Add TT cutoffs

src/search/negamax.rs

@@ -9,7 +9,7 @@ use super::{
     move_ordering::score_move,
     quiescence::quiescence,
     time::TimeInfo,
-    transposition_table::TTEntry,
+    transposition_table::{NodeType, TTEntry},
 };
 
 pub fn negamax(
@@ -30,8 +30,7 @@ pub fn negamax(
     }
 
     if depth == 0 {
-        let q_score =
-            quiescence(game, alpha, beta, time, nodes).map_err(|e| anyhow!("{e}"))?;
+        let q_score = quiescence(game, alpha, beta, time, nodes).map_err(|e| anyhow!("{e}"))?;
         return Ok(q_score);
     }
 
@@ -40,9 +39,22 @@ pub fn negamax(
     let mut best_score = MIN_SCORE;
     let mut best_move = None;
     let mut moves = game.board.pseudo_moves_all();
-    let tt_move = game.tt.lookup(game.hash).and_then(|entry| entry.mv);
+    let tt_entry = game.tt.lookup(game.hash);
 
-    moves.sort_unstable_by_key(|mv| score_move(&game.mailbox, *mv, tt_move));
+    if let Some(tt_entry) = tt_entry {
+        if plies > 0 && tt_entry.hash == game.hash && tt_entry.depth >= depth {
+            match tt_entry.node_type {
+                NodeType::Exact => return Ok(tt_entry.score),
+                NodeType::LowerBound if tt_entry.score >= beta => return Ok(tt_entry.score),
+                NodeType::UpperBound if tt_entry.score <= alpha => return Ok(tt_entry.score),
+                _ => (),
+            }
+        }
+    }
+
+    moves.sort_unstable_by_key(|mv| {
+        score_move(&game.mailbox, *mv, tt_entry.and_then(|entry| entry.mv))
+    });
 
     for mv in moves {
         game.make_move(&mv);
@@ -57,15 +69,7 @@ pub fn negamax(
         let score = if legal_moves == 1 {
             -negamax(game, -beta, -alpha, depth - 1, plies + 1, time, nodes)?
         } 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)?;
             if score > alpha && score < beta {
                 score = -negamax(game, -beta, -alpha, depth - 1, plies + 1, time, nodes)?;
             }
@@ -95,7 +99,15 @@ pub fn negamax(
         return Ok(0);
     }
 
-    game.tt.insert(TTEntry::new(game.hash, best_move));
+    let node_type = match best_score {
+        s if s >= beta => NodeType::LowerBound,
+        s if s <= alpha => NodeType::UpperBound,
+        _ => NodeType::Exact,
+    };
+
+    game.tt.insert(TTEntry::new(
+        game.hash, best_move, depth, best_score, node_type,
+    ));
     Ok(best_score)
 }