/// <summary>
/// AlphaBeta Pruning algorithm adapted from genric AlphaBeta Pseudo-code: https://en.wikipedia.org/wiki/Alpha%E2%80%93beta_pruning
/// </summary>
/// <param name="game">Current game object (containing up-to-date board simulation)</param>
/// <param name="alpha">Alpha value</param>
/// <param name="beta">Beta value</param>
/// <param name="maximizingPlayer">Maximizing player (True when it is THIS AI's turn in the simulation)</param>
/// <returns></returns>
private float AlphaBetaPrune(Game game, float alpha, float beta, bool maximizingPlayer)
{
// Check the current game state and if it is completed, return the (maximizing player's) value associated with the game outcome
GameState state = game.GetGameState();
if (state.IsCompleted)
{
if (state.Winner == this)
{
return 1f + (1f - state.TotalMoves / 9f);
}
else if (state.Winner == null)
{
return 0;
}
else
{
return -1f - (1f - state.TotalMoves / 9f); ;
}
}
// If the game was not completed by the previous move, continue recursing through possible moves
float value;
if (maximizingPlayer)
{
value = float.MinValue;
foreach (Square square in game.Board.Squares)
{
if (square.Owner == null)
{
square.Owner = this;
value = Math.Max(value, AlphaBetaPrune(game, alpha, beta, false));
alpha = Math.Max(alpha, value);
square.Owner = null;
if (beta <= alpha)
{
break;
}
}
}
}
else
{
value = float.MaxValue;
foreach (Square square in game.Board.Squares)
{
if (square.Owner == null)
{
square.Owner = _playerOpponent;
value = Math.Min(value, AlphaBetaPrune(game, alpha, beta, true));
beta = Math.Min(beta, value);
square.Owner = null;
if (beta <= alpha)
{
break;
}
}
}
}
return value;
}
