Add one killer move per ply

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 {