Add Null-move pruning

src/search/iterative_deepening.rs

@@ -18,12 +18,15 @@ pub fn iterative_deepening(
 
     for depth in 1..=max_depth {
         if time.exceed_soft_limit() {
-            write_response(&mut io::stdout(), &Response::Info("Soft limit exceeded in negamax".to_string()))?;
+            write_response(
+                &mut io::stdout(),
+                &Response::Info("Soft limit exceeded in negamax".to_string()),
+            )?;
             return Ok(best_move);
         }
 
         let mut nodes = 0;
-        let score = negamax::negamax(game, MIN_SCORE, MAX_SCORE, depth, 0, time, &mut nodes);
+        let score = negamax::negamax(game, MIN_SCORE, MAX_SCORE, depth, 0, time, &mut nodes, true);
 
         if let Err(e) = score {
             write_response(&mut io::stdout(), &Response::Info(format!("{e}")))?;
@@ -36,7 +39,7 @@ pub fn iterative_deepening(
             &mut io::stdout(),
             &log_depth_results(
                 depth,
-                time.instant.elapsed().as_millis() as u64,
+                time.instant.elapsed().as_secs(),
                 nodes,
                 time.nps(nodes),
                 score?,

src/search/negamax.rs

@@ -20,6 +20,7 @@ pub fn negamax(
     plies: u8,
     time: &TimeInfo,
     nodes: &mut u64,
+    do_nmp: bool,
 ) -> Result<i32> {
     if time.exceed_hard_limit() {
         bail!("Hard limit exceeded in negamax");
@@ -41,6 +42,25 @@ pub fn negamax(
         return Ok(q_score);
     }
 
+    if plies != 0 && !in_check && depth >= 3 && do_nmp && game.board.non_pawn_materials() > 0 {
+        game.make_null_move();
+        let score = -negamax(
+            game,
+            -beta,
+            -beta + 1,
+            depth - 3,
+            plies + 1,
+            time,
+            nodes,
+            false,
+        )?;
+        game.unmake_null_move();
+
+        if score >= beta {
+            return Ok(score);
+        }
+    }
+
     let mut legal_moves = 0;
     let mut best_score = MIN_SCORE;
     let mut best_move = None;
@@ -78,11 +98,20 @@ pub fn negamax(
         *nodes += 1;
 
         let score = if legal_moves == 1 {
-            -negamax(game, -beta, -alpha, depth - 1, plies + 1, time, nodes)?
+            -negamax(game, -beta, -alpha, depth - 1, plies + 1, time, nodes, true)?
         } else {
-            let mut score = -negamax(game, -alpha - 1, -alpha, depth - 1, plies + 1, time, nodes)?;
+            let mut score = -negamax(
+                game,
+                -alpha - 1,
+                -alpha,
+                depth - 1,
+                plies + 1,
+                time,
+                nodes,
+                true,
+            )?;
             if score > alpha && score < beta {
-                score = -negamax(game, -beta, -alpha, depth - 1, plies + 1, time, nodes)?;
+                score = -negamax(game, -beta, -alpha, depth - 1, plies + 1, time, nodes, true)?;
             }
             score
         };
@@ -147,16 +176,20 @@ mod tests {
 
         let e3f2 = Move::new(Square::E3, Square::F2);
         let time_info = TimeInfo::new(std::time::Instant::now(), TIME, INC);
-        negamax(&mut game, MIN_SCORE, MAX_SCORE, 2, 0, &time_info, &mut 0)
-            .expect("Expected a search result");
+        negamax(
+            &mut game, MIN_SCORE, MAX_SCORE, 5, 0, &time_info, &mut 0, true,
+        )
+        .expect("Expected a search result");
 
         assert_eq!(e3f2, game.tt.lookup(game.hash).unwrap().mv.unwrap());
 
         let mut game = from_fen(FEN_MATE_IN_1[1]).unwrap();
 
         let e3f2 = Move::new(Square::E3, Square::F2);
-        negamax(&mut game, MIN_SCORE, MAX_SCORE, 2, 0, &time_info, &mut 0)
-            .expect("Expected a search result");
+        negamax(
+            &mut game, MIN_SCORE, MAX_SCORE, 5, 0, &time_info, &mut 0, true,
+        )
+        .expect("Expected a search result");
 
         assert_eq!(e3f2, game.tt.lookup(game.hash).unwrap().mv.unwrap());
 

src/search/transposition_table.rs

@@ -79,8 +79,10 @@ mod tests {
         let mut game = from_fen(FEN).unwrap();
         let time_info = TimeInfo::new(std::time::Instant::now(), 30000, 100);
 
-        negamax(&mut game, MIN_SCORE, MAX_SCORE, 2, 0, &time_info, &mut 0)
-            .expect("Expected a search result");
+        negamax(
+            &mut game, MIN_SCORE, MAX_SCORE, 2, 0, &time_info, &mut 0, true,
+        )
+        .expect("Expected a search result");
 
         dbg!(time_info.instant.elapsed());