/// <summary>
/// AlphaBeta algorithm.Calculates best line in given depth
/// </summary>
/// <param name="alpha">Lowest value</param>
/// <param name="beta">highest value</param>
/// <param name="ply"></param>
/// <param name="depth"></param>
/// <param name="pv">Holds principal variation</param>
/// <param name="nullmove"></param>
/// <param name="nodeCount">Holds value of calculated moves</param>
/// <returns>Returning value is the score of best line</returns>
int AlphaBeta(int alpha, int beta, int ply, int depth, List <Move> pv, NullMove nullmove, ref int nodeCount)
{
//if time out or exit requested after 1st iteration,so leave thinking.
if ((!HaveTime() || Exit) && iterationPly > 1)
{
return(0);
}
nodeCount++;
var moves = Board.GenerateMoves();
if (!moves.Any())
{
return(-Board.GetCheckMateOrStaleMateScore(ply));
}
if (ply <= 0)
{
return(QuiescenceSearch(alpha, beta, ref nodeCount));
}
var localpv = new List <Move>();
var pvSearch = false;
#region Null Move Prunning
if (nullmove == NullMove.Enabled && !Board.IsInCheck())
{
int R = 2;
Board.ToggleSide();
int score = -AlphaBeta(-beta, -beta + 1, ply - 1 - R, depth + 1, localpv, NullMove.Disabled, ref nodeCount);
Board.ToggleSide();
if (score >= beta)
{
return(score);
}
}
#endregion
var sortedMoves = SortMoves(moves, depth);
foreach (var move in sortedMoves)
{
Board.MakeMove(move);
int score;
if (Board.threeFoldRepetetion.IsThreeFoldRepetetion)
{
score = Board.Draw;
}
else
{
#region Late Move Reduction
if (!Board.IsInCheck())
{
score = -AlphaBeta(-alpha - 1, -alpha, ply - 2, depth + 1, localpv, NullMove.Enabled, ref nodeCount);
}
else
{
score = alpha + 1;
}
#endregion
if (score > alpha)
{
if (pvSearch)
{
score = -AlphaBeta(-alpha - 1, -alpha, ply - 1, depth + 1, localpv, NullMove.Enabled, ref nodeCount);
if (score > alpha && score < beta)
{
score = -AlphaBeta(-beta, -alpha, ply - 1, depth + 1, localpv, NullMove.Enabled, ref nodeCount);
}
}
else
{
score = -AlphaBeta(-beta, -alpha, ply - 1, depth + 1, localpv, NullMove.Enabled, ref nodeCount);
}
}
}
Board.TakeBackMove(move);
if (score >= beta)
{
killerMoves.Add(move, depth);
return(beta);//beta cut-off
}
if (score > alpha)
{
historyMoves.AddMove(move);
pvSearch = true;
alpha = score;
pv.Clear();
pv.Add(move);
pv.AddRange(localpv);
}
}
return(alpha);
}