public int Search(int alpha, int beta, int depth)
{
Metrics.Nodes++;
PvLength[Ply] = Ply;
var inCheck = MyBoard.InCheck(MyBoard.SideToMove);
int ext = inCheck ? 1 : 0;
if (MyGameState.GameBoard.History.IsPositionDrawn(MyBoard.HashKey))
{
return(CurrentDrawScore);
}
if (Ply >= MAX_DEPTH)
{
TakeBack();
return(Evaluator.Evaluate(MyBoard, -10000, 10000));
}
if ((Metrics.Nodes & 65535) == 65535)
{
Interrupt();
if (MyGameState.TimeUp)
{
return(alpha);
}
}
Move bestMove = null;
if (depth + ext <= 0)
{
return(Quiesce(alpha, beta));
}
//first let's look for a transposition
var entry = TranspositionTable.Instance.Read(MyBoard.HashKey);
if (entry != null)
{
//we have a hit from the TTable
if (entry.Depth >= depth)
{
if (entry.Type == (byte)EntryType.CUT && entry.Score >= beta)
{
return(beta);
}
else if (entry.Type == (byte)EntryType.ALL && entry.Score <= alpha)
{
return(alpha);
}
else if (entry.Type == (byte)EntryType.PV)
{
return(entry.Score);
}
}
}
int mateThreat = 0;
var myPieceCount = MyBoard.PieceCount(MyBoard.SideToMove);
//next try a Null Move
if (Ply > 0 &&
depth > 1 &&
alpha == beta - 1 &&
!inCheck &&
!MyBoard.History[Ply - 1].IsNullMove &&
myPieceCount > 0 &&
(myPieceCount > 2 || depth < 7))
{
Metrics.NullMoveTries++;
MakeNullMove();
var nullReductionDepth = depth > 6 ? 4 : 3;
int nmScore;
if (depth - nullReductionDepth - 1 > 0)
{
nmScore = -Search(-beta, 1 - beta, depth - nullReductionDepth - 1);
}
else
{
nmScore = -Quiesce(-beta, 1 - beta);
}
UnmakeNullMove();
if (MyGameState.TimeUp)
{
return(alpha);
}
if (nmScore >= beta)
{
Metrics.NullMoveFailHigh++;
TranspositionTable.Instance.Store(MyBoard.HashKey, null, depth, nmScore, EntryType.CUT);
return(nmScore);
}
}
var moves = MoveGenerator
.GenerateMoves(MyBoard)
.OrderByDescending((m) => OrderMove(m, entry));
Move lastMove = null;
int lmr = 0, nonCaptureMoves = 0, movesSearched = 0;
foreach (var m in moves)
{
bool fprune = false;
int score;
if (!Make(m))
{
continue;
}
var justGaveCheck = MyBoard.InCheck(MyBoard.SideToMove);
var capture = ((m.Bits & (byte)MoveBits.Capture) != 0);
if (!capture && (entry == null || entry.MoveValue != m.Value)) // don't count the hash move as a non-capture
{
++nonCaptureMoves; // while it might not be a capture, the point
}
// here is to start counting after generated captures
var passedpawnpush = (m.Bits & (byte)MoveBits.Pawn) > 0 && (Evaluator.PassedPawnMask[MyBoard.SideToMove ^ 1, m.To] & MyBoard.Pawns[MyBoard.SideToMove]) == 0;
//LATE MOVE REDUCTIONS
if (ext == 0 && //no extension
!inCheck && //i am not in check at this node
!justGaveCheck && //the move we just made does not check the opponent
mateThreat == 0 && //no mate threat detected
!passedpawnpush && //do not reduce/prune passed pawn pushes
nonCaptureMoves > 0) //start reducing after the winning captures
{
if (depth > 2)
{
lmr = movesSearched > 2 ? 2 : 1; // start reducing depth if we aren't finding anything useful
}
//FUTILITY PRUNING
else if (depth < 3 && alpha > -9900 && beta < 9900)
{
if (depth == 2 && -Evaluator.EvaluateMaterial(MyBoard) + Evaluator.PieceValues[(int)Piece.Rook] <= alpha)
{
Metrics.EFPrune++;
fprune = true;
}
else if (depth == 1 && -Evaluator.EvaluateMaterial(MyBoard) + Evaluator.PieceValues[(int)Piece.Knight] <= alpha)
{
Metrics.FPrune++;
fprune = true;
}
}
}
if (!fprune)
{
//if we don't yet have a move, then search full window (PV Node)
if (bestMove == null)
{
score = -Search(-beta, -alpha, depth - 1 - lmr + ext);
}
else //otherwise, use a zero window
{
//zero window search
score = -Search(-alpha - 1, -alpha, depth - 1 - lmr + ext);
if (score > alpha)
{
//this move might be better than our current best move
//we have to research with full window
score = -Search(-beta, -alpha, depth - 1 - lmr + ext);
if (score > alpha && lmr > 0)
{
//let's research again without the lmr
Metrics.LMRResearch++;
score = -Search(-beta, -alpha, depth - 1);
}
}
}
}
else
{
score = -Quiesce(-beta, -alpha);
}
TakeBack();
++movesSearched;
if (MyGameState.TimeUp)
{
return(alpha);
}
if (score >= beta)
{
SearchFailHigh(m, score, depth, entry);
if (lastMove == null)
{
Metrics.FirstMoveFailHigh++;
}
return(score);
}
if (score > alpha)
{
alpha = score;
bestMove = m;
// PV Node
//update the PV
UpdatePv(bestMove);
//Add to hashtable
TranspositionTable.Instance.Store(
MyBoard.HashKey, bestMove, depth, alpha, TranspositionTableEntry.EntryType.PV);
}
lastMove = m;
}
//check for mate
if (lastMove == null)
{
//we can't make a move. check for mate or stalemate.
if (inCheck)
{
return(-10000 + Ply);
}
else
{
return(CurrentDrawScore);
}
}
if (bestMove == null)
{
//ALL NODE
TranspositionTable.Instance.Store(
MyBoard.HashKey, null, depth, alpha,
TranspositionTableEntry.EntryType.ALL);
}
return(alpha);
}