private int QEval(Board position, int ply, SearchWindow window)
{
NodesVisited++;
if (Aborted)
{
return(0);
}
Color color = position.SideToMove;
bool inCheck = position.IsChecked(color);
//if inCheck we can't use standPat, need to escape check!
if (!inCheck)
{
int standPatScore = position.Score + _mobilityBonus;
//Cut will raise alpha and perform beta cutoff when standPatScore is too good
if (window.Cut(standPatScore, color))
{
return(GetScore(window, color));
}
}
int expandedNodes = 0;
//play remaining captures (or any moves if king is in check)
foreach (Board child in inCheck ? Playmaker.Play(position) : Playmaker.PlayCaptures(position))
{
expandedNodes++;
//recursively evaluate the resulting position (after the capture) with QEval
int score = QEval(child, ply + 1, window);
//Cut will raise alpha and perform beta cutoff when the move is too good
if (window.Cut(score, color))
{
break;
}
}
//checkmate?
if (expandedNodes == 0 && inCheck)
{
return(Evaluation.Checkmate(color, ply));
}
//stalemate?
if (expandedNodes == 0 && !LegalMoves.HasMoves(position))
{
return(0);
}
//can't capture. We return the 'alpha' which may have been raised by "stand pat"
return(GetScore(window, color));
}
private (int Score, Move[] PV) EvalPosition(Board position, int ply, int depth, SearchWindow window)
{
if (depth <= 0)
{
_mobilityBonus = Evaluation.ComputeMobility(position);
return(QEval(position, ply, window), Array.Empty <Move>());
}
NodesVisited++;
if (Aborted)
{
return(0, Array.Empty <Move>());
}
Color color = position.SideToMove;
bool isChecked = position.IsChecked(color);
//if the previous iteration found a mate we check the first few plys without null move to try and find the shortest mate or escape
bool allowNullMove = Evaluation.IsCheckmate(Score) ? (ply > Depth / 4) : true;
//should we try null move pruning?
if (allowNullMove && depth >= 2 && !isChecked && window.CanFailHigh(color))
{
const int R = 2;
//evaluate the position at reduced depth with a null-window around beta
SearchWindow beta = window.GetUpperBound(color);
//skip making a move
Board nullChild = Playmaker.PlayNullMove(position);
(int score, _) = EvalPositionTT(nullChild, ply + 1, depth - R - 1, beta);
//is the evaluation "too good" despite null-move? then don't waste time on a branch that is likely going to fail-high
//if the static eval look much worse the alpha also skip it
if (window.FailHigh(score, color))
{
return(score, Array.Empty <Move>());
}
}
//do a regular expansion...
Move[] pv = Array.Empty <Move>();
int expandedNodes = 0;
foreach ((Move move, Board child) in Playmaker.Play(position, depth, _killers, _history))
{
expandedNodes++;
bool interesting = expandedNodes == 1 || isChecked || child.IsChecked(child.SideToMove);
//some near the leaves that appear hopeless can be skipped without evaluation
if (depth <= 4 && !interesting)
{
//if the static eval look much worse the alpha also skip it
int futilityMargin = (int)color * depth * MAX_GAIN_PER_PLY;
if (window.FailLow(child.Score + futilityMargin, color))
{
continue;
}
}
//moves after the PV node are unlikely to raise alpha.
//avoid a full evaluation by searching with a null-sized window around alpha first
//...we expect it to fail low but if it does not we have to research it!
if (depth >= 2 && expandedNodes > 1)
{
//non-tactical late moves are searched at a reduced depth to make this test even faster!
int R = (interesting || expandedNodes < 4) ? 0 : 2;
(int score, _) = EvalPositionTT(child, ply + 1, depth - R - 1, window.GetLowerBound(color));
if (window.FailLow(score, color))
{
continue;
}
}
//this move is expected to raise alpha so we search it at full depth!
var eval = EvalPositionTT(child, ply + 1, depth - 1, window);
if (window.FailLow(eval.Score, color))
{
_history.Bad(position, move, depth);
continue;
}
//the position has a new best move and score!
Transpositions.Store(position.ZobristHash, depth, ply, window, eval.Score, move);
//set the PV to this move, followed by the PV of the childnode
pv = Merge(move, eval.PV);
//...and maybe we even get a beta cutoff
if (window.Cut(eval.Score, color))
{
//we remember killers like hat!
if (position[move.ToSquare] == Piece.None)
{
_history.Good(position, move, depth);
_killers.Add(move, depth);
}
return(GetScore(window, color), pv);
}
}
//checkmate or draw?
if (expandedNodes == 0)
{
return(position.IsChecked(color) ? Evaluation.Checkmate(color, ply) : 0, Array.Empty <Move>());
}
return(GetScore(window, color), pv);
}