feat: store PV in a separate table

2 files changed, 44 insertions, 20 deletions

diff --git a/src/grossmeister/search.rs b/src/grossmeister/search.rs
index c81c586..372fecc 100644
--- a/src/grossmeister/search.rs
+++ b/src/grossmeister/search.rs
@@ -47,6 +47,12 @@ impl Grossmeister {
             return 0.0
         }
 
+        // Mate distance pruning
+        let mating_score = Grossmeister::MDP(&mut alpha, &mut beta, root_distance);
+        if mating_score != 0.0 {
+            return mating_score
+        }
+
         if let Some(transposition) = self.transposition() {
             if transposition.depth >= depth_left {
                 match transposition.node_type {
@@ -77,12 +83,6 @@ impl Grossmeister {
             node_type: NodeType::All,
         };
 
-        // Mate distance pruning
-        let mating_score = Grossmeister::MDP(&mut alpha, &mut beta, root_distance);
-        if mating_score != 0.0 {
-            return mating_score
-        }
-
         if depth_left == 0 {
             return self.quiscence(alpha, beta, root_distance)
         }
@@ -167,6 +167,12 @@ impl Grossmeister {
             return 0.0
         }
 
+        // Mate distance pruning
+        let mating_score = Grossmeister::MDP(&mut alpha, &mut beta, root_distance);
+        if mating_score != 0.0 {
+            return mating_score
+        }
+
         if let Some(transposition) = self.transposition() {
             match transposition.node_type {
                 NodeType::PV => {
@@ -195,14 +201,6 @@ impl Grossmeister {
             node_type: NodeType::All,
         };
 
-        // Mate distance pruning
-        let mating_score = Grossmeister::MDP(&mut alpha, &mut beta, root_distance);
-        if mating_score != 0.0 {
-            transposition.score = mating_score;
-            return mating_score
-        }
-
-
         let mut tactical_only = false;
         if !self.board.is_king_in_check(color) {
             // If we are not in check, we can evaluate stand pat
@@ -263,17 +261,17 @@ impl Grossmeister {
         alpha
     }
 
-    fn reconstruct_pv(&mut self, depth: u8) -> Vec<Move> {
+    fn collect_pv(&mut self, depth: u8) -> Vec<Move> {
         let mut pv = Vec::with_capacity(depth as usize);
 
         if depth > 0 {
-            if let Some(transposition) = self.transposition().cloned() {
+            if let Some(transposition) = self.pv().cloned() {
                 if let Some(mov) = transposition.mov {
                     if transposition.depth < depth {
-                        println!("info string found lower-depth {:?}-node ({}) than expected ({}). Probably bad", transposition.node_type, transposition.depth, depth);
+                        println!("info string found lower-depth {:?} node ({}) than expected ({}). Probably bad", transposition.node_type, transposition.depth, depth);
                     }
                     if transposition.depth > depth {
-                        println!("info string found higher-depth {:?}-node ({}) than expected ({}). Good for us?", transposition.node_type, transposition.depth, depth);
+                        println!("info string found higher-depth {:?} node ({}) than expected ({}). Good for us?", transposition.node_type, transposition.depth, depth);
                     }
                     let ep_target_before = self.board.ep_target;
                     let castling_rights_before = self.board.castling_rights;
@@ -281,12 +279,14 @@ impl Grossmeister {
                     let captured = self.board.make_move(mov);
 
                     pv.push(mov);
-                    let mut subtree_pv = self.reconstruct_pv(transposition.depth - 1);
+                    let mut subtree_pv = self.collect_pv(depth - 1);
                     pv.append(&mut subtree_pv);
 
                     self.board.unmake_move(mov, captured, ep_target_before, castling_rights_before, hash_before);
 
                 }
+            } else {
+                println!("No PV node found for hash {}", self.board.hash);
             }
         }
 
@@ -336,7 +336,7 @@ impl Grossmeister {
             }
 
             best_score = score;
-            pv = self.reconstruct_pv(depth);
+            pv = self.collect_pv(depth);
 
             // Print UCI info
             print!("info depth {} score ", depth);
diff --git a/src/grossmeister/ttable.rs b/src/grossmeister/ttable.rs
index b1eeec3..b40bac1 100644
--- a/src/grossmeister/ttable.rs
+++ b/src/grossmeister/ttable.rs
@@ -62,8 +62,12 @@ impl TranspositionTable {
 
 #[derive(Debug, Default, Clone)]
 pub struct MasterTable {
+    /// Always contains the most recent transposition
     always_replace: TranspositionTable,
+    /// Stores highest-depth entries
     depth_preferred: TranspositionTable,
+    /// Used to collect Principal Variation. Not used in search
+    pv: TranspositionTable,
 }
 
 impl Grossmeister {
@@ -81,6 +85,26 @@ impl Grossmeister {
         } {
             self.transposition_table.depth_preferred.set(self.board.hash, transposition)
         }
+
+        // Store PV/Cut nodes in PV table
+        // Note: Cut nodes are probably only relevant in close-to-mate situations
+        if match transposition.node_type {
+            NodeType::PV => true, // Always replace PV nodes
+            NodeType::Cut => {
+                match self.transposition_table.pv.get(self.board.hash) {
+                    Some(existing_transposition) => existing_transposition.node_type != NodeType::PV,
+                    None => true,
+                }
+            }
+            _ => false,
+        } {
+            self.transposition_table.pv.set(self.board.hash, transposition)
+        }
+    }
+
+    /// Extract current node information from PV table
+    pub fn pv(&self) -> Option<&TranspositionTableItem> {
+        self.transposition_table.pv.get(self.board.hash)
     }
 
     pub fn table_full(&self) -> u64 {