/// <summary>
/// Decides the best move for the player given by the color parameter, with a search
/// of the given depth.
/// </summary>
public static ColoredBitBoard GetBestMove(ChessBoard board, int depth, ChessPieceColors color, GenerateMoveDelegate GenerateMoves, GetMaterialDelegate GetMaterialValue)
{
Debug.Assert(board != null);
Debug.Assert(depth > 0);
Debug.Assert(GenerateMoves != null);
Debug.Assert(GetMaterialValue != null);
//Alpha, have to add one to change symbol later otherwise overflow.
double a = int.MinValue + 1;
//Beta value.
double b = int.MaxValue;
double val = 0;
ColoredBitBoard bestMove = null;
List <ColoredBitBoard> moves = GenerateMoves(board, color);
var ttEntry = new TranspositionEntry(board.BoardHash.Key, depth, val, false, EntryType.Exact);
foreach (ColoredBitBoard move in moves)
{
board.Update(move);
if (color == ChessPieceColors.Black)
{
val = -NegaMaxAlgorithm(board, depth - 1, -b, -a, ChessPieceColors.White, GenerateMoves, GetMaterialValue);
}
else if (color == ChessPieceColors.White)
{
val = -NegaMaxAlgorithm(board, depth - 1, -b, -a, ChessPieceColors.Black, GenerateMoves, GetMaterialValue);
}
if (val > a)
{
a = val;
bestMove = move;
ttEntry.BestMove = move;
ttEntry.Score = a;
}
board.Undo();
}
TranspositionTable.TranspositionCache.Add(ttEntry);
return(bestMove);
}
/// <summary>
/// Implementation of the NegaMax algorithm.
/// <param name="board"></param>
/// <param name="depth"></param>
/// <param name="a"></param>
/// <param name="b"></param>
/// <param name="color"></param>
/// <param name="GenerateMoves"></param>
/// <param name="GetMaterialValue"></param>
/// <returns></returns>
public static double NegaMaxAlgorithm(
ChessBoard board,
int depth, double a, double b,
ChessPieceColors color,
GenerateMoveDelegate GenerateMoves,
GetMaterialDelegate GetMaterialValue
)
{
ColoredBitBoard bestMove = null;
var hashBeforeTermCondition = board.BoardHash.Key;
var termCondition = IsTerminal(board, color, GenerateMoves);
Debug.Assert(hashBeforeTermCondition == board.BoardHash.Key);
if (termCondition == TerminalConditions.Win ||
termCondition == TerminalConditions.Draw ||
depth == 0)
{
return((int)color * (GetMaterialValue(board)));
}
else
{
bool usingTT = false;
ulong hash_beforeMove = board.BoardHash.Key;
var ttEntry = TranspositionTable.TranspositionCache[board.BoardHash.Key];
if (ttEntry != null &&
ttEntry.Hash == board.BoardHash.Key &&
ttEntry.Depth >= depth
)
{
if (ttEntry.NodeType == EntryType.Alpha && ttEntry.Score <= a)
{
return(a);
}
if (ttEntry.NodeType == EntryType.Beta && ttEntry.Score >= b)
{
return(b);
}
if (ttEntry.NodeType == EntryType.Exact)
{
return(ttEntry.Score);
}
usingTT = true;
}
TranspositionEntry newEntry = new TranspositionEntry(board.BoardHash.Key, depth, 0, false, EntryType.Alpha);
List <ColoredBitBoard> moves = new List <ColoredBitBoard>();
moves = GenerateMoves(board, color);
foreach (ColoredBitBoard move in moves)
{
double val = 0;
board.Update(move);
var hashBeforeRec = board.BoardHash.Key;
if (color == ChessPieceColors.White)
{
val = -NegaMaxAlgorithm(board, depth - 1, -b, -a, ChessPieceColors.Black, GenerateMoves, GetMaterialValue);
}
else if (color == ChessPieceColors.Black)
{
val = -NegaMaxAlgorithm(board, depth - 1, -b, -a, ChessPieceColors.White, GenerateMoves, GetMaterialValue);
}
Debug.Assert(hashBeforeRec == board.BoardHash.Key);
board.Undo();
usingTT = false;
if (val >= b)
{
if (!usingTT)
{
newEntry.NodeType = EntryType.Beta;
newEntry.Score = val;
TranspositionTable.TranspositionCache.Add(newEntry);
Debug.Assert(hash_beforeMove == board.BoardHash.Key, "Board hash changed during update/undo");
}
return(val);
}
if (val >= a)
{
a = val;
bestMove = move;
newEntry.NodeType = EntryType.Exact;
}
Debug.Assert(hash_beforeMove == board.BoardHash.Key, "Board hash changed during update/undo");
}
if (!usingTT)
{
if (newEntry.NodeType == EntryType.Exact)
{
newEntry.BestMove = bestMove;
}
newEntry.Score = a;
TranspositionTable.TranspositionCache.Add(newEntry);
}
return(a);
}
}