chess/src/main/java/dev/kske/chess/game/ai/MoveProcessor.java

package dev.kske.chess.game.ai;

import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.concurrent.Callable;

import dev.kske.chess.board.*;
import dev.kske.chess.board.Piece.Color;

/**
 * Implements a basic minimax move search algorithm for testing purposes.<br>
 * <br>
 * Project: <strong>Chess</strong><br>
 * File: <strong>MoveProcessor.java</strong><br>
 * Created: <strong>08.07.2019</strong><br>
 *
 * @since Chess v0.1-alpha
 * @author Kai S. K. Engelbart
 */
public class MoveProcessor implements Callable<ProcessingResult> {

	private final Board board;
	private final List<Move> rootMoves;
	private final Color color;
	private final int maxDepth;
	private final int alphaBetaThreshold;

	private Move bestMove;

	private static final Map<Class<? extends Piece>, int[][]> positionScores;

	static {
		positionScores = new HashMap<>();
		positionScores.put(
			King.class,
			new int[][] {
				new int[] {
					-3, -4, -4, -5, -5, -4, -4, -3
				}, new int[] {
					-3, -4, -4, -5, -4, -4, -4, -3
				},
				new int[] {
					-3, -4, -4, -5, -4, -4, -4, -3
				},
				new int[] {
					-3, -4, -4, -5, -4, -4, -4, -3
				},
				new int[] {
					-2, -3, -3, -2, -2, -2, -2, -1
				},
				new int[] {
					-1, -2, -2, -2, -2, -2, -2, -1
				},
				new int[] {
					2, 2, 0, 0, 0, 0, 2, 2
				},
				new int[] {
					2, 3, 1, 0, 0, 1, 3, 2
				}
			}
		);
		positionScores.put(
			Queen.class,
			new int[][] {
				new int[] {
					-2, -1, -1, -1, -1, -1, -1, -2
				}, new int[] {
					-1, 0, 0, 0, 0, 0, 0, -1
				},
				new int[] {
					-1, 0, 1, 1, 1, 1, 0, -1
				},
				new int[] {
					-1, 0, 1, 1, 1, 1, 0, -1
				},
				new int[] {
					0, 0, 1, 1, 1, 1, 0, -1
				},
				new int[] {
					-1, 1, 1, 1, 1, 1, 0, -1
				},
				new int[] {
					-1, 0, 1, 0, 0, 0, 0, -1
				},
				new int[] {
					-2, -1, -1, -1, -1, -1, -1, -2
				}
			}
		);
		positionScores.put(
			Rook.class,
			new int[][] {
				new int[] {
					0, 0, 0, 0, 0, 0, 0, 0
				}, new int[] {
					1, 1, 1, 1, 1, 1, 1, 1
				}, new int[] {
					-1, 0, 0, 0, 0, 0, 0, -1
				},
				new int[] {
					-1, 0, 0, 0, 0, 0, 0, -1
				},
				new int[] {
					-1, 0, 0, 0, 0, 0, 0, -1
				},
				new int[] {
					-1, 0, 0, 0, 0, 0, 0, -1
				},
				new int[] {
					-1, 0, 0, 0, 0, 0, 0, -1
				},
				new int[] {
					0, 0, 0, 1, 1, 0, 0, 0
				}
			}
		);
		positionScores.put(
			Knight.class,
			new int[][] {
				new int[] {
					-5, -4, -3, -3, -3, -3, -4, -5
				}, new int[] {
					-4, -2, 0, 0, 0, 0, -2, -4
				},
				new int[] {
					-3, 0, 1, 2, 2, 1, 0, -3
				},
				new int[] {
					-3, 1, 2, 2, 2, 2, 1, -3
				},
				new int[] {
					-3, 0, 2, 2, 2, 2, 0, -1
				},
				new int[] {
					-3, 1, 1, 2, 2, 1, 1, -3
				},
				new int[] {
					-4, -2, 0, 1, 1, 0, -2, -4
				},
				new int[] {
					-5, -4, -3, -3, -3, -3, -4, -5
				}
			}
		);
		positionScores.put(
			Bishop.class,
			new int[][] {
				new int[] {
					-2, -1, -1, -1, -1, -1, -1, 2
				}, new int[] {
					-1, 0, 0, 0, 0, 0, 0, -1
				},
				new int[] {
					-1, 0, 1, 1, 1, 1, 0, -1
				},
				new int[] {
					-1, 1, 1, 1, 1, 1, 1, -1
				},
				new int[] {
					-1, 0, 1, 1, 1, 1, 0, -1
				},
				new int[] {
					-1, 1, 1, 1, 1, 1, 1, -1
				},
				new int[] {
					-1, 1, 0, 0, 0, 0, 1, -1
				},
				new int[] {
					-2, -1, -1, -1, -1, -1, -1, -2
				}
			}
		);
		positionScores.put(
			Pawn.class,
			new int[][] {
				new int[] {
					0, 0, 0, 0, 0, 0, 0, 0
				}, new int[] {
					5, 5, 5, 5, 5, 5, 5, 5
				}, new int[] {
					1, 1, 2, 3, 3, 2, 1, 1
				},
				new int[] {
					0, 0, 1, 3, 3, 1, 0, 0
				},
				new int[] {
					0, 0, 0, 2, 2, 0, 0, 0
				},
				new int[] {
					0, 0, -1, 0, 0, -1, 0, 0
				},
				new int[] {
					0, 1, 1, -2, -2, 1, 1, 0
				},
				new int[] {
					0, 0, 0, 0, 0, 0, 0, 0
				}
			}
		);
	}

	/**
	 * Creates an instance of {@link MoveProcessor}.
	 *
	 * @param board              the board to search
	 * @param rootMoves          the moves on which the search is based
	 * @param color              the color for which to search
	 * @param maxDepth           the maximal recursion depth to search to
	 * @param alphaBetaThreshold the threshold necessary to continue a search
	 *                           for a
	 *                           specific move
	 */
	public MoveProcessor(
		Board board, List<Move> rootMoves, Color color, int maxDepth,
		int alphaBetaThreshold
	) {
		this.board = board;
		this.rootMoves = rootMoves;
		this.color = color;
		this.maxDepth = maxDepth;
		this.alphaBetaThreshold = alphaBetaThreshold;
	}

	@Override
	public ProcessingResult call() throws Exception {
		int score = miniMax(board, rootMoves, color, 0);
		return new ProcessingResult(bestMove, score);
	}

	private int miniMax(Board board, List<Move> moves, Color color, int depth) {
		int bestValue = Integer.MIN_VALUE;
		for (Move move : moves) {
			board.move(move);
			int teamValue = evaluate(board, color);
			int enemyValue = evaluate(board, color.opposite());
			int valueChange = teamValue - enemyValue;

			if (depth < maxDepth && valueChange >= alphaBetaThreshold)
				valueChange -= miniMax(
					board,
					board.getMoves(color.opposite()),
					color.opposite(),
					depth + 1
				);

			if (valueChange > bestValue) {
				bestValue = valueChange;
				if (depth == 0)
					bestMove = move;
			}
			board.revert();
		}
		return bestValue;
	}

	/**
	 * Evaluated a board.
	 *
	 * @param board the board to evaluate
	 * @param color The color to evaluate for
	 * @return a positive number representing how good the position is
	 */
	private int evaluate(Board board, Color color) {
		int score = 0;
		for (int i = 0; i < 8; i++)
			for (int j = 0; j < 8; j++)
				if (
					board.getBoardArr()[i][j] != null
						&& board.getBoardArr()[i][j].getColor() == color
				) {
					score += board.getBoardArr()[i][j].getValue();
					if (
						positionScores
							.containsKey(board.getBoardArr()[i][j].getClass())
					)
						score += positionScores.get(
							board.getBoardArr()[i][j].getClass()
						)[i][color == Color.WHITE ? j : 7 - j];
				}
		return score;
	}
}