feat: pv search

Improves nodes searched by a lot - https://web.archive.org/web/20071030220825/http://www.brucemo.com/compchess/programming/pvs.htm
znxftw · Dec 17, 2024 · 784894a · 784894a
1 parent a0445d4
commit 784894a
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Showing 2 changed files with 29 additions and 8 deletions.
diff --git a/Rudim.Test/UnitTest/Board/TraversalTest.cs b/Rudim.Test/UnitTest/Board/TraversalTest.cs
@@ -13,10 +13,10 @@ public class TraversalTest
         // This helps keep track if certain optimizations are good enough to make up for the extra time spent
         // Compare time spent with and without the change before updating the keys
         [Theory]
-        [InlineData(Helpers.StartingFEN, 3916083,5, 8)]
-        [InlineData(Helpers.EndgameFEN, 2327956,45, 9)]
-        [InlineData(Helpers.AdvancedMoveFEN, 10012236,1520, 8)]
-        [InlineData(Helpers.KiwiPeteFEN, 11101517,-18, 8)]
+        [InlineData(Helpers.StartingFEN, 3804303,5, 8)]
+        [InlineData(Helpers.EndgameFEN, 1130154,45, 9)]
+        [InlineData(Helpers.AdvancedMoveFEN, 6689081,1520, 8)]
+        [InlineData(Helpers.KiwiPeteFEN, 10605420,-18, 8)]
         public void ShouldTraverseDeterministically(string position, int expectedNodes, int expectedScore, int depth)
         {
             Global.Reset();

diff --git a/Rudim/Search/Negamax.cs b/Rudim/Search/Negamax.cs
@@ -37,6 +37,7 @@ private static int Search(BoardState boardState, int depth, int alpha, int beta,
 
             int originalAlpha = alpha;
             int ply = _searchDepth - depth;
+            bool foundPv = false;
             Move bestEvaluation = Move.NoMove;
             Nodes++;
 
@@ -55,7 +56,9 @@ private static int Search(BoardState boardState, int depth, int alpha, int beta,
                     boardState.UnmakeMove(move);
                     continue;
                 }
-                int score = -Search(boardState, depth - 1, -beta, -alpha, cancellationToken);
+
+                int score = SearchDeeper(boardState, depth, alpha, beta, cancellationToken, foundPv);
+
                 numberOfLegalMoves++;
 
                 boardState.UnmakeMove(move);
@@ -65,14 +68,14 @@ private static int Search(BoardState boardState, int depth, int alpha, int beta,
                 }
                 if (score > alpha)
                 {
-                    AlphaUpdate(out alpha, score, move, out bestEvaluation);
+                    AlphaUpdate(out alpha, score, move, out bestEvaluation, out foundPv);
                 }
             }
 
             if (numberOfLegalMoves == 0)
             {
                 if (boardState.IsInCheck(boardState.SideToMove))
-                    return -Constants.MaxCentipawnEval + (_searchDepth - depth);
+                    return -Constants.MaxCentipawnEval + ply;
                 return 0;
             }
 
@@ -82,10 +85,28 @@ private static int Search(BoardState boardState, int depth, int alpha, int beta,
             return alpha;
         }
 
-        private static void AlphaUpdate(out int alpha, int score, Move move, out Move bestEvaluation)
+        private static int SearchDeeper(BoardState boardState, int depth, int alpha, int beta,
+            CancellationToken cancellationToken, bool foundPv)
+        {
+            int score;
+            if (foundPv)
+            {
+                score = -Search(boardState, depth - 1, -alpha - 1, -alpha, cancellationToken);
+                if (score > alpha && score < beta) 
+                    score = -Search(boardState,depth - 1, -beta, -alpha, cancellationToken);
+            }
+            else
+            {
+                score = -Search(boardState, depth - 1, -beta, -alpha, cancellationToken);
+            }
+            return score;
+        }
+
+        private static void AlphaUpdate(out int alpha, int score, Move move, out Move bestEvaluation, out bool foundPv)
         {
             alpha = score;
             bestEvaluation = move;
+            foundPv = true;
         }
 
         private static int BetaCutoff(int beta, Move move, int ply)