Make 'half_move_clock' a 'u32'

It *could* be set to a high value due to e.g: starting the engine in the
middle of a game.

Moving from a `u8` to a `u32` does not change the size of the type, so
let's just do that.

Use that opportunity to fix the comment about the number of
*half-moves* (it's 50 moves *per player*).

src/board/chess_board/builder.rs

@@ -8,9 +8,10 @@ pub struct ChessBoardBuilder {
     // Same fields as [ChessBoard].
     castle_rights: [CastleRights; Color::NUM_VARIANTS],
     en_passant: Option<Square>,
-    half_move_clock: u8,
-    total_plies: u32,
+    half_move_clock: u32,
     side: Color,
+    // 1-based, a turn is *two* half-moves (i.e: both players have played).
+    turn_count: u32,
 }
 
 impl ChessBoardBuilder {
@@ -20,8 +21,8 @@ impl ChessBoardBuilder {
             castle_rights: [CastleRights::NoSide; 2],
             en_passant: Default::default(),
             half_move_clock: Default::default(),
-            total_plies: Default::default(),
             side: Color::White,
+            turn_count: 1,
         }
     }
 
@@ -40,13 +41,13 @@ impl ChessBoardBuilder {
         self
     }
 
-    pub fn with_half_move_clock(&mut self, clock: u8) -> &mut Self {
+    pub fn with_half_move_clock(&mut self, clock: u32) -> &mut Self {
         self.half_move_clock = clock;
         self
     }
 
-    pub fn with_total_plies(&mut self, plies: u32) -> &mut Self {
-        self.total_plies = plies;
+    pub fn with_turn_count(&mut self, count: u32) -> &mut Self {
+        self.turn_count = count;
         self
     }
 
@@ -90,8 +91,8 @@ impl TryFrom<ChessBoardBuilder> for ChessBoard {
             castle_rights,
             en_passant,
             half_move_clock,
-            total_plies,
             side,
+            turn_count,
         } = builder;
 
         for square in Square::iter() {
@@ -103,6 +104,8 @@ impl TryFrom<ChessBoardBuilder> for ChessBoard {
             combined_occupancy |= square;
         }
 
+        let total_plies = (turn_count - 1) * 2 + if side == Color::White { 0 } else { 1 };
+
         let board = ChessBoard {
             piece_occupancy,
             color_occupancy,
@@ -146,7 +149,7 @@ mod test {
 
         builder
             .with_half_move_clock(board.half_move_clock())
-            .with_total_plies(board.total_plies())
+            .with_turn_count(board.total_plies() / 2 + 1)
             .with_current_player(board.current_player());
 
         builder

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

@@ -24,7 +24,7 @@ pub struct ChessBoard {
     /// `Some(target_square)` if a double-step move was made.
     en_passant: Option<Square>,
     /// The number of half-turns without either a pawn push or capture.
-    half_move_clock: u8, // Should never go higher than 50.
+    half_move_clock: u32, // Should *probably* never go higher than 100.
     /// The number of half-turns so far.
     total_plies: u32, // Should be plenty.
     /// The current player turn.
@@ -36,7 +36,7 @@ pub struct ChessBoard {
 pub struct NonReversibleState {
     castle_rights: [CastleRights; Color::NUM_VARIANTS],
     en_passant: Option<Square>,
-    half_move_clock: u8, // Should never go higher than 50.
+    half_move_clock: u32, // Should *probably* never go higher than 100.
 }
 
 impl ChessBoard {
@@ -105,7 +105,7 @@ impl ChessBoard {
 
     /// Return the number of half-turns without either a pawn push or a capture.
     #[inline(always)]
-    pub fn half_move_clock(&self) -> u8 {
+    pub fn half_move_clock(&self) -> u32 {
         self.half_move_clock
     }
 
@@ -208,8 +208,13 @@ 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() {
+        if 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 = {

src/fen.rs

@@ -139,24 +139,21 @@ impl FromFen for ChessBoard {
             builder.with_castle_rights(castle_rights[color.index()], color);
         }
 
-        let side = Color::from_fen(side_to_move)?;
-        builder.with_current_player(side);
+        builder.with_current_player(FromFen::from_fen(side_to_move)?);
 
-        if let Some(square) = Option::<Square>::from_fen(en_passant_square)? {
+        if let Some(square) = FromFen::from_fen(en_passant_square)? {
             builder.with_en_passant(square);
         };
 
         let half_move_clock = half_move_clock
-            .parse::<u8>()
+            .parse::<_>()
             .map_err(|_| FenError::InvalidFen)?;
         builder.with_half_move_clock(half_move_clock);
 
         let full_move_counter = full_move_counter
-            .parse::<u32>()
+            .parse::<_>()
             .map_err(|_| FenError::InvalidFen)?;
-        builder.with_total_plies(
-            (full_move_counter - 1) * 2 + if side == Color::White { 0 } else { 1 },
-        );
+        builder.with_turn_count(full_move_counter);
 
         {
             let mut rank: usize = 8;
@@ -175,7 +172,7 @@ impl FromFen for ChessBoard {
                             file += digit.to_digit(10).unwrap() as usize;
                             continue;
                         }
-                        _ => Piece::from_fen(&c.to_string())?,
+                        _ => FromFen::from_fen(&c.to_string())?,
                     };
 
                     // Only need to worry about underflow since those are `usize` values.