Add total plie count validation

src/board/chess_board/error.rs

@@ -23,6 +23,8 @@ pub enum InvalidError {
     ErroneousCombinedOccupancy,
     /// Half-move clock is higher than total number of plies.
     HalfMoveClockTooHigh,
+    /// The total plie count does not match the current player.
+    IncoherentPlieCount,
 }
 
 impl std::fmt::Display for InvalidError {
@@ -45,6 +47,7 @@ impl std::fmt::Display for InvalidError {
                 "The pre-computed combined occupancy boards does not match the other boards."
             }
             Self::HalfMoveClockTooHigh => "Half-move clock is higher than total number of plies.",
+            Self::IncoherentPlieCount => "The total plie count does not match the current player.",
         };
         write!(f, "{}", error_msg)
     }

src/board/chess_board/mod.rs

@@ -208,6 +208,11 @@ impl ChessBoard {
 
     /// Validate the state of the board. Return Err([InvalidError]) if an issue is found.
     pub fn validate(&self) -> Result<(), InvalidError> {
+        // The current plie count should be odd on white's turn, and vice-versa.
+        if self.total_plies() % 2 != self.current_player().index() as u32 {
+            return Err(InvalidError::IncoherentPlieCount);
+        }
+
         // Make sure the clocks are in agreement.
         if u32::from(self.half_move_clock()) > self.total_plies() {
             return Err(InvalidError::HalfMoveClockTooHigh);
@@ -428,6 +433,22 @@ mod test {
         assert!(default_position.is_valid());
     }
 
+    #[test]
+    fn invalid_incoherent_plie_count() {
+        let position = {
+            let mut builder = ChessBoardBuilder::new();
+            builder[Square::E1] = Some((Piece::King, Color::White));
+            builder[Square::E8] = Some((Piece::King, Color::Black));
+            let mut board = TryInto::<ChessBoard>::try_into(builder).unwrap();
+            board.total_plies = 1;
+            board
+        };
+        assert_eq!(
+            position.validate().err().unwrap(),
+            InvalidError::IncoherentPlieCount,
+        );
+    }
+
     #[test]
     fn invalid_half_moves_clock() {
         let res = {