/// <summary>
/// Search all capture positions, to help avoid the Horizon effect.
/// </summary>
private int Quiescence(int alpha, int beta, ref S_SearchInfo info)
{
Debug.Assert(BoardOperations.CheckBoard(board));
if ((info.Nodes & 2047) == 0)
{
CheckUp(ref info);
}
info.Nodes++;
// If position is a draw.
if ((IsRepetition() || board.FiftyMoves >= 100) && board.Ply != 0)
{
return(0);
}
int score = Evaluate.Position(board); // Stand_pat.
if (board.Ply > Variables.MAX_DEPTH - 1)
{
return(score);
}
if (score >= beta)
{
return(beta);
}
if (score > alpha)
{
alpha = score;
}
MoveList list = new MoveList();
MoveGen.GenerateAllMoves(board, list, true); // Only capture moves
int oldAlpha = alpha;
score = -infinite;
int legal = 0; // Will increment when we find a legal move.
int bestMove = Variables.NO_MOVE;
int PvMove = PvTable.Probe(board);
for (int i = 0; i < list.Count; ++i)
{
PickNextMove(i, list);
var move = list.Moves[i].Move;
if (!MakeMove.Make_Move(board, move))
{
continue;
}
legal++;
score = -Quiescence(-beta, -alpha, ref info);
MakeMove.TakeMove(board); // Take back the made move.
if (info.Stopped)
{
return(0);
}
// We have a new alpha or beta cutoff.
if (score > alpha)
{
bool isCaptureMove = (move & MoveOperations.MoveFlagCapture) != 0;
// beta cutoff?
if (score >= beta)
{
if (legal == 1)
{
info.Fhf++; // We searched the best move first.
}
info.Fh++;
return(beta);
}
// Alpha cutoff
alpha = score;
bestMove = move;
}
}
if (alpha != oldAlpha)
{
PvTable.StoreMove(board, bestMove);
}
return(alpha);
}
public int AlphaBeta(int alpha, int beta, int depth, ref S_SearchInfo info, bool DoNull)
{
Debug.Assert(BoardOperations.CheckBoard(board));
if (depth == 0)
{
return(Quiescence(alpha, beta, ref info));
}
if ((info.Nodes & 2047) == 0)
{
CheckUp(ref info);
}
info.Nodes++;
// If position is a draw.
if ((IsRepetition() || board.FiftyMoves >= 100) && board.Ply != 0)
{
return(0);
}
if (board.Ply > Variables.MAX_DEPTH - 1)
{
return(Evaluate.Position(board));
}
bool kingInCheck = Attack.IsSqAttacked(board.KingSq[board.Side], board.Side ^ 1, board);
// If king is in check, search deeper to get out of check.
if (kingInCheck)
{
depth++;
// The two following lines are possibly ERROR.
long timeInc = (info.StopTime - info.StartTime) * (1 / 2);
info.StopTime += timeInc;
}
MoveList list = new MoveList();
MoveGen.GenerateAllMoves(board, list, false);
int oldAlpha = alpha;
int score = -infinite;
int legal = 0; // Will increment when we find a legal move.
int bestMove = Variables.NO_MOVE;
int PvMove = PvTable.Probe(board);
// Prioritize Principle Variation move if it's found.
if (PvMove != Variables.NO_MOVE)
{
for (int i = 0; i < list.Count; ++i)
{
var move = list.Moves[i].Move;
if (move == PvMove)
{
list.Moves[i].Score = 2000000;
break;
}
}
}
for (int i = 0; i < list.Count; ++i)
{
PickNextMove(i, list);
var move = list.Moves[i].Move;
if (!MakeMove.Make_Move(board, move))
{
continue;
}
legal++;
score = -AlphaBeta(-beta, -alpha, depth - 1, ref info, true);
MakeMove.TakeMove(board); // Take back the made move.
if (info.Stopped)
{
return(0); // Back up to the root if times up.
}
// We have a new alpha or beta cutoff.
if (score > alpha)
{
bool isCaptureMove = (move & MoveOperations.MoveFlagCapture) != 0;
// beta cutoff?
if (score >= beta)
{
if (legal == 1)
{
info.Fhf++; // We searched the best move first.
}
info.Fh++;
// If beta cutoff, but no capture move.
if (!isCaptureMove)
{
board.SearchKillers[1, board.Ply] = board.SearchKillers[0, board.Ply];
board.SearchKillers[0, board.Ply] = move;
}
return(beta);
}
// Alpha cutoff
alpha = score;
bestMove = move;
if (!isCaptureMove)
{
int from = MoveOperations.FromSq(move);
int to = MoveOperations.ToSq(move);
board.SearchHistory[board[from], to] += depth; // Prioritizes move near the root of the tree.
}
}
}
// If we haven't had any legal moves.
if (legal == 0)
{
// If in check with no legal moves checkmate.
if (kingInCheck)
{
return(-mate + board.Ply); // Return the amount of moves it takes to mate.
// Returning in this way, allows the method to "prefer" the fastest checkmate combination.
}
else
{
return(0); // Stalemate.
}
}
if (alpha != oldAlpha)
{
PvTable.StoreMove(board, bestMove);
}
return(alpha);
}