[board] Implement ply and full move counters on Position

Pipe these numbers through the Builders

board/src/position/position.rs

@@ -18,6 +18,9 @@ pub struct Position {
     pieces: PieceBitBoards,
     en_passant_square: Option<Square>,
     sight: [OnceCell<BitBoard>; 2],
+
+    half_move_counter: u16,
+    full_move_number: u16,
 }
 
 impl Position {
@@ -28,6 +31,8 @@ impl Position {
             pieces: PieceBitBoards::default(),
             en_passant_square: None,
             sight: [OnceCell::new(), OnceCell::new()],
+            half_move_counter: 0,
+            full_move_number: 1,
         }
     }
 
@@ -57,6 +62,8 @@ impl Position {
             pieces: PieceBitBoards::new([white_pieces, black_pieces]),
             en_passant_square: None,
             sight: [OnceCell::new(), OnceCell::new()],
+            half_move_counter: 0,
+            full_move_number: 1,
         }
     }
 
@@ -64,6 +71,14 @@ impl Position {
         self.color_to_move
     }
 
+    pub fn move_number(&self) -> u16 {
+        self.full_move_number
+    }
+
+    pub fn ply_counter(&self) -> u16 {
+        self.half_move_counter
+    }
+
     /// Returns true if the player has the right to castle on the given side of
     /// the board.
     ///
@@ -191,6 +206,8 @@ impl Position {
         flags: Flags,
         pieces: PieceBitBoards,
         en_passant_square: Option<Square>,
+        half_move_counter: u16,
+        full_move_number: u16,
     ) -> Self {
         Self {
             color_to_move: player_to_move,
@@ -198,6 +215,8 @@ impl Position {
             en_passant_square,
             pieces,
             sight: [OnceCell::new(), OnceCell::new()],
+            half_move_counter: 0,
+            full_move_number: 1,
         }
     }