Add one killer move per ply

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],
         }
     }