/// <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);
}
/// <summary>
/// Carries out a narrow Quiescent search where only capure moves are searched.
/// </summary>
/// <param name="alpha">Current alpha value.</param>
/// <param name="beta">Current beta value.</param>
/// <returns>The score of a board.</returns>
private int QuiescentSearch(int alpha, int beta)
{
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;
QuiescentFlag quiescentFlag = QuiescentFlag.AlphaScore;
if (m_quiescentTable.ProbeScore(m_board.BoardHash(false), alpha, beta, ref score))
{
return(score);
}
score = m_evaluator.EvaluatePosition(m_board);
if (score >= beta)
{
m_quiescentTable.RecordHash(m_board.BoardHash(false), QuiescentFlag.BetaScore, beta);
return(beta);
}
if (score > alpha)
{
alpha = score;
quiescentFlag = QuiescentFlag.ExactScore;
}
MoveOrganizer currentMoves = new MoveOrganizer();
m_board.GeneratePseudoLegalMoves(currentMoves);
currentMoves.Sort(m_captureMoveCompare);
foreach (Move move in currentMoves)
{
if (!move.IsCaptureMove)
{
break;
}
if (move.Execute(m_board))
{
score = -QuiescentSearch(-beta, -alpha);
move.UnExecute(m_board);
if (score >= beta)
{
m_quiescentTable.RecordHash(m_board.BoardHash(false), QuiescentFlag.BetaScore, beta);
return(beta);
}
if (score > alpha)
{
alpha = score;
quiescentFlag = QuiescentFlag.ExactScore;
}
}
}
m_quiescentTable.RecordHash(m_board.BoardHash(false), quiescentFlag, alpha);
return(alpha);
}
/// <summary>
/// Starts a iterative depending search, returning the move engine is to play.
/// </summary>
/// <param name="board">Board to find move for.</param>
/// <param name="moveOrganizer">move organizer to where best move is to be found in.</param>
/// <param name="aimSearchTime">The search time to aim for. Actual search time might be between this and maxSearchTime.</param>
/// <param name="maxSearchTime">The maximum amount of seconds to seach for move.</param>
/// <param name="maxSearchDepth">Maximum depth for full width search.</param>
/// <returns>Move considered best.</returns>
public Move FindBestMove(Board board, MoveOrganizer moveOrganizer, TimeSpan aimSearchTime, TimeSpan maxSearchTime, int maxSearchDepth)
{
moveOrganizer.Sort(m_captureMoveCompare);
List <ScoreMove> scoredMoves = new List <ScoreMove>(moveOrganizer.Count);
foreach (Move move in moveOrganizer)
{
scoredMoves.Add(new ScoreMove(move));
}
if (scoredMoves.Count == 1)
{
return(scoredMoves[0].Move);
}
m_alphaBetaTable.Open();
m_quiescentTable.Open();
m_searchRollback = m_externalAbort = false;
m_board = board;
m_searchStartTime = DateTime.Now;
m_searchMaxTime = maxSearchTime;
m_nodesVisited = 0;
m_nextRollbackCheck = ROLLBACK_INTERVAL_COUNT;
bool continueSearch = true;
for (m_searchDepth = 0; m_searchDepth <= maxSearchDepth && continueSearch; ++m_searchDepth)
{
int movesSearched = 0;
int alpha = m_evaluator.AlphaValue;
int beta = -m_evaluator.AlphaValue;
foreach (ScoreMove scoreMove in scoredMoves)
{
scoreMove.Move.Execute(m_board);
if (movesSearched == 0)
{
scoreMove.TemporaryScore = -AlphaBetaSearch(-beta, -alpha, m_searchDepth, false, true);
}
else //search in the same way as we do with pv moves by narrowing together alpha and beta
{
scoreMove.TemporaryScore = -AlphaBetaSearch(-alpha - 1, -alpha, m_searchDepth, false, false);
if ((scoreMove.TemporaryScore > alpha) && (scoreMove.TemporaryScore < beta))
{
scoreMove.TemporaryScore = -AlphaBetaSearch(-beta, -alpha, m_searchDepth, false, true);
}
}
scoreMove.Move.UnExecute(m_board);
if (scoreMove.TemporaryScore > alpha)
{
alpha = scoreMove.TemporaryScore - 1;
}
if (m_searchRollback || m_externalAbort || (scoreMove.ValidScore >= -m_evaluator.MateValue))
{
continueSearch = false;
break;
}
++movesSearched;
}
if (movesSearched == scoredMoves.Count)
{
foreach (ScoreMove scoreMove in scoredMoves)
{
scoreMove.ValidScore = scoreMove.TemporaryScore;
}
}
scoredMoves.Sort(m_scoreMoveCompare);
OutputWriter.Write("Depth: " + m_searchDepth + ", Score: " + scoredMoves[0].ValidScore);
if ((DateTime.Now - m_searchStartTime) >= aimSearchTime)
{
continueSearch = false;
}
}
OutputWriter.Write("Nodes: " + m_nodesVisited);
OutputWriter.Write("Time: " + (DateTime.Now - m_searchStartTime).ToString());
if (m_externalAbort)
{
OutputWriter.Write("Search aborted");
return(null);
}
int canditates;
for (canditates = 1; canditates < scoredMoves.Count; ++canditates)
{
if (m_scoreMoveCompare.Compare(scoredMoves[0], scoredMoves[canditates]) != 0)
{
break;
}
}
Random selector = new Random();
int idx = selector.Next(canditates);
OutputWriter.Write("Selecting: " + scoredMoves[idx].Move.From + " " + scoredMoves[idx].Move.To + " from " + scoredMoves.Count + " candidates");
return(scoredMoves[idx].Move);
}