Index killers by ply

src/search/movepick.rs

@@ -18,7 +18,7 @@ pub struct MovePicker<const QUIESCE: bool> {
     stage: MoveStage,
     position: Position,
     table: Option<Arc<SearchTable>>,
-    depth: Option<Depth>,
+    ply: Depth,
 }
 
 impl MovePicker<true> {
@@ -34,7 +34,7 @@ impl MovePicker<true> {
             stage: MoveStage::CapturesAndPromotions,
             position: *position,
             table: None,
-            depth: None,
+            ply: 0,
         }
     }
 }
@@ -48,7 +48,7 @@ impl std::iter::Iterator for MovePicker<true> {
 }
 
 impl MovePicker<false> {
-    pub fn new(position: &Position, table: Arc<SearchTable>, depth: Depth, shuffle: bool) -> Self {
+    pub fn new(position: &Position, table: Arc<SearchTable>, ply: Depth, shuffle: bool) -> Self {
         let moves = if !shuffle {
             if let Some(hash_move) = table.get_hash_move(position) {
                 vec![(hash_move, ValueScore::MAX)]
@@ -69,7 +69,7 @@ impl MovePicker<false> {
             stage: if !shuffle { MoveStage::HashMove } else { MoveStage::All },
             position: *position,
             table: Some(table),
-            depth: Some(depth),
+            ply,
         }
     }
 }
@@ -97,7 +97,7 @@ impl std::iter::Iterator for MovePicker<false> {
                 self.stage = MoveStage::NonCaptures;
                 let all_non_capture_moves = self.position.moves(MoveStage::NonCaptures);
 
-                let killers = self.table.as_ref().unwrap().get_killers(self.depth.unwrap());
+                let killers = self.table.as_ref().unwrap().get_killers(self.ply);
                 self.moves = decorate_moves_with_score(&all_non_capture_moves, |mov| {
                     if killers[1] == Some(mov) || killers[0] == Some(mov) {
                         Piece::Queen.value()

src/search/pvs.rs

@@ -241,7 +241,7 @@ fn pvs<const ROOT: bool, const MAIN_THREAD: bool, const ALLOW_NMR: bool>(
 
     // Prepare move generation and sorting. This is lazy and works in stages.
     let mut picker =
-        MovePicker::<false>::new(position, table.clone(), depth, ROOT && !MAIN_THREAD).peekable();
+        MovePicker::<false>::new(position, table.clone(), ply, ROOT && !MAIN_THREAD).peekable();
 
     // Detect checkmate and stalemate
     if picker.peek().is_none() {
@@ -314,7 +314,7 @@ fn pvs<const ROOT: bool, const MAIN_THREAD: bool, const ALLOW_NMR: bool>(
                 if MAIN_THREAD && mov.flag().is_quiet() {
                     // Killer moves are prioritized in move ordering.
                     // It assumes that similar "refutation" moves at siblings will be useful.
-                    table.put_killer_move(depth, mov);
+                    table.put_killer_move(ply, mov);
                 }
 
                 // This position is now far too good to be true.

src/search/table.rs

@@ -164,7 +164,7 @@ impl TranspositionTable {
 
 pub struct SearchTable {
     transposition: RwLock<TranspositionTable>,
-    killer_moves: [AtomicU16; 2 * (MAX_DEPTH + 1) as usize],
+    killer_moves: [AtomicU16; 3 * (MAX_DEPTH + 1) as usize],
 }
 
 impl SearchTable {
@@ -255,8 +255,8 @@ impl SearchTable {
         }
     }
 
-    pub fn put_killer_move(&self, depth: Depth, mov: Move) {
-        let index = 2 * depth as usize;
+    pub fn put_killer_move(&self, ply: Depth, mov: Move) {
+        let index = 2 * ply as usize;
         if self.load_killer(index).is_none() {
             self.store_killer(index, mov);
         } else if self.load_killer(index + 1).is_none() {
@@ -270,8 +270,8 @@ impl SearchTable {
         }
     }
 
-    pub fn get_killers(&self, depth: Depth) -> [Option<Move>; 2] {
-        let index = 2 * depth as usize;
+    pub fn get_killers(&self, ply: Depth) -> [Option<Move>; 2] {
+        let index = 2 * ply as usize;
         [self.load_killer(index), self.load_killer(index + 1)]
     }