/// <summary>
/// alpha beta algorithm
/// boardcoordlist- each item in this map contains a new board (representing a new state) and a list of 2 coords representing the move that brought to this new state
/// mapBoardSrcDestCap- each item in this map contains the new board and source- dest coords (like above) and a list of captured coordinate (in case the new board is a result of a capture)
/// </summary>
/// <param name="board"></param>
/// <param name="depth"></param>
/// <param name="alpha"></param>
/// <param name="beta"></param>
/// <param name="player"></param>
/// <param name="maxplayer"></param>
/// <param name="srcCoord"></param>
/// <param name="destCoord"></param>
/// <param name="updateBoard"></param>
/// <param name="captures"></param>
/// <returns></returns>
public int AlphaBeta(Board board, int depth, int alpha, int beta, Player player, bool maxplayer,
ref Coordinate srcCoord, ref Coordinate destCoord, ref Board updateBoard,
ref IList<Coordinate> captures)
{
var robj = new Rules();
if (depth == 0 || rule.IsBoardLeaf(player, board)) // is node a terminal node
{
var obj = new HeuristicFunction();
return obj.Evaluate(board, player); //return Heurisitic value of node
}
//Finds if there are any captures
IDictionary<IList<Coordinate>, IList<Coordinate>> capturesAvailable = robj.FindCaptures(board, player);
IDictionary<Board, IList<Coordinate>> boardCoordsList;
//Calculating board according to current state (with or without captures)
if (capturesAvailable.Count > 0)
{
boardCoordsList = robj.CreateNewBoards(board, capturesAvailable);
}
else
{
boardCoordsList = robj.CalculateNewBoardsFromCoordinates(board, player);
}
//values which save capture list , source, detination on the chosen move
var minsrcCoord = new Coordinate();
var mindestCoord = new Coordinate();
var maxsrcCoord = new Coordinate();
var maxdestCoord = new Coordinate();
var maxCapturesList = new List<Coordinate>();
var minCapturesList = new List<Coordinate>();
var minBoard = new Board();
var maxBoard = new Board();
//player
if (maxplayer)
{
foreach (var newState in boardCoordsList)
{
Coordinate newSrcCoord = newState.Value[0];
Coordinate newDestCoord = newState.Value[1];
IList<Coordinate> capturesList = robj.MapContainsCoords(capturesAvailable, newSrcCoord, newDestCoord);
IList<Coordinate> tempCapList = new List<Coordinate>();
int res = AlphaBeta(newState.Key, depth - 1, alpha, beta, board.GetOpponent(player), !maxplayer,
ref newSrcCoord, ref newDestCoord, ref updateBoard, ref tempCapList);
if (res > alpha)
{
alpha = res;
maxsrcCoord = newState.Value[0];
maxdestCoord = newState.Value[1];
maxBoard = newState.Key.Copy();
maxCapturesList = new List<Coordinate>(capturesList);
}
if (beta <= alpha)
{
break;
}
if (capturesList.Count > 0)
{
capturesAvailable.Remove(capturesList);
}
}
srcCoord = maxsrcCoord;
destCoord = maxdestCoord;
updateBoard = maxBoard.Copy();
captures = maxCapturesList;
return alpha;
}
//opponent
else
{
foreach (var newState in boardCoordsList)
{
Coordinate newSrcCoord = newState.Value[0];
Coordinate newDestCoord = newState.Value[1];
IList<Coordinate> capturesList = robj.MapContainsCoords(capturesAvailable, newSrcCoord, newDestCoord);
IList<Coordinate> tempCapList = new List<Coordinate>();
int res = AlphaBeta(newState.Key, depth - 1, alpha, beta, board.GetOpponent(player), !maxplayer,
ref newSrcCoord, ref newDestCoord, ref updateBoard, ref tempCapList);
if (res < beta)
{
beta = res;
minsrcCoord = newState.Value[0];
mindestCoord = newState.Value[1];
minBoard = newState.Key.Copy();
minCapturesList = new List<Coordinate>(capturesList);
}
if (beta <= alpha)
{
break;
}
if (capturesList.Count > 0)
{
capturesAvailable.Remove(capturesList);
}
}
srcCoord = minsrcCoord;
destCoord = mindestCoord;
updateBoard = minBoard.Copy();
captures = minCapturesList;
return beta;
}
}
