/// <summary>
/// Recursively carries out a full width alpha beta search untill the bottom is reached.
/// From then a Quiescent search is carried out to avoid horrison effects.
/// </summary>
/// <param name="alpha">This is the best score that can be forced by some means. Anything worth less than this is of no use, because there is a strategy that is known to result in a score of alpha.</param>
/// <param name="beta">Beta is the worst thing that the opponent has to endure. If the search finds something that returns a score of beta or better, it's too good, so the side to move is not going to get a chance to use this strategy.</param>
/// <param name="remainingDepth">Remaining depth to search before bottom is reached.</param>
/// <param name="allowNullMove">Is a null move allowed at this search depth.</param>
/// <param name="allowTranspotitionTable">Is it allowed to use the transpotition table to probe for scores.</param>
/// <returns>The score of a board.</returns>
private int AlphaBetaSearch(int alpha, int beta, int remainingDepth, bool allowNullMove, bool allowTranspotitionTable)
{
if (m_searchRollback)
{
return(beta);
}
//At interval check if time is up and rollback if so.
if (m_nodesVisited == m_nextRollbackCheck)
{
if (AbortingSearch())
{
m_alphaBetaTable.Close();
m_quiescentTable.Close();
m_searchRollback = true;
return(beta);
}
m_nextRollbackCheck += ROLLBACK_INTERVAL_COUNT;
}
++m_nodesVisited;
int score = 0;
Move pvMove = null;
AlphaBetaFlag alphaBetaFlag = AlphaBetaFlag.AlphaScore;
if (m_board.State.NonHitAndPawnMovesPlayed >= 100)
{
return(0);
}
if (m_board.BoardHistoryFrequency() >= 3)
{
return(0);
}
//don't allow transposition table before both players has passed the above repetetion draw checks.
if (allowTranspotitionTable && (m_searchDepth - remainingDepth) > 0)
{
if (m_alphaBetaTable.ProbeScore(m_board.BoardHash(false), alpha, beta, remainingDepth, ref score))
{
if (Math.Abs(score) != Math.Abs(m_evaluator.MateValue)) //don't probe a mate value as this actuall might be a mate far down in the tree, and we want to find the mate at lowest depth.
{
return(score);
}
}
}
if (remainingDepth == 0)
{
score = QuiescentSearch(alpha, beta);
if (allowTranspotitionTable)
{
m_alphaBetaTable.RecordHash(m_board.BoardHash(false), AlphaBetaFlag.ExactScore, remainingDepth, score, pvMove);
}
return(score);
}
if (allowNullMove)
{
//Carry out a null move which is a move where nothing is actually moved. This makes the opponent move
//twice in a row. The idea is then to search to a reduced depth and if nothing good come out of this
//(for the opponent) the move is considered garbage and we cut it off.
NullMove nullMove = new NullMove(m_board, m_evaluator);
if (nullMove.Execute())
{
int remDepth = (remainingDepth > NULL_REDUCTION) ? remainingDepth - 1 - NULL_REDUCTION : 0;
score = -AlphaBetaSearch(-beta, -beta + 1, remDepth, false, false);
nullMove.UnExecute();
if (score >= beta)
{
return(beta);
}
}
}
MoveOrganizer currentMoves = new MoveOrganizer(m_alphaBetaTable.ProbePvMove(m_board.BoardHash(false)));
m_board.GeneratePseudoLegalMoves(currentMoves);
currentMoves.Sort(m_captureMoveCompare);
bool validMoveFound = false;
foreach (Move move in currentMoves)
{
if (move.Execute(m_board))
{
validMoveFound = true;
if (pvMove == null)
{
score = -AlphaBetaSearch(-beta, -alpha, remainingDepth - 1, true, allowTranspotitionTable);
}
else
{
score = -AlphaBetaSearch(-alpha - 1, -alpha, remainingDepth - 1, true, false);
if ((score > alpha) && (score < beta))
{
score = -AlphaBetaSearch(-beta, -alpha, remainingDepth - 1, true, allowTranspotitionTable);
}
}
move.UnExecute(m_board);
if (score >= beta)
{
if (allowTranspotitionTable)
{
m_alphaBetaTable.RecordHash(m_board.BoardHash(false), AlphaBetaFlag.BetaScore, remainingDepth, beta, pvMove);
}
return(beta);
}
if (score > alpha)
{
alpha = score;
pvMove = move;
alphaBetaFlag = AlphaBetaFlag.ExactScore;
}
}
}
if (!validMoveFound)
{
if (m_board.ColorToPlayIsCheck())
{
alpha = m_evaluator.MateValue; //Checkmate
}
else
{
alpha = 0; //Stalemate
}
}
if (allowTranspotitionTable)
{
m_alphaBetaTable.RecordHash(m_board.BoardHash(false), alphaBetaFlag, remainingDepth, alpha, pvMove);
}
return(alpha);
}