//evaluates exchanges on a square initiated by stm
public static int Eval(Board b, Move m, int stm)
{
ulong[] attackers = new ulong[2];
int score = 0;
var sq = m.To;
var pieceVal = Evaluator.PieceValueOnSquare(b, sq);
var attacks = MoveGenerator.Attacks(b, sq);
var mystm = stm ^ 1;
attackers[mystm] = attacks & b.Pieces[mystm];
attackers[stm] = attacks & b.Pieces[stm];
//change the pieceVal to be the new piece
//if there is a promotion, then stm is already up material
if (m.Promotion > 0)
{
pieceVal = Evaluator.PieceValues[m.Promotion];
score = pieceVal - 100; //lose the initial pawn
}
else
{
pieceVal = Evaluator.PieceValues[(int)Move.GetPiece((MoveBits)m.Bits)];
}
//add in any material taken on the first capture
if ((m.Bits & (byte)MoveBits.Capture) > 0)
{
score += Evaluator.PieceValueOnSquare(b, m.To);
}
//remove the first Move, unless this was a straight pawn push
if (!((m.Bits & (byte)MoveBits.Pawn) > 0 && Math.Abs(m.From - m.To) == 8))
{
attackers[stm] ^= BitMask.Mask[m.From];
}
while (attackers[mystm] > 0)
{
var tempAttackers = attackers[mystm];
var leastValuable = int.MaxValue;
int leastValSq = -1;
bool promote, promoting = false;
//make a pass over the attackers to find the lowest valued piece
while (tempAttackers > 0)
{
promote = false;
var attack_sq = tempAttackers.BitScanForward();
tempAttackers ^= BitMask.Mask[attack_sq];
var attackerVal = Evaluator.PieceValueOnSquare(b, attack_sq);
//if we found a pawn capturing onto the 8th rank, we need to consider promotion
if (attackerVal == 100 && (sq.Rank() == 0 || sq.Rank() == 7))
{
//we will treat this attacker as if it were a queen
attackerVal = Evaluator.PieceValues[(int)Piece.Queen];
promote = true;
}
if (attackerVal < leastValuable)
{
promoting = promote;
leastValSq = attack_sq;
leastValuable = attackerVal;
}
}
//now we have the least valuable attacker
//remove whatever value is on the square
if (mystm == stm)
{
score += pieceVal;
if (promoting)
{
score += leastValuable - 100;//lose the promoting pawn, add in queen
}
}
else
{
score -= pieceVal;
if (promoting)
{
score -= leastValuable + 100;//lose the promoting pawn, add in queen
}
}
//update the attacked piece
pieceVal = leastValuable;
//remove the attacker
attackers[mystm] ^= BitMask.Mask[leastValSq];
//now we check for x-ray attacks
//the idea here is that if any rooks, queens, or bishops attack the temporary
//square from the same direction that the temporary square is attacking the target
//then we have an xray attacker
var direction = MoveGenerator.Directions[leastValSq, sq];
ulong xrayAttacks = 0;
switch (Math.Abs(direction))
{
case 1:
case 8:
if (((b.Rooks[mystm] | b.Queens[mystm]) & Board.FileMask[sq.File()]) > 0)
{
xrayAttacks = MoveGenerator.RookAttacks(b, leastValSq) & (b.Rooks[mystm] | b.Queens[mystm]) & attackers[mystm];
}
break;
case 7:
case 9:
xrayAttacks = MoveGenerator.BishopAttacks(b, leastValSq) & (b.Bishops[mystm] | b.Queens[mystm]) & attackers[mystm];
break;
}
while (xrayAttacks > 0)
{
var xraySq = xrayAttacks.BitScanForward();
xrayAttacks ^= BitMask.Mask[xraySq];
if (MoveGenerator.Directions[xraySq, sq] == direction)
{
//put this piece in main attackers bitboard
attackers[mystm] |= BitMask.Mask[xraySq];
}
}
//switch stm
mystm ^= 1;
}
return(score);
}
