[position] Implement FromFen for Position, Piece, and Color

I can now create Positions from FEN strings!

position/src/position/builders/position_builder.rs

@@ -13,7 +13,8 @@ pub struct Builder {
     player_to_move: Color,
     flags: Flags,
     pieces: BTreeMap<Square, Piece>,
-    kings: [Square; 2],
+    kings: [Option<Square>; 2],
+    en_passant_square: Option<Square>,
     ply_counter: u16,
     move_number: u16,
 }
@@ -23,6 +24,18 @@ impl Builder {
         Self::default()
     }
 
+    pub(crate) fn empty() -> Self {
+        Self {
+            player_to_move: Color::default(),
+            flags: Flags::default(),
+            pieces: BTreeMap::default(),
+            kings: [None, None],
+            en_passant_square: None,
+            ply_counter: 0,
+            move_number: 1,
+        }
+    }
+
     pub fn from_position(position: &Position) -> Self {
         let pieces = BTreeMap::from_iter(
             position
@@ -38,7 +51,8 @@ impl Builder {
             player_to_move: position.player_to_move(),
             flags: position.flags(),
             pieces,
-            kings: [white_king, black_king],
+            kings: [Some(white_king), Some(black_king)],
+            en_passant_square: position.en_passant_square(),
             ply_counter: position.ply_counter(),
             move_number: position.move_number(),
         }
@@ -59,6 +73,11 @@ impl Builder {
         self
     }
 
+    pub fn en_passant_square(&mut self, square: Option<Square>) -> &mut Self {
+        self.en_passant_square = square;
+        self
+    }
+
     pub fn place_piece(&mut self, piece: PlacedPiece) -> &mut Self {
         let square = piece.square();
         let shape = piece.shape();
@@ -66,8 +85,11 @@ impl Builder {
         if shape == Shape::King {
             let color = piece.color();
             let color_index: usize = color as usize;
-            self.pieces.remove(&self.kings[color_index]);
-            self.kings[color_index] = square;
+
+            if let Some(king_square) = self.kings[color_index] {
+                self.pieces.remove(&king_square);
+            }
+            self.kings[color_index] = Some(square);
         }
 
         self.pieces.insert(square, *piece.piece());
@@ -75,6 +97,17 @@ impl Builder {
         self
     }
 
+    pub fn player_can_castle(&mut self, color: Color, castle: Castle) -> &mut Self {
+        self.flags
+            .set_player_has_right_to_castle_flag(color, castle);
+        self
+    }
+
+    pub fn no_castling_rights(&mut self) -> &mut Self {
+        self.flags.clear_all_castling_rights();
+        self
+    }
+
     pub fn build(&self) -> Position {
         let pieces = PieceBitBoards::from_iter(
             self.pieces
@@ -93,7 +126,7 @@ impl Builder {
                     .get(&starting_squares.rook)
                     .is_some_and(|piece| piece.shape() == Shape::Rook);
                 let king_is_on_starting_square =
-                    self.kings[color as usize] == starting_squares.king;
+                    self.kings[color as usize] == Some(starting_squares.king);
 
                 if !king_is_on_starting_square || !has_rook_on_starting_square {
                     flags.clear_player_has_right_to_castle_flag(color, castle);
@@ -105,7 +138,7 @@ impl Builder {
             self.player_to_move,
             flags,
             pieces,
-            None,
+            self.en_passant_square,
             self.ply_counter,
             self.move_number,
         )
@@ -139,7 +172,8 @@ impl Default for Builder {
             player_to_move: Color::White,
             flags: Flags::default(),
             pieces: pieces,
-            kings: [white_king_square, black_king_square],
+            kings: [Some(white_king_square), Some(black_king_square)],
+            en_passant_square: None,
             ply_counter: 0,
             move_number: 1,
         }

position/src/position/flags.rs

@@ -24,6 +24,10 @@ impl Flags {
     pub(super) fn clear_player_has_right_to_castle_flag(&mut self, color: Color, castle: Castle) {
         self.0 &= !(1 << Self::player_has_right_to_castle_flag_offset(color, castle));
     }
+
+    pub(super) fn clear_all_castling_rights(&mut self) {
+        self.0 &= 0b11111100;
+    }
 }
 
 impl fmt::Debug for Flags {

position/src/position/position.rs

@@ -269,8 +269,8 @@ impl Position {
             pieces,
             sight: [OnceCell::new(), OnceCell::new()],
             moves: OnceCell::new(),
-            half_move_counter: 0,
-            full_move_number: 1,
+            half_move_counter,
+            full_move_number,
         }
     }