Add one killer move per ply

src/board/fen.rs

@@ -2,7 +2,7 @@ use crate::board::board::{Board, Color, PieceType};
 use crate::board::game::Game;
 use crate::board::state::{Castle, State};
 use crate::search::transposition_table::TranspositionTable;
-use crate::search::MAX_TT_SIZE;
+use crate::search::{MAX_DEPTH, MAX_TT_SIZE};
 use String as FenError;
 
 pub fn from_fen(fen: &str) -> Result<Game, FenError> {
@@ -37,6 +37,7 @@ pub fn from_fen(fen: &str) -> Result<Game, FenError> {
         mailbox,
         hash,
         tt: TranspositionTable::new(MAX_TT_SIZE),
+        killer: [None; MAX_DEPTH as usize],
     })
 }
 

src/board/game.rs

@@ -1,7 +1,7 @@
 use crate::{
     board::fen::from_fen,
     movegen::r#move::{Move, MoveType},
-    search::{transposition_table::TranspositionTable, MAX_TT_SIZE},
+    search::{transposition_table::TranspositionTable, MAX_DEPTH, MAX_TT_SIZE},
 };
 use String as FenError;
 
@@ -29,6 +29,7 @@ pub struct Game {
     pub mailbox: Mailbox,
     pub hash: ZobristHash,
     pub tt: TranspositionTable,
+    pub killer: [Option<Move>; MAX_DEPTH as usize],
 }
 
 impl Game {
@@ -39,6 +40,7 @@ impl Game {
             mailbox: Mailbox::from_board(&Board::startpos()),
             hash: zobrist_keys().calculate_hash(&Board::startpos()),
             tt: TranspositionTable::new(MAX_TT_SIZE),
+            killer: [None; MAX_DEPTH as usize],
         }
     }
 

src/movegen/move.rs

@@ -173,6 +173,13 @@ impl Move {
     pub fn parse_into_str(&self) -> String {
         format!("{self:?}")
     }
+
+    pub const fn is_capture(&self) -> bool {
+        matches!(
+            self.move_type,
+            MoveType::Capture | MoveType::PromotionCapture(_)
+        )
+    }
 }
 
 #[cfg(test)]

src/search/move_ordering.rs

@@ -1,10 +1,19 @@
 use crate::{board::mailbox::Mailbox, movegen::r#move::Move};
 
-pub fn score_move(mailbox: &Mailbox, mv: Move, tt_move: Option<Move>) -> i32 {
+pub fn score_move(
+    mailbox: &Mailbox,
+    mv: Move,
+    killer_move: Option<Move>,
+    tt_move: Option<Move>,
+) -> i32 {
     if Some(mv) == tt_move {
         return -100;
     }
 
+    if Some(mv) == killer_move {
+        return -90;
+    }
+
     let aggressor = mailbox.piece_at(mv.src).expect("No aggressor found.");
     mailbox.piece_at(mv.dst).map_or(100, |victim| {
         aggressor.0.idx() as i32 - (victim.0.idx() * 8) as i32
@@ -30,11 +39,11 @@ mod tests {
         let castle = Move::with_type(Square::E1, Square::C1, MoveType::Castle);
 
         let mut moves = vec![castle, queen_takes_pawn, pawn_takes_queen];
-        moves.sort_unstable_by_key(|mv| score_move(&game.mailbox, *mv, None));
+        moves.sort_unstable_by_key(|mv| score_move(&game.mailbox, *mv, None, None));
 
         assert_eq!(moves, vec![pawn_takes_queen, queen_takes_pawn, castle]);
 
-        moves.sort_unstable_by_key(|mv| score_move(&game.mailbox, *mv, Some(castle)));
+        moves.sort_unstable_by_key(|mv| score_move(&game.mailbox, *mv, None, Some(castle)));
 
         assert_eq!(moves, vec![castle, pawn_takes_queen, queen_takes_pawn]);
 

src/search/negamax.rs

@@ -53,7 +53,12 @@ pub fn negamax(
     }
 
     moves.sort_unstable_by_key(|mv| {
-        score_move(&game.mailbox, *mv, tt_entry.and_then(|entry| entry.mv))
+        score_move(
+            &game.mailbox,
+            *mv,
+            game.killer[plies as usize],
+            tt_entry.and_then(|entry| entry.mv),
+        )
     });
 
     for mv in moves {
@@ -88,6 +93,9 @@ pub fn negamax(
         }
 
         if score >= beta {
+            if !mv.is_capture() {
+                game.killer[plies as usize] = Some(mv);
+            }
             break;
         }
     }

src/search/quiescence.rs

@@ -42,7 +42,12 @@ pub fn quiescence(
     }
 
     moves.sort_unstable_by_key(|mv| {
-        score_move(&game.mailbox, *mv, tt_entry.and_then(|entry| entry.mv))
+        score_move(
+            &game.mailbox,
+            *mv,
+            None,
+            tt_entry.and_then(|entry| entry.mv),
+        )
     });
 
     for mv in moves {