Just maintain a halfmove_number

src/board.rs

@@ -27,7 +27,7 @@ struct InternalState {
     keys: Keys,
     en_passant: Square,
     castling: Castling,
-    halfmove_clock: u8,
+    fiftymove_clock: u8,
     material: i32,
     plies_from_null: usize,
     repetition: i32,
@@ -42,13 +42,12 @@ struct InternalState {
 
 #[derive(Clone)]
 pub struct Board {
-    side_to_move: Color,
     pieces: [Bitboard; PieceType::NUM],
     colors: [Bitboard; Color::NUM],
     mailbox: [Piece; Square::NUM],
     state: InternalState,
     state_stack: Vec<InternalState>,
-    fullmove_number: usize,
+    halfmove_number: usize,
     castling_rights: [u8; Square::NUM],
     castling_path: [Bitboard; 16],
     castling_threat: [Bitboard; 16],
@@ -65,23 +64,23 @@ impl Board {
         self.frc
     }
 
-    pub const fn side_to_move(&self) -> Color {
-        self.side_to_move
+    pub const fn fullmove_number(&self) -> usize {
+        self.halfmove_number / 2
     }
 
-    pub const fn fullmove_number(&self) -> usize {
-        self.fullmove_number
+    pub fn side_to_move(&self) -> Color {
+        Color::new((self.halfmove_number & 1) as u8)
     }
 
-    pub fn halfmove_clock_bucket(&self) -> usize {
-        (self.halfmove_clock().saturating_sub(8) as usize / 8).min(15)
+    pub fn fiftymove_clock_bucket(&self) -> usize {
+        (self.fiftymove_clock().saturating_sub(8) as usize / 8).min(15)
     }
 
     pub fn hash(&self) -> u64 {
         // To mitigate Graph History Interaction (GHI) problems, the hash key is changed
         // every 8 plies to distinguish between positions that would otherwise appear
         // identical to the transposition table.
-        self.state.keys.full() ^ ZOBRIST.halfmove_clock[self.halfmove_clock_bucket()]
+        self.state.keys.full() ^ ZOBRIST.fiftymove_clock[self.fiftymove_clock_bucket()]
     }
 
     pub const fn pawn_key(&self) -> u64 {
@@ -133,8 +132,8 @@ impl Board {
         self.state.castling
     }
 
-    pub const fn halfmove_clock(&self) -> u8 {
-        self.state.halfmove_clock
+    pub const fn fiftymove_clock(&self) -> u8 {
+        self.state.fiftymove_clock
     }
 
     pub const fn material(&self) -> i32 {
@@ -193,14 +192,6 @@ impl Board {
         self.mailbox[mv.from()]
     }
 
-    pub const fn advance_fullmove_counter(&mut self) {
-        self.fullmove_number += self.side_to_move() as usize;
-    }
-
-    pub const fn retreat_fullmove_counter(&mut self) {
-        self.fullmove_number -= self.side_to_move() as usize;
-    }
-
     pub const fn set_frc(&mut self, frc: bool) {
         self.frc = frc;
     }
@@ -258,12 +249,12 @@ impl Board {
     }
 
     pub fn has_repeated(&self) -> bool {
-        let end = self.state.plies_from_null.min(self.state.halfmove_clock as usize);
+        let end = self.state.plies_from_null.min(self.state.fiftymove_clock as usize);
         self.state_stack.iter().rev().take(end.saturating_sub(3)).any(|s| s.repetition != 0)
     }
 
     pub fn draw_by_fifty_move_rule(&self) -> bool {
-        self.halfmove_clock() >= 100 && (!self.in_check() || self.has_legal_moves())
+        self.fiftymove_clock() >= 100 && (!self.in_check() || self.has_legal_moves())
     }
 
     /// Checks if the position is a known draw by material, fifty-move or repetition.
@@ -278,7 +269,7 @@ impl Board {
     ///
     /// <http://web.archive.org/web/20201107002606/https://marcelk.net/2013-04-06/paper/upcoming-rep-v2.pdf>
     pub fn upcoming_repetition(&self, ply: usize) -> bool {
-        let half_moves = self.state.plies_from_null.min(self.state.halfmove_clock as usize);
+        let half_moves = self.state.plies_from_null.min(self.state.fiftymove_clock as usize);
         if half_moves < 3 {
             return false;
         }
@@ -558,13 +549,12 @@ impl Board {
 impl Default for Board {
     fn default() -> Self {
         Self {
-            side_to_move: Color::White,
             state: InternalState::default(),
             pieces: [Bitboard::default(); PieceType::NUM],
             colors: [Bitboard::default(); Color::NUM],
             mailbox: [Piece::None; Square::NUM],
             state_stack: Vec::with_capacity(2048),
-            fullmove_number: 0,
+            halfmove_number: 0,
             castling_rights: [0b1111; Square::NUM],
             castling_path: [Bitboard::default(); 16],
             castling_threat: [Bitboard::default(); 16],

src/board/makemove.rs

@@ -3,7 +3,7 @@ use crate::types::{Move, MoveKind, Piece, PieceType, Square};
 
 impl Board {
     pub fn make_null_move(&mut self) {
-        self.side_to_move = !self.side_to_move;
+        self.halfmove_number += 1;
         self.state_stack.push(self.state);
 
         self.state.keys.toggle_side();
@@ -21,7 +21,7 @@ impl Board {
     }
 
     pub fn undo_null_move(&mut self) {
-        self.side_to_move = !self.side_to_move();
+        self.halfmove_number -= 1;
         self.state = self.state_stack.pop().unwrap();
     }
 
@@ -32,7 +32,7 @@ impl Board {
         let from = mv.from();
         let to = mv.to();
         let piece = self.piece_on(from);
-        let stm = self.side_to_move;
+        let stm = self.side_to_move();
 
         self.state_stack.push(self.state);
 
@@ -49,9 +49,9 @@ impl Board {
         self.state.plies_from_null += 1;
 
         if mv.kind() == MoveKind::Capture || piece.piece_type() == PieceType::Pawn {
-            self.state.halfmove_clock = 0;
+            self.state.fiftymove_clock = 0;
         } else {
-            self.state.halfmove_clock += 1;
+            self.state.fiftymove_clock += 1;
         }
 
         if mv.is_castling() {
@@ -113,8 +113,7 @@ impl Board {
             self.state.material += promotion.value() - PieceType::Pawn.value();
         }
 
-        self.advance_fullmove_counter();
-        self.side_to_move = !self.side_to_move;
+        self.halfmove_number += 1;
 
         self.state.castling.raw &= self.castling_rights[from] & self.castling_rights[to];
         self.state.keys.toggle_castling(self.state.castling);
@@ -124,7 +123,7 @@ impl Board {
 
         self.state.repetition = 0;
 
-        let end = self.state.plies_from_null.min(self.halfmove_clock() as usize);
+        let end = self.state.plies_from_null.min(self.fiftymove_clock() as usize);
 
         if end >= 4 {
             let mut idx = self.state_stack.len() as isize - 4;
@@ -146,13 +145,12 @@ impl Board {
     }
 
     pub fn undo_move(&mut self, mv: Move) {
-        self.side_to_move = !self.side_to_move;
-        self.retreat_fullmove_counter();
+        self.halfmove_number -= 1;
 
         let from = mv.from();
         let to = mv.to();
         let mover = self.remove_piece(to);
-        let stm = self.side_to_move;
+        let stm = self.side_to_move();
 
         if mv.is_castling() {
             let (rook_from, rook_to) = self.get_castling_rook(to);

src/board/parser.rs

@@ -45,7 +45,7 @@ impl Board {
             }
         }
 
-        board.side_to_move = match parts.next() {
+        let side_to_move = match parts.next() {
             Some("w") => Color::White,
             Some("b") => Color::Black,
             _ => return Err(ParseFenError::InvalidActiveColor),
@@ -54,8 +54,9 @@ impl Board {
         board.set_castling(parts.next().unwrap());
 
         board.state.en_passant = parts.next().unwrap_or_default().try_into().unwrap_or_default();
-        board.state.halfmove_clock = parts.next().unwrap_or_default().parse().unwrap_or_default();
-        board.fullmove_number = parts.next().unwrap_or_default().parse().unwrap_or_default();
+        board.state.fiftymove_clock = parts.next().unwrap_or_default().parse().unwrap_or_default();
+        let fullmove_number: usize = parts.next().unwrap_or_default().parse().unwrap_or_default();
+        board.halfmove_number = (2 * fullmove_number) + side_to_move as usize;
 
         board.update_threats();
         board.update_hash_keys();
@@ -144,15 +145,15 @@ impl Board {
         }
 
         fen.push(' ');
-        fen.push_str(&self.side_to_move.to_string());
+        fen.push_str(&self.side_to_move().to_string());
         fen.push(' ');
         fen.push_str(&self.state.castling.to_string(self));
         fen.push(' ');
         fen.push_str(&self.state.en_passant.to_string());
         fen.push(' ');
-        fen.push_str(&self.state.halfmove_clock.to_string());
+        fen.push_str(&self.state.fiftymove_clock.to_string());
         fen.push(' ');
-        fen.push_str(&self.fullmove_number.to_string());
+        fen.push_str(&self.fullmove_number().to_string());
         fen
     }
 

src/evaluation.rs

@@ -5,7 +5,7 @@ pub fn correct_eval(td: &ThreadData, raw_eval: i32, correction_value: i32) -> i3
         + td.optimism[td.board.side_to_move()] * (1543 + td.board.material()))
         / 26663;
 
-    eval = eval * (200 - td.board.halfmove_clock() as i32) / 200;
+    eval = eval * (200 - td.board.fiftymove_clock() as i32) / 200;
 
     eval += correction_value;
 

src/search.rs

@@ -329,7 +329,7 @@ fn search<NODE: NodeType>(
     let mut depth = depth.min(MAX_PLY as i32 - 1);
 
     let hash = td.board.hash();
-    let entry = td.shared.tt.read(hash, td.board.halfmove_clock(), ply);
+    let entry = td.shared.tt.read(hash, td.board.fiftymove_clock(), ply);
 
     let mut tt_depth = 0;
     let mut tt_move = Move::NULL;
@@ -365,7 +365,7 @@ fn search<NODE: NodeType>(
                 update_continuation_histories(td, ply, td.board.moved_piece(tt_move), tt_move.to(), cont_bonus);
             }
 
-            if td.board.halfmove_clock() < 90 {
+            if td.board.fiftymove_clock() < 90 {
                 return tt_score;
             }
         }
@@ -376,7 +376,7 @@ fn search<NODE: NodeType>(
     if !NODE::ROOT
         && !excluded
         && !td.shared.stop_probing_tb.load(Ordering::Relaxed)
-        && td.board.halfmove_clock() == 0
+        && td.board.fiftymove_clock() == 0
         && td.board.castling().raw() == 0
         && td.board.occupancies().popcount() <= tb::size()
         && let Some(outcome) = tb::probe(&td.board)
@@ -1169,7 +1169,7 @@ fn qsearch<NODE: NodeType>(td: &mut ThreadData, mut alpha: i32, beta: i32, ply:
     }
 
     let hash = td.board.hash();
-    let entry = td.shared.tt.read(hash, td.board.halfmove_clock(), ply);
+    let entry = td.shared.tt.read(hash, td.board.fiftymove_clock(), ply);
 
     let mut tt_score = Score::NONE;
     let mut tt_bound = Bound::None;
@@ -1321,7 +1321,7 @@ fn qsearch<NODE: NodeType>(td: &mut ThreadData, mut alpha: i32, beta: i32, ply:
 
 fn eval_correction(td: &ThreadData, ply: isize) -> i32 {
     let stm = td.board.side_to_move();
-    let bucket = td.board.halfmove_clock_bucket();
+    let bucket = td.board.fiftymove_clock_bucket();
     let corrhist = td.corrhist();
 
     (corrhist.pawn.get(stm, td.board.pawn_key(), bucket)
@@ -1342,7 +1342,7 @@ fn eval_correction(td: &ThreadData, ply: isize) -> i32 {
 
 fn update_correction_histories(td: &mut ThreadData, depth: i32, diff: i32, ply: isize) {
     let stm = td.board.side_to_move();
-    let bucket = td.board.halfmove_clock_bucket();
+    let bucket = td.board.fiftymove_clock_bucket();
     let corrhist = td.corrhist();
     let bonus = (146 * depth * diff / 128).clamp(-4449, 2659);
 

src/tb.rs

@@ -113,7 +113,7 @@ pub fn rank_rootmoves(td: &mut ThreadData) {
             td.board.pieces(PieceType::Bishop).0,
             td.board.pieces(PieceType::Knight).0,
             td.board.pieces(PieceType::Pawn).0,
-            td.board.halfmove_clock() as u32,
+            td.board.fiftymove_clock() as u32,
             0,
             ep_square,
             td.board.side_to_move() == Color::White,
@@ -140,7 +140,7 @@ pub fn rank_rootmoves(td: &mut ThreadData) {
             td.board.pieces(PieceType::Bishop).0,
             td.board.pieces(PieceType::Knight).0,
             td.board.pieces(PieceType::Pawn).0,
-            td.board.halfmove_clock() as u32,
+            td.board.fiftymove_clock() as u32,
             0,
             ep_square,
             td.board.side_to_move() == Color::White,

src/types/zobrist.rs

@@ -6,7 +6,7 @@ pub struct Zobrist {
     pub en_passant: [u64; 64],
     pub castling: [u64; 16],
     pub side: u64,
-    pub halfmove_clock: [u64; 16],
+    pub fiftymove_clock: [u64; 16],
 }
 
 pub const ZOBRIST: Zobrist = {