direction),
+                    Direction::$direction,
+                )
+            };
+        }
+
+        let square = self.square();
+        let target_bitboard: BitBoard = target.into();
+
+        let ray_and_direction = match self.shape() {
+            Shape::Bishop => [
+                ray!(square, NorthEast),
+                ray!(square, SouthEast),
+                ray!(square, SouthWest),
+                ray!(square, NorthWest),
+            ]
+            .into_iter()
+            .find(|(&ray, _)| !(target_bitboard & ray).is_empty()),
+            Shape::Rook => [
+                ray!(square, North),
+                ray!(square, East),
+                ray!(square, South),
+                ray!(square, West),
+            ]
+            .into_iter()
+            .find(|(&ray, _)| !(target_bitboard & ray).is_empty()),
+            Shape::Queen => [
+                ray!(square, North),
+                ray!(square, NorthEast),
+                ray!(square, East),
+                ray!(square, SouthEast),
+                ray!(square, South),
+                ray!(square, SouthWest),
+                ray!(square, West),
+                ray!(square, NorthWest),
+            ]
+            .into_iter()
+            .find(|(&ray, _)| !(target_bitboard & ray).is_empty()),
+            _ => None,
+        };
+
+        if let Some((ray, direction)) = ray_and_direction {
+            let first_occupied_square = match direction {
+                Direction::East
+                | Direction::NorthWest
+                | Direction::NorthEast
+                | Direction::North => ray.first_occupied_square_trailing(),
+                Direction::West
+                | Direction::SouthWest
+                | Direction::SouthEast
+                | Direction::South => ray.first_occupied_square(),
+            };
+
+            if let Some(occupied_square) = first_occupied_square {
+                let remainder = BitBoard::ray(target, direction);
+                return Some(ray & !remainder);
+            }
+        }
+
+        None
+    }
+}
+
 #[cfg(test)]
 mod tests {
+    use super::*;
+    use chessfriend_bitboard::bitboard;
+    use chessfriend_core::{piece, Square};
+
     macro_rules! sight_test {
         ($test_name:ident, $position:expr, $piece:expr, $bitboard:expr) => {
             #[test]
@@ -201,6 +277,12 @@ mod tests {
         };
     }
 
+    #[test]
+    fn pawns_and_knights_cannot_make_rays() {
+        assert_eq!(piece!(White Pawn on F7).ray_to_square(Square::E8), None);
+        assert_eq!(piece!(White Knight on F6).ray_to_square(Square::E8), None);
+    }
+
     mod pawn {
         use crate::test_position;
         use chessfriend_bitboard::{bitboard, BitBoard};
@@ -273,20 +355,25 @@ mod tests {
     }
 
     mod bishop {
-        use chessfriend_bitboard::bitboard;
-        use chessfriend_core::piece;
+        use super::*;
 
         sight_test!(
             c2_bishop,
             piece!(Black Bishop on C2),
             bitboard!(D1, B3, A4, B1, D3, E4, F5, G6, H7)
         );
+
+        #[test]
+        fn ray_to_square() {
+            let generated_ray = piece!(White Bishop on C5).ray_to_square(Square::E7);
+            let expected_ray = bitboard![D6, E7];
+            assert_eq!(generated_ray, Some(expected_ray));
+        }
     }
 
     mod rook {
+        use super::*;
         use crate::test_position;
-        use chessfriend_bitboard::bitboard;
-        use chessfriend_core::piece;
 
         sight_test!(
             g3_rook,
@@ -304,6 +391,25 @@ mod tests {
             piece!(White Rook on E4),
             bitboard!(A4, B4, C4, D4, F4, G4, H4, E2, E3, E5, E6, E7)
         );
+
+        #[test]
+        fn ray_to_square() {
+            let generated_ray = piece!(White Rook on C2).ray_to_square(Square::C6);
+            let expected_ray = bitboard![C3, C4, C5, C6];
+            assert_eq!(generated_ray, Some(expected_ray));
+
+            let generated_ray = piece!(White Rook on D2).ray_to_square(Square::H2);
+            let expected_ray = bitboard![E2, F2, G2, H2];
+            assert_eq!(generated_ray, Some(expected_ray));
+
+            let generated_ray = piece!(White Rook on G6).ray_to_square(Square::B6);
+            let expected_ray = bitboard![B6, C6, D6, E6, F6];
+            assert_eq!(generated_ray, Some(expected_ray));
+
+            let generated_ray = piece!(White Rook on A6).ray_to_square(Square::A3);
+            let expected_ray = bitboard![A5, A4, A3];
+            assert_eq!(generated_ray, Some(expected_ray));
+        }
     }
 
     mod king {