Add one killer move per ply

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;
         }
     }