cs8803p4/src/model/comPlayer/generator/AlphaBetaMoveGenerator.java

package model.comPlayer.generator;

import java.util.Arrays;
import java.util.List;

import model.Board;
import model.BoardScorer;
import model.Move;
import model.SearchResult;

public class AlphaBetaMoveGenerator implements MoveGenerator {
	private static final int DEFAULT_RECURSIVE_PLAYS = 1;
	private int lookahead = DEFAULT_RECURSIVE_PLAYS;
	private final BoardScorer scorer = new BoardScorer();
	private final ValidMoveGenerator validMoveGenerator = new ValidMoveGenerator();

	@Override
	public Move genMove(Board board, boolean asHuman) {

		if (!asHuman) {
			return getMaxValue(board, asHuman, lookahead * 2,
					Integer.MIN_VALUE, Integer.MAX_VALUE).move;
		} else {
			return getMinValue(board, asHuman, lookahead * 2,
					Integer.MIN_VALUE, Integer.MAX_VALUE).move;
		}
	}

	/**
	 * AlphaBetaMoveGenerator2 does not support this method.
	 */
	@Override
	public List<Move> genMoves(Board board, boolean asHuman, int nMoves) {
		Move[] doNothing = new Move[] { Move.NONE };
		return Arrays.asList(doNothing);
	}

	private SearchResult getMaxValue(Board board, boolean asHuman,
			int recursionLevel, int alpha, int beta) {
		if (recursionLevel < 1) {
			return new SearchResult(Move.NONE, scorer.getScore(board));
		}

		List<Move> validMoves = validMoveGenerator.genMoves(board, asHuman,
				MoveGenerator.ALL_MOVES);

		SearchResult bestResult = new SearchResult(Move.NONE, Integer.MIN_VALUE);

		if (validMoves.size() == 0) {
			return bestResult;
		}

		for (Move nextMove : validMoves) {
			Board nextBoard = new Board(board);

			if (!nextBoard.playTile(nextMove.getCell(), nextMove.getColor())) {
				throw new RuntimeException(
						"Illegal move attempted during search!");
			}

			SearchResult searchResult = new SearchResult(nextMove, getMinValue(
					nextBoard, !asHuman, recursionLevel - 1, alpha, beta).score);

			if (searchResult.compareTo(bestResult) > 0) {
				bestResult = searchResult;
			}

			if (bestResult.score >= beta) {
				return bestResult;
			}

			alpha = Math.max(alpha, bestResult.score);
		}

		return bestResult;
	}

	private SearchResult getMinValue(Board board, boolean asHuman,
			int recursionLevel, int alpha, int beta) {
		if (recursionLevel < 1) {
			return new SearchResult(Move.NONE, scorer.getScore(board));
		}

		List<Move> validMoves = validMoveGenerator.genMoves(board, asHuman,
				MoveGenerator.ALL_MOVES);

		SearchResult bestResult = new SearchResult(Move.NONE, Integer.MAX_VALUE);

		if (validMoves.size() == 0) {
			return bestResult;
		}

		for (Move nextMove : validMoves) {
			Board nextBoard = new Board(board);

			if (!nextBoard.playTile(nextMove.getCell(), nextMove.getColor())) {
				throw new RuntimeException(
						"Illegal move attempted during search!");
			}

			SearchResult searchResult = new SearchResult(nextMove, getMaxValue(
					nextBoard, !asHuman, recursionLevel - 1, alpha, beta).score);

			if (searchResult.compareTo(bestResult) < 0) {
				bestResult = searchResult;
			}

			if (bestResult.score <= alpha) {
				return bestResult;
			}

			beta = Math.min(beta, bestResult.score);
		}

		return bestResult;
	}
	@Override
	public boolean setLookahead(int lookahead) {
		this.lookahead = lookahead;
		return true;
	}
}