Refactor en-passant updates in 'ChessBoard'

src/board/chess_board/builder.rs

@@ -106,7 +106,7 @@ impl TryFrom<ChessBoardBuilder> for ChessBoard {
 
         let total_plies = (turn_count - 1) * 2 + if side == Color::White { 0 } else { 1 };
 
-        let board = ChessBoard {
+        let mut board = ChessBoard {
             piece_occupancy,
             color_occupancy,
             combined_occupancy,
@@ -115,7 +115,9 @@ impl TryFrom<ChessBoardBuilder> for ChessBoard {
             half_move_clock,
             total_plies,
             side,
+            hash: 0,
         };
+        board.hash = board.compute_hash();
 
         board.validate()?;
         Ok(board)

src/board/chess_board/mod.rs

@@ -1,6 +1,6 @@
 use crate::movegen;
 
-use super::{Bitboard, CastleRights, Color, File, Move, Piece, Rank, Square};
+use super::{zobrist, Bitboard, CastleRights, Color, File, Move, Piece, Rank, Square};
 
 mod builder;
 pub use builder::*;
@@ -29,6 +29,8 @@ pub struct ChessBoard {
     total_plies: u32, // Should be plenty.
     /// The current player turn.
     side: Color,
+    /// Zobrist hash for the board.
+    hash: u64,
 }
 
 /// The state which can't be reversed when doing/un-doing a [Move].
@@ -123,12 +125,42 @@ impl ChessBoard {
         self.compute_checkers(self.current_player())
     }
 
+    /// Return the Zobrist hash for the board.
+    #[inline(always)]
+    pub fn hash(&self) -> u64 {
+        self.hash
+    }
+
+    /// Compute the Zobrist hash for the board.
+    fn compute_hash(&self) -> u64 {
+        let mut res = 0;
+
+        // The side-to-move is only toggled on Black, and otherwise 0.
+        if self.current_player() == Color::Black {
+            res ^= zobrist::side_to_move();
+        }
+        if let Some(square) = self.en_passant() {
+            res ^= zobrist::en_passant(square.file());
+        }
+        res ^= zobrist::castling_rights(self.castle_rights);
+        for piece in Piece::iter() {
+            for color in Color::iter() {
+                for square in self.occupancy(piece, color) {
+                    res ^= zobrist::moved_piece(color, piece, square);
+                }
+            }
+        }
+
+        res
+    }
+
     /// Quickly add/remove a piece on the [Bitboard]s that are part of the [ChessBoard] state.
     #[inline(always)]
     fn xor(&mut self, color: Color, piece: Piece, square: Square) {
         *self.piece_occupancy_mut(piece) ^= square;
         *self.color_occupancy_mut(color) ^= square;
         self.combined_occupancy ^= square;
+        self.hash ^= zobrist::moved_piece(color, piece, square);
     }
 
     /// Compute the change of [CastleRights] from moving/taking a piece.
@@ -140,9 +172,22 @@ impl ChessBoard {
             (Piece::King, _) => CastleRights::NoSide,
             _ => return,
         };
-        if new_rights != original {
-            *self.castle_rights_mut(color) = new_rights;
-        }
+        self.hash ^= zobrist::castling_rights(self.castle_rights);
+        *self.castle_rights_mut(color) = new_rights;
+        self.hash ^= zobrist::castling_rights(self.castle_rights);
+    }
+
+    fn update_en_passant(&mut self, en_passant: Option<Square>) {
+        self.hash ^= self
+            .en_passant()
+            .map(Square::file)
+            .map(zobrist::en_passant)
+            .unwrap_or(0);
+        self.hash ^= en_passant
+            .map(Square::file)
+            .map(zobrist::en_passant)
+            .unwrap_or(0);
+        self.en_passant = en_passant;
     }
 
     /// Play the given [Move], return a copy of the board with the resulting state.
@@ -182,15 +227,16 @@ impl ChessBoard {
         } else {
             self.half_move_clock += 1;
         }
-        if is_double_step {
+        let en_passant = if is_double_step {
             let target_square = Square::new(
                 chess_move.destination().file(),
                 self.current_player().third_rank(),
             );
-            self.en_passant = Some(target_square);
+            Some(target_square)
         } else {
-            self.en_passant = None;
-        }
+            None
+        };
+        self.update_en_passant(en_passant);
         self.update_castling(self.current_player(), move_piece, chess_move.start().file());
         if let Some(piece) = captured_piece {
             self.xor(opponent, piece, chess_move.destination());
@@ -203,6 +249,7 @@ impl ChessBoard {
         self.xor(self.current_player(), move_piece, chess_move.destination());
         self.total_plies += 1;
         self.side = !self.side;
+        self.hash ^= zobrist::side_to_move();
 
         state
     }
@@ -212,8 +259,12 @@ impl ChessBoard {
     #[inline(always)]
     pub fn unplay_move(&mut self, chess_move: Move, previous: NonReversibleState) {
         // Restore non-revertible state
-        self.castle_rights = previous.castle_rights;
-        self.en_passant = previous.en_passant;
+        if self.castle_rights != previous.castle_rights {
+            self.hash ^= zobrist::castling_rights(self.castle_rights);
+            self.hash ^= zobrist::castling_rights(previous.castle_rights);
+            self.castle_rights = previous.castle_rights;
+        }
+        self.update_en_passant(previous.en_passant);
         self.half_move_clock = previous.half_move_clock;
 
         let move_piece = Piece::iter()
@@ -231,6 +282,7 @@ impl ChessBoard {
         self.xor(!self.current_player(), move_piece, chess_move.start());
         self.total_plies -= 1;
         self.side = !self.side;
+        self.hash ^= zobrist::side_to_move();
     }
 
     /// Return true if the current state of the board looks valid, false if something is definitely
@@ -421,7 +473,7 @@ impl ChessBoard {
 /// "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1" FEN string
 impl Default for ChessBoard {
     fn default() -> Self {
-        Self {
+        let mut res = Self {
             piece_occupancy: [
                 // King
                 Square::E1 | Square::E8,
@@ -449,7 +501,10 @@ impl Default for ChessBoard {
             half_move_clock: 0,
             total_plies: 0,
             side: Color::White,
-        }
+            hash: 0,
+        };
+        res.hash = res.compute_hash();
+        res
     }
 }
 

src/board/mod.rs

@@ -27,3 +27,6 @@ pub use rank::*;
 
 pub mod square;
 pub use square::*;
+
+pub mod zobrist;
+pub use zobrist::*;

src/board/zobrist.rs

@@ -0,0 +1,978 @@
+// vim: foldmethod=marker
+use super::{CastleRights, Color, File, Piece, Square};
+
+type Hash = u64;
+
+type EnPassantHashesType = [Hash; File::NUM_VARIANTS];
+
+type CastlingHashesType = [[Hash; CastleRights::NUM_VARIANTS]; CastleRights::NUM_VARIANTS];
+
+type PieceHashesType = [[[Hash; Square::NUM_VARIANTS]; Piece::NUM_VARIANTS]; Color::NUM_VARIANTS];
+
+// region:sourcegen {{{
+static BLACK_TO_MOVE: Hash = 64934999470316615;
+
+static EN_PASSANT_HASHES: EnPassantHashesType = [
+    15459456780870779090,
+    13715484424881807779,
+    17718572936700675021,
+    14587996314750246637,
+    6677028061025754322,
+    13431580652429297681,
+    10925346160689749684,
+    14880644562654141744,
+];
+
+static CASTLING_HASHES: CastlingHashesType = [
+    [
+        1045274958981515586,
+        5392148996167941106,
+        14319713487061152634,
+        15981422700840256970,
+    ],
+    [
+        11438431953559128973,
+        15165640837126078406,
+        9682374909294934964,
+        3746638112738371362,
+    ],
+    [
+        13000030342210934332,
+        12451924122435870810,
+        1313989399406774579,
+        3743948554594595520,
+    ],
+    [
+        1870378950492858291,
+        9613620781770901799,
+        1269273018318959782,
+        13819231333457059,
+    ],
+];
+
+static PIECE_HASHES: PieceHashesType = [
+    [
+        [
+            15846310122511714997,
+            15565321750356536561,
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+            12051949271418111299,
+            6735994487591330742,
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+        [
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+        ],
+        [
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+            9001292467502639703,
+            5302703806177083024,
+            11829855060898447362,
+            7792871150005700771,
+        ],
+        [
+            1520093331355966943,
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+        ],
+        [
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+        ],
+        [
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+            61217168966658081,
+        ],
+        [
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+            6812119576048580698,
+            6424206435528040580,
+            487857718885957481,
+            13371986807426798615,
+        ],
+    ],
+];
+// endregion:sourcegen }}}
+
+/// Return the Zobrist hash for a [Piece] of a given [Color] on a given [Square].
+pub fn moved_piece(color: Color, piece: Piece, square: Square) -> Hash {
+    PIECE_HASHES[color.index()][piece.index()][square.index()]
+}
+
+/// Return the Zobrist hash for the [CastleRights] for a [Color].
+pub fn castling_rights(rights: [CastleRights; Color::NUM_VARIANTS]) -> Hash {
+    CASTLING_HASHES[rights[0].index()][rights[1].index()]
+}
+
+/// Return the Zobrist hash for the [File] of an en-passant capture.
+pub fn en_passant(file: File) -> Hash {
+    EN_PASSANT_HASHES[file.index()]
+}
+
+/// Return the Zobrist hash for the given side-to-move being [Color::Black].
+pub fn side_to_move() -> Hash {
+    BLACK_TO_MOVE
+}
+
+#[cfg(test)]
+mod test {
+    use std::fmt::{Display, Write as _};
+
+    use super::*;
+    use crate::utils::SimpleRng;
+
+    fn split_twice<'a>(
+        text: &'a str,
+        start_marker: &str,
+        end_marker: &str,
+    ) -> Option<(&'a str, &'a str, &'a str)> {
+        let (prefix, rest) = text.split_once(start_marker)?;
+        let (mid, suffix) = rest.split_once(end_marker)?;
+        Some((prefix, mid, suffix))
+    }
+
+    fn array_string<T: Display>(indent_level: usize, values: &[T]) -> String {
+        let inner = || -> Result<String, std::fmt::Error> {
+            let mut res = String::new();
+
+            writeln!(&mut res, "[")?;
+            for val in values {
+                let indent = (indent_level + 1) * 4;
+                writeln!(&mut res, "{:indent$}{},", "", val)?;
+            }
+            let indent = indent_level * 4;
+            write!(&mut res, "{:indent$}]", "")?;
+
+            Ok(res)
+        };
+
+        inner().unwrap()
+    }
+
+    #[test]
+    fn regen_zobrist_hashes() {
+        let mut rng = SimpleRng::new();
+
+        macro_rules! rng_iter {
+            ($outer:expr, $( $inner:expr ),+) => {{
+                let mut res = vec![];
+                for _ in $outer {
+                    res.push(rng_iter!($($inner),+));
+                }
+                res
+            }};
+            ($iter:expr) => {{
+                let mut res = vec![];
+                for _ in $iter {
+                    res.push(rng.gen());
+                }
+                res
+            }};
+        }
+
+        let black_to_move = rng.gen();
+        let en_passant = rng_iter!(File::iter());
+        let castle_rights = rng_iter!(CastleRights::iter(), CastleRights::iter());
+        let move_piece = rng_iter!(Color::iter(), Piece::iter(), Square::iter());
+
+        let original_text = std::fs::read_to_string(file!()).unwrap();
+        let new_text = {
+            let start_marker = "// region:sourcegen {{{\n";
+            let end_marker = "// endregion:sourcegen }}}\n";
+            let (prefix, _, suffix) =
+                split_twice(&original_text, start_marker, end_marker).unwrap();
+
+            let black_to_move = format!("static BLACK_TO_MOVE: Hash = {};\n", black_to_move);
+            let en_passant = format!(
+                "static EN_PASSANT_HASHES: EnPassantHashesType = {};\n",
+                array_string(0, &en_passant[..]),
+            );
+            let castle_rights = {
+                let strings: Vec<_> = castle_rights
+                    .iter()
+                    .map(|v| array_string(1, &v[..]))
+                    .collect();
+                format!(
+                    "static CASTLING_HASHES: CastlingHashesType = {};\n",
+                    array_string(0, &strings[..])
+                )
+            };
+            let move_piece = {
+                let strings: Vec<_> = move_piece
+                    .iter()
+                    .map(|v| {
+                        let strings: Vec<_> = v.iter().map(|v| array_string(2, &v[..])).collect();
+                        array_string(1, &strings[..])
+                    })
+                    .collect();
+                format!(
+                    "static PIECE_HASHES: PieceHashesType = {};\n",
+                    array_string(0, &strings[..])
+                )
+            };
+            let hashes = format!("{black_to_move}\n{en_passant}\n{castle_rights}\n{move_piece}");
+            format!("{prefix}{start_marker}{hashes}{end_marker}{suffix}")
+        };
+
+        if new_text != original_text {
+            std::fs::write(file!(), new_text).unwrap();
+            panic!("source was not up-to-date")
+        }
+    }
+}