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lithanderlithander
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Added history heuristic and a simple late move reduction. Many variants are possible. Currently any move that raises alpha (and not only beta cutoffs) are remembered as good moves and the reduction is only applied during the null-window search that proofs a node is below alpha. Research happens at full depth.
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10 files changed

+310
-12
lines changed

10 files changed

+310
-12
lines changed

‎MinimalChess/History.cs‎

Lines changed: 73 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,73 @@
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using System;
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using System.Collections.Generic;
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using System.Linq;
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using System.Text;
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using System.Threading.Tasks;
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namespace MinimalChess
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{
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public static class History
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{
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private const int Squares = 64;
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private const int Pieces = 12;
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private static int[,] Positive = new int[Squares, Pieces];
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private static int[,] Negative = new int[Squares, Pieces];
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public static int Max
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{
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get
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{
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int max = 0;
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for (int square = 0; square < Squares; square++)
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for (int piece = 0; piece < Pieces; piece++)
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{
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max = Math.Max(max, Positive[square, piece]);
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max = Math.Max(max, Negative[square, piece]);
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}
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return max;
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}
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}
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public static void Clear()
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{
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for (int square = 0; square < Squares; square++)
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for (int piece = 0; piece < Pieces; piece++)
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{
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Positive[square, piece] = 0;
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Negative[square, piece] = 0;
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}
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}
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public static void Shrink()
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{
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for (int square = 0; square < Squares; square++)
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for(int piece = 0; piece < Pieces; piece++)
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{
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Positive[square, piece] /= 2;
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Negative[square, piece] /= 2;
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}
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}
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public static void Good(Piece piece, int square, int depth)
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{
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int iPiece = ((byte)piece >> 1) - 2; //BlackPawn = 0...
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Positive[square, iPiece] += depth * depth;
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}
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public static void Bad(Piece piece, int square, int depth)
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{
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int iPiece = ((byte)piece >> 1) - 2; //BlackPawn = 0...
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Negative[square, iPiece] += depth * depth;
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}
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public static float Value(Piece piece, int square)
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{
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int iPiece = ((byte)piece >> 1) - 2; //BlackPawn = 0...
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float a = Positive[square, iPiece] + 1;
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float b = Negative[square, iPiece] + 2;
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return a / b;
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}
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}
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}

‎MinimalChess/IterativeSearch.cs‎

Lines changed: 12 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -25,6 +25,7 @@ public IterativeSearch(Board board, long maxNodes = long.MaxValue)
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_root = new Board(board);
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_killers = new KillerMoves(4);
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_maxNodes = maxNodes;
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History.Clear();
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}
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public IterativeSearch(int searchDepth, Board board) : this(board)
@@ -40,6 +41,7 @@ public void SearchDeeper(Func<bool> killSwitch = null)
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Depth++;
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_killers.Resize(Depth);
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History.Shrink();
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StorePVinTT(PrincipalVariation, Depth);
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_killSwitch = new KillSwitch(killSwitch);
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(Score, PrincipalVariation) = EvalPosition(_root, Depth, SearchWindow.Infinite);
@@ -79,6 +81,8 @@ private void StorePVinTT(Move[] pv, int depth)
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Color color = position.SideToMove;
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bool isChecked = position.IsChecked(color);
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int futilityMargin = (int)color * depth * MAX_GAIN_PER_PLY;
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//should we try null move pruning?
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if (depth >= 2 && !isChecked)
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{
@@ -101,25 +105,30 @@ private void StorePVinTT(Move[] pv, int depth)
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expandedNodes++;
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//moves after the PV node are unlikely to raise alpha. skip those that appear clearly futile!
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int futilityMargin = (int)color * depth * MAX_GAIN_PER_PLY;
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if (expandedNodes > 1 && depth <= 4 && !isChecked && window.FailLow(child.Score + futilityMargin, color) && !child.IsChecked(child.SideToMove))
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bool reduce = expandedNodes > 1 && !isChecked && (move.Promotion < Piece.Queen) && !child.IsChecked(child.SideToMove);
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if (reduce && depth <= 4 && window.FailLow(child.Score + futilityMargin, color))
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continue;
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//moves after the PV node are unlikely to raise alpha. searching with a null-sized window can save a lot of nodes
109113
if (expandedNodes > 1 && depth >= 3)
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{
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//we can save a lot of nodes by searching with "null window" first, proving cheaply that the score is below alpha...
116+
int R = reduce && expandedNodes >= 4 ? 1 : 0;
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SearchWindow nullWindow = window.GetLowerBound(color);
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var nullResult = EvalPositionTT(child, depth - 1, nullWindow);
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var nullResult = EvalPositionTT(child, depth - 1 - R, nullWindow);
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if (nullWindow.FailLow(nullResult.Score, color))
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continue;
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}
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//this node may raise alpha!
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var eval = EvalPositionTT(child, depth - 1, window);
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if (window.FailLow(eval.Score, color))
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{
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History.Bad(position[move.FromSquare], move.ToSquare, depth);
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continue;
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}
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History.Good(position[move.FromSquare], move.ToSquare, depth);
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Transpositions.Store(position.ZobristHash, depth, window, eval.Score, move);
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//store the PV beginning with move, followed by the PV of the childnode
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pv = Merge(move, eval.PV);

‎MinimalChess/MoveCollection.cs‎

Lines changed: 9 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -63,5 +63,14 @@ int Score(Move move)
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}
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Sort((a, b) => Score(b).CompareTo(Score(a)));
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}
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public void SortHistory(Board context)
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{
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float Score(Move move)
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{
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return History.Value(context[move.FromSquare], move.ToSquare);
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}
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Sort((a, b) => Score(b).CompareTo(Score(a)));
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}
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}
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}

‎MinimalChess/Playmaker.cs‎

Lines changed: 6 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -1,4 +1,5 @@
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using System.Collections.Generic;
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using System;
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using System.Collections.Generic;
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namespace MinimalChess
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{
@@ -32,7 +33,10 @@ public static class Playmaker
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}
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//4. Play quiet moves that aren't known killers
35-
foreach (var move in MoveList.Quiets(position))
36+
var quiets = MoveList.Quiets(position);
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if(depth >= 3)
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quiets.SortHistory(position);
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foreach (var move in quiets)
3640
if (!killers.Contains(depth, move))
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{
3842
var nextPosition = new Board(position, move);

‎MinimalChess/SearchWindow.cs‎

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Original file line numberDiff line numberDiff line change
@@ -2,13 +2,13 @@
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{
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public struct SearchWindow
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{
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public static SearchWindow Infinite = new SearchWindow(short.MinValue, short.MaxValue);
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public static SearchWindow Infinite = new(short.MinValue, short.MaxValue);
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public int Floor;//Alpha
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public int Ceiling;//Beta
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10-
public SearchWindow UpperBound => new SearchWindow(Ceiling - 1, Ceiling);
11-
public SearchWindow LowerBound => new SearchWindow(Floor, Floor + 1);
10+
public SearchWindow UpperBound => new(Ceiling - 1, Ceiling);
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public SearchWindow LowerBound => new(Floor, Floor + 1);
1212
//used to quickly determine that a move is not improving the score for color.
1313
public SearchWindow GetLowerBound(Color color) => color == Color.White ? LowerBound : UpperBound;
1414
//used to quickly determine that a move is too good and will not be allowed by the opponent .

‎MinimalChessBoard/MinimalChessBoard.csproj‎

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@@ -27,6 +27,9 @@
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<None Update="wac-revised.epd">
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<CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
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</None>
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<None Update="wac.epd">
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<CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
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</None>
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<None Update="zugzwang.epd">
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<CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
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</None>

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