//
// This function uses look ahead to evaluate all valid moves for a
// given player color and returns the best move it can find.
//
private ComputerMove GetBestMove(Board board, int color, int depth, int alpha, int beta)
{
// Initialize the best move.
ComputerMove bestMove = new ComputerMove(-1, -1);
bestMove.rank = -color * ReversiForm.maxRank;
// Find out how many valid moves we have so we can initialize the
// mobility score.
int validMoves = board.GetValidMoveCount(color);
// Start at a random position on the board. This way, if two or
// more moves are equally good, we'll take one of them at random.
Random random = new Random();
int rowStart = random.Next(8);
int colStart = random.Next(8);
// Check all valid moves.
int i, j;
for (i = 0; i < 8; i++)
for (j = 0; j < 8; j++)
{
// Get the row and column.
int row = (rowStart + i) % 8;
int col = (colStart + j) % 8;
if (board.IsValidMove(color, row, col))
{
// Update the progress bar for each move when on the
// first look ahead depth level.
if (depth == 1)
this.BeginInvoke(new UpdateStatusProgressDelegate(this.UpdateStatusProgress));
// Make the move.
ComputerMove testMove = new ComputerMove(row, col);
Board testBoard = new Board(board);
testBoard.MakeMove(color, testMove.row, testMove.col);
int score = testBoard.WhiteCount - testBoard.BlackCount;
// Check the board.
int nextColor = -color;
int forfeit = 0;
bool isEndGame = false;
int opponentValidMoves = testBoard.GetValidMoveCount(nextColor);
if (opponentValidMoves == 0)
{
// The opponent cannot move, count the forfeit.
forfeit = color;
// Switch back to the original color.
nextColor = -nextColor;
// If that player cannot make a move either, the
// game is over.
if (!testBoard.HasAnyValidMove(nextColor))
isEndGame = true;
}
// If we reached the end of the look ahead (end game or
// max depth), evaluate the board and set the move
// rank.
if (isEndGame || depth == this.lookAheadDepth)
{
// For an end game, max the ranking and add on the
// final score.
if (isEndGame)
{
// Negative value for black win.
if (score < 0)
testMove.rank = -ReversiForm.maxRank + score;
// Positive value for white win.
else if (score > 0)
testMove.rank = ReversiForm.maxRank + score;
// Zero for a draw.
else
testMove.rank = 0;
}
// It's not an end game so calculate the move rank.
else
testMove.rank =
this.forfeitWeight * forfeit +
this.frontierWeight * (testBoard.BlackFrontierCount - testBoard.WhiteFrontierCount) +
this.mobilityWeight * color * (validMoves - opponentValidMoves) +
this.stabilityWeight * (testBoard.WhiteSafeCount - testBoard.BlackSafeCount) +
score;
}
// Otherwise, perform a look ahead.
else
{
ComputerMove nextMove = this.GetBestMove(testBoard, nextColor, depth + 1, alpha, beta);
// Pull up the rank.
testMove.rank = nextMove.rank;
// Forfeits are cumulative, so if the move did not
// result in an end game, add any current forfeit
// value to the rank.
if (forfeit != 0 && Math.Abs(testMove.rank) < ReversiForm.maxRank)
testMove.rank += forfeitWeight * forfeit;
// Adjust the alpha and beta values, if necessary.
if (color == Board.White && testMove.rank > beta)
beta = testMove.rank;
if (color == Board.Black && testMove.rank < alpha)
alpha = testMove.rank;
}
// Perform a cutoff if the rank is outside tha alpha-beta range.
if (color == Board.White && testMove.rank > alpha)
{
testMove.rank = alpha;
return testMove;
}
if (color == Board.Black && testMove.rank < beta)
{
testMove.rank = beta;
return testMove;
}
// If this is the first move tested, assume it is the
// best for now.
if (bestMove.row < 0)
bestMove = testMove;
// Otherwise, compare the test move to the current
// best move and take the one that is better for this
// color.
else if (color * testMove.rank > color * bestMove.rank)
bestMove = testMove;
}
}
// Return the best move found.
return bestMove;
}
private static void Main(string[] args)
{
Board b = new Board();
while (true)
{
bool playing = true;
b.SetForNewGame();
bool isWhite = true;
while (playing)
{
isWhite = !isWhite;
Board.Color player = isWhite ? Board.Color.White : Board.Color.Black;
Console.Clear();
Console.Write(" ");
for (int i = 0; i < 8; i++)
{
Console.Write($" {i + 1}");
}
Console.Write(" X");
Console.WriteLine();
for (int y = 0; y < 8; y++)
{
for (int x = 0; x < 8; x++)
{
Console.Write(x == 0 ? $"{y + 1} " : " ");
Console.ForegroundColor =
b.IsValidMove(player, y, x) ? ConsoleColor.Green : ConsoleColor.Red;
Console.Write(GetChar(b.GetSquareContents(y, x)));
}
Console.ResetColor();
Console.WriteLine();
}
Console.WriteLine("Y");
if (!b.HasAnyValidMove(player))
{
if (!b.HasAnyValidMove(Board.Invert(player)))
{
Console.WriteLine(b.WhiteCount == b.BlackCount
? "Tie"
: $"{(b.WhiteCount > b.BlackCount ? "White" : "Black")} won");
Console.ReadKey();
playing = false;
}
continue;
}
Console.WriteLine($"Current player: {(isWhite ? "White (+)" : "Black (-)")}");
Console.WriteLine($"{b.GetValidMoveCount(player)} moves possible");
int nX;
int nY;
bool first = true;
do
{
if (!first)
{
Console.WriteLine("Invalid move");
}
first = false;
Console.Write("x> ");
nX = int.Parse(Console.ReadKey().KeyChar.ToString()) - 1;
Console.WriteLine();
Console.Write("y> ");
nY = int.Parse(Console.ReadKey().KeyChar.ToString()) - 1;
Console.WriteLine();
} while (!b.IsValidMove(player, nY, nX));
b.MakeMove(player, nY, nX);
}
}
}
//
// This function uses look ahead to evaluate all valid moves for a
// given player color and returns the best move it can find.
//
private ComputerMove minimax(Board board, int color, int alpha, int beta, int depth = 1)
{
// Initialize the best move.
ComputerMove bestMove = new ComputerMove(-1, -1);
bestMove.rank = -color * int.MaxValue;
// Start at a random position on the board. This way, if two or
// more moves are equally good, we'll take one of them at random.
Random random = new Random();
int rowStart = random.Next(10);
int colStart = random.Next(10);
// Check every square on the board and try to perform
// each and every move (up to the given depth)
// to calculate best move for the current player.
// We are certain that there are valid moves at this point
// as we wouldn't be here if there weren't any
// checks are performed in the StartTurn function.
for (int i = 0; i < 10; ++i)
for (int j = 0; j < 10; ++j)
{
// Get the row and column.
int row = (rowStart + i) % 10;
int col = (colStart + j) % 10;
if (board.IsValidMove(color, row, col))
{
// We found a valid move now we copy the board
// and try to make that move on the new board
// to evaluate its weight.
Board tempBoard = new Board(board);
tempBoard.MakeMove(color, row, col);
// Holds the current move being tested.
ComputerMove moveBeingChecked = new ComputerMove(row, col);
// Holds the color ID of a player that has no mobility
// Initialized to 0 in case both are mobile.
int forfeit = 0;
// Holds the color ID of the next player.
int nextPlayer = -color;
// A flag that indicates whether either of
// the players is mobile or if game is over.
bool gameOver = false;
// Just like in StartTurn, after passing a turn
// to the next player due to no mobility for the other
// we need to check if the new player is mobile
// if not then neither can move and game is over.
int opponentMobility = tempBoard.GetValidMoveCount(nextPlayer);
if (opponentMobility == 0)
{
forfeit = nextPlayer;
nextPlayer = color;
if (!tempBoard.HasAnyValidMove(color))
gameOver = true;
}
if (depth >= lookAheadDepth || gameOver)
{
// Initialize AI Parameters.
if (this.currentColor == Board.White)
{
moveBeingChecked.rank = heuristicFunction1(tempBoard, forfeit, color, opponentMobility);
if (tempBoard.EmptyCount > 0 && Board.isCorner(row, col))
moveBeingChecked.rank += 200;
}
else moveBeingChecked.rank = heuristicFunction2(tempBoard);
}
else
{
ComputerMove nextMove = minimax(tempBoard, nextPlayer, alpha, beta, ++depth);
moveBeingChecked.rank = nextMove.rank;
// Adjust the alpha and beta values, if necessary.
if (color == Board.White && moveBeingChecked.rank > beta)
beta = moveBeingChecked.rank;
if (color == Board.Black && moveBeingChecked.rank < alpha)
alpha = moveBeingChecked.rank;
}
// If the alpha-beta pruning is enabled
// perform a cut off if necessary.
if (alphaBeta)
{
if (color == Board.White && moveBeingChecked.rank > alpha)
{
moveBeingChecked.rank = alpha;
return moveBeingChecked;
}
if (color == Board.Black && moveBeingChecked.rank < beta)
{
moveBeingChecked.rank = beta;
return moveBeingChecked;
}
}
// If this is the first move tested, assume it is the
// best for now. otherwise, compare the test move
// to the current best move and take the one that
// is better for this color.
if (bestMove.row < 0)
bestMove = moveBeingChecked;
else if (color * moveBeingChecked.rank < color * bestMove.rank)
bestMove = moveBeingChecked;
}
}
// Return the best move found.
return bestMove;
}