[board, position] Add a color argument to opposing_sight

New convention: active_color_ methods operate on the active color of the Board.
Methods without that prefix take a color parameter and operate on that.

Refactor opposing_sight to do this.

board/src/castle.rs

@@ -61,7 +61,7 @@ impl Board {
         }
 
         // King cannot pass through check.
-        let opposing_sight = self.opposing_sight();
+        let opposing_sight = self.opposing_sight(active_color);
         let opposing_pieces_can_see_castling_path =
             (parameters.check & opposing_sight).is_populated();
         if opposing_pieces_can_see_castling_path {

board/src/sight.rs

@@ -44,14 +44,17 @@ impl Board {
             .fold(BitBoard::empty(), BitOr::bitor)
     }
 
+    pub fn active_color_opposing_sight(&self) -> BitBoard {
+        self.opposing_sight(self.active_color)
+    }
+
     /// A [`BitBoard`] of all squares visible by colors that oppose the given color.
-    pub fn opposing_sight(&self) -> BitBoard {
+    pub fn opposing_sight(&self, color: Color) -> BitBoard {
         // TODO: Probably want to implement a caching layer here.
-        let active_color = self.active_color;
         Color::ALL
             .into_iter()
             .filter_map(|c| {
-                if c == active_color {
+                if c == color {
                     None
                 } else {
                     Some(self.friendly_sight(c))
@@ -223,7 +226,7 @@ mod tests {
             Black Rook on E7,
         );
 
-        let sight = pos.opposing_sight();
+        let sight = pos.active_color_opposing_sight();
         assert_eq!(sight, bitboard![A7 B7 C7 D7 F7 G7 H7 E8 E6 E5 E4]);
     }
 

position/src/position/position.rs

@@ -81,8 +81,8 @@ impl Position {
     }
 
     /// A [`BitBoard`] of all squares visible by colors that oppose the given color.
-    pub fn opposing_sight(&self) -> BitBoard {
-        self.board.opposing_sight()
+    pub fn active_color_opposing_sight(&self) -> BitBoard {
+        self.board.active_color_opposing_sight()
     }
 }