/// <summary>
/// Returns a list of all pieces belonging to a current player that are still on the board
/// </summary>
public List <Piece> GetPlayersPieces(Program.Square currentPlayer)
{
var currentPlayersPieces = new List <Piece>();
// set playersKing to either RedKing or WhiteKing
var playersKing = currentPlayer == Program.Square.Red ? Program.Square.RedKing : Program.Square.WhiteKing;
// scan the board to identify squares where current player's pieces are
for (var row = 0; row < Size; row++)
{
for (var col = 0; col < Size; col++)
{
// if current row,column coordinate within the board holds current player's piece
if (_board[row, col] == currentPlayer)
{
currentPlayersPieces.Add(new Piece(currentPlayer, false, row, col));
}
else if (_board[row, col] == playersKing)
{
currentPlayersPieces.Add(new Piece(playersKing, true, row, col));
}
}
}
return(currentPlayersPieces);
}
public Piece(Program.Square type, bool isKing, int x, int y)
{
Type = type;
IsKing = isKing;
X = x;
Y = y;
}
/// <summary>
/// Changes the location of a piece using the move provided
/// </summary>
public void MovePiece(Move move)
{
var fromRow = move.FromRow;
var fromCol = move.FromCol;
var toRow = move.ToRow;
var toCol = move.ToCol;
Program.Square pieceType = _board[fromRow, fromCol];
// if move is more than 2 rows forwards/backwards, then it's illegal
if (toRow - fromRow > 2 || toRow - fromRow < -2)
{
Console.WriteLine("You can't move further than 2 rows");
}
else
{
// if the piece has reached either first or last row, it becomes a king
if (toRow == 0 && pieceType == Program.Square.Red || toRow == 7 && pieceType == Program.Square.White)
{
SetKing(pieceType, toRow, toCol);
_board[fromRow, fromCol] = Program.Square.EmptyDark;
}
else
{
// move piece and replace it with an empty dark square
_board[toRow, toCol] = _board[fromRow, fromCol];
_board[fromRow, fromCol] = Program.Square.EmptyDark;
}
}
}
/// <summary>
/// Checks whether provided move is valid
/// </summary>
private bool IsMovePermitted(Program.Square currentPiece, Move move)
{
// can't move outside the board
if (move.ToRow < 0 | move.ToRow > 7 | move.ToCol < 0 | move.ToCol > 7)
{
return(false);
}
// can't move onto a light square
if (_board[move.ToRow, move.ToCol] == Program.Square.EmptyLight)
{
return(false);
}
if (currentPiece == Program.Square.Red)
{
// can't move backwards
if (move.ToRow >= move.FromRow)
{
return(false);
}
// can only move to an empty dark square
if (GetPieceAt(move.CoordinateTo) != Program.Square.EmptyDark)
{
return(false);
}
return(true);
}
if (currentPiece == Program.Square.White)
{
// can't move backwards
if (move.ToRow <= move.FromRow)
{
return(false);
}
// can only move to an empty dark square
if (GetPieceAt(move.CoordinateTo) != Program.Square.EmptyDark)
{
return(false);
}
return(true);
}
if (currentPiece == Program.Square.WhiteKing || currentPiece == Program.Square.RedKing)
{
// can only move to an empty dark square
if (GetPieceAt(move.CoordinateTo) != Program.Square.EmptyDark)
{
return(false);
}
return(true);
}
return(false);
}
/// <summary>
/// Makes a regular red or white piece a king once it's reached the opposite end row of the board
/// </summary>
private void SetKing(Program.Square currentPlayer, int rowTo, int colTo)
{
if (currentPlayer == Program.Square.Red && rowTo == 0)
{
_board[rowTo, colTo] = Program.Square.RedKing;
}
if (currentPlayer == Program.Square.White && rowTo == 7)
{
_board[rowTo, colTo] = Program.Square.WhiteKing;
}
}
/// <summary>
/// Returns current state of the board, ie. how pieces are positioned at this particular stage
/// </summary>
public Program.Square[,] GetState()
{
var newArr = new Program.Square[8, 8];
Array.Copy(_board, newArr, _board.Length);
/* for (var row = 0; row < 8; row++)
* for (var col = 0; col < 8; col++)
* newArr[row, col] = _board[row, col];*/
return(newArr);
}
/// <summary>
/// Evaluates all possible moves that AI player can make during its current turn in order to
/// find the highest score associated with one of these moves
/// </summary>
/// <param name="board"></param>
/// <param name="state"></param>
/// <param name="depth"></param>
/// <param name="alpha"></param>
/// <param name="beta"></param>
/// <returns>Returns the highest scored out of all possible moves</returns>
private double Negamax(Board board, State state, int depth, double alpha, double beta)
{
List <Piece> pieces = board.GetPlayersPieces(state.CurrentPlayer);
HashSet <Move> legalJumps = board.GetLegalJumps(pieces);
HashSet <Move> legalMoves = board.GetLegalMoves(pieces);
legalMoves.UnionWith(legalJumps);
// evaluate the state of the board in order to obtain the score if depth has reached zero
if (depth == 0)
{
return(Evaluate(state));
}
Program.Square currentPlayer = state.CurrentPlayer;
Board boardCopy;
State newState;
foreach (var move in legalMoves)
{
boardCopy = board.DeepCopy();
if (move.IsJump)
{
boardCopy.DoJump(move);
}
else
{
boardCopy.MovePiece(move);
}
newState = new State(boardCopy.GetState(), Program.ChangeTurn(currentPlayer));
double newScore = -Negamax(boardCopy, newState, depth - 1, -beta, -alpha);
// alpha-beta cut-off
if (newScore >= beta)
{
return(newScore);
}
if (newScore > alpha)
{
alpha = newScore;
}
}
return(alpha);
}
/// <summary>
/// Evaluates the state of the board provided by Negamax method by calculating the difference in number of
/// both players' pieces, either from Red or from White player's perspective
/// </summary>
/// <param name="state"></param>
/// <returns>Returns the score for a move which is the difference between the number of players' pieces
/// left on the board once the move has been applied</returns>
private static int Evaluate(State state)
{
int red = 0;
int white = 0;
Program.Square currentPlayer = state.CurrentPlayer;
Program.Square[,] board = state.BoardState;
for (var row = 0; row < 8; row++)
{
for (var col = 0; col < 8; col++)
{
if (board[row, col] == Program.Square.Red)
{
red++;
}
if (board[row, col] == Program.Square.White)
{
white++;
}
if (board[row, col] == Program.Square.RedKing)
{
red += 2;
}
if (board[row, col] == Program.Square.WhiteKing)
{
white += 2;
}
}
}
// return the difference between the number of red and white pieces if calculating for red player
if (currentPlayer == Program.Square.Red || currentPlayer == Program.Square.RedKing)
{
return(red - white);
}
// return the difference between the number of white and red pieces if calculating for white player
return(white - red);
}
/// <summary>
/// Retrieves the best move that AI player can perform that is associated with the highest score returned
/// by Negamax method
/// </summary>
/// <param name="board"></param>
/// <param name="state"></param>
/// <returns>Returns the best move out of all legal moves for AI player</returns>
public Move GetBestMove(Board board, State state)
{
List <Piece> pieces = board.GetPlayersPieces(state.CurrentPlayer);
HashSet <Move> legalJumps = board.GetLegalJumps(pieces);
HashSet <Move> legalMoves = board.GetLegalMoves(pieces);
legalMoves.UnionWith(legalJumps);
var bestMoves = new List <Move>();
// analyze all possible moves three turns ahead while searching for best move for this turn
int depth = 3;
double alpha = Double.MinValue;
Program.Square currentPlayer = state.CurrentPlayer;
Board boardCopy;
State newState;
var rand = new Random();
foreach (var move in legalMoves)
{
// make a deep copy of the board to perform a move without changing the original board
boardCopy = board.DeepCopy();
if (move.IsJump)
{
boardCopy.DoJump(move);
}
else
{
boardCopy.MovePiece(move);
}
// create a new state of the game using copy of the original board after move has been performed
newState = new State(boardCopy.GetState(), Program.ChangeTurn(currentPlayer));
// get score for the move that has been applied to copy of the board
double newScore = -Negamax(boardCopy, newState, depth - 1, Double.MinValue, -alpha);
//Console.WriteLine("From {0} To {1} = {2}", move.CoordinateFrom, move.CoordinateTo, newScore);
// if new score is better than alpha, it becomes a new alpha (highest score found so far)
if (newScore > alpha)
{
alpha = newScore;
bestMoves.Clear();
bestMoves.Add(move);
}
// if score for this move is equal to score of best move found so far, add it to the list of best moves
else if (newScore == alpha)
{
bestMoves.Add(move);
}
}
// remove all regular moves from the list of best moves if any jumps were found
if (bestMoves.Exists(other => other.IsJump))
{
bestMoves.RemoveAll(notJump => !notJump.IsJump);
}
return(bestMoves[rand.Next(bestMoves.Count)]);
}
public State(Program.Square[,] state, Program.Square currentPlayer)
{
BoardState = state;
CurrentPlayer = currentPlayer;
}
/// <summary>
/// Checks whether provided jump is valid
/// </summary>
private bool CanJump(Piece piece, Move move)
{
Program.Square currentPlayer = piece.Type;
Program.Square opponent;
Program.Square opponentsKing;
var toRow = move.ToRow;
var toCol = move.ToCol;
var colDirection = move.FromCol - move.ToCol;
var rowDirection = move.FromRow - move.ToRow;
// can't jump outside the board
if (move.ToRow < 0 | move.ToRow > 7 | move.ToCol < 0 | move.ToCol > 7)
{
return(false);
}
// set opponent and opponentsKing variables to opposite colour to the currentPlayer's pieces
if (currentPlayer == Program.Square.Red || currentPlayer == Program.Square.RedKing)
{
opponent = Program.Square.White;
opponentsKing = Program.Square.WhiteKing;
}
else
{
opponent = Program.Square.Red;
opponentsKing = Program.Square.RedKing;
}
if (currentPlayer == Program.Square.Red && !piece.HasJustJumped)
{
// checking jumps to the right
if (colDirection == -2)
{
// checking if a jump to the right can be made to the row below
if (GetPieceAt(new Point(toRow, toCol)) == Program.Square.EmptyDark &&
(GetPieceAt(new Point(toRow + 1, toCol - 1)) == opponent ||
GetPieceAt(new Point(toRow + 1, toCol - 1)) == opponentsKing))
{
return(true);
}
return(false);
}
// checking jumps to the left
if (colDirection == 2)
{
// checking if a jump to the left can be made to the row below
if (GetPieceAt(new Point(toRow, toCol)) == Program.Square.EmptyDark &&
(GetPieceAt(new Point(toRow + 1, toCol + 1)) == opponent ||
GetPieceAt(new Point(toRow + 1, toCol + 1)) == opponentsKing))
{
return(true);
}
return(false);
}
}
if (currentPlayer == Program.Square.White && !piece.HasJustJumped)
{
// checking jumps to the right
if (colDirection == -2)
{
// checking if a jump to the right can be made to the row above
if (GetPieceAt(new Point(toRow, toCol)) == Program.Square.EmptyDark &&
(GetPieceAt(new Point(toRow - 1, toCol - 1)) == opponent ||
GetPieceAt(new Point(toRow - 1, toCol - 1)) == opponentsKing))
{
return(true);
}
return(false);
}
// checking jumps to the left
if (colDirection == 2)
{
// checking if a jump to the left can be made to the row above
if (GetPieceAt(new Point(toRow, toCol)) == Program.Square.EmptyDark &&
(GetPieceAt(new Point(toRow - 1, toCol + 1)) == opponent ||
GetPieceAt(new Point(toRow - 1, toCol + 1)) == opponentsKing))
{
return(true);
}
return(false);
}
}
// king and regular piece that has just jumped can jump in all directions
if (currentPlayer == Program.Square.RedKing || currentPlayer == Program.Square.WhiteKing || piece.HasJustJumped)
{
// checking jumps to the right
if (colDirection == -2)
{
// checking jumps to the row above
if (rowDirection == -2)
{
if (GetPieceAt(new Point(toRow, toCol)) == Program.Square.EmptyDark &&
(GetPieceAt(new Point(toRow - 1, toCol - 1)) == opponent ||
GetPieceAt(new Point(toRow - 1, toCol - 1)) == opponentsKing))
{
return(true);
}
}
// checking jumps to the row below
if (rowDirection == 2)
{
if (GetPieceAt(new Point(toRow, toCol)) == Program.Square.EmptyDark &&
(GetPieceAt(new Point(toRow + 1, toCol - 1)) == opponent ||
GetPieceAt(new Point(toRow + 1, toCol - 1)) == opponentsKing))
{
return(true);
}
}
return(false);
}
// checking jumps to the left
if (colDirection == 2)
{
// checking jumps to the row above
if (rowDirection == -2)
{
if (GetPieceAt(new Point(toRow, toCol)) == Program.Square.EmptyDark &&
(GetPieceAt(new Point(toRow - 1, toCol + 1)) == opponent ||
GetPieceAt(new Point(toRow - 1, toCol + 1)) == opponentsKing))
{
return(true);
}
}
// checking jumps to the row below
if (rowDirection == 2)
{
if (GetPieceAt(new Point(toRow, toCol)) == Program.Square.EmptyDark &&
(GetPieceAt(new Point(toRow + 1, toCol + 1)) == opponent ||
GetPieceAt(new Point(toRow + 1, toCol + 1)) == opponentsKing))
{
return(true);
}
}
return(false);
}
}
return(false);
}