chess/src/main/java/dev/kske/chess/board/FENString.java

package dev.kske.chess.board;

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

import dev.kske.chess.board.Piece.Color;
import dev.kske.chess.exception.ChessException;

/**
 * Project: <strong>Chess</strong><br>
 * File: <strong>FENString.java</strong><br>
 * Created: <strong>20 Oct 2019</strong><br>
 * <br>
 * Represents a FEN string and enables parsing an existing FEN string or
 * serializing a {@link Board} to one.
 *
 * @since Chess v0.5-alpha
 * @author Kai S. K. Engelbart
 */
public class FENString {

	private Board board;
	private String piecePlacement, castlingAvailability;
	private int halfmoveClock, fullmoveNumber;
	private Color activeColor;
	private Position enPassantTargetSquare;

	/**
	 * Constructs a {@link FENString} representing the starting position
	 * {@code rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1}.
	 */
	public FENString() {
		board = new Board();
		piecePlacement = "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR";
		activeColor = Color.WHITE;
		castlingAvailability = "KQkq";
		halfmoveClock = 0;
		fullmoveNumber = 1;
	}

	/**
	 * Constructs a {@link FENString} by parsing an existing string.
	 *
	 * @param fen the FEN string to parse
	 * @throws ChessException if the FEN string contains invalid syntax
	 */
	public FENString(String fen) throws ChessException {
		// Check fen string against regex
		Pattern fenPattern = Pattern.compile(
			"^(?<piecePlacement>(?:[1-8nbrqkpNBRQKP]{1,8}\\/){7}[1-8nbrqkpNBRQKP]{1,8}) (?<activeColor>[wb]) (?<castlingAvailability>-|[KQkq]{1,4}) (?<enPassantTargetSquare>-|[a-h][1-8]) (?<halfmoveClock>\\d+) (?<fullmoveNumber>\\d+)$"
		);
		Matcher matcher = fenPattern.matcher(fen);
		if (!matcher.find())
			throw new ChessException(
				"FEN string does not match pattern " + fenPattern.pattern()
			);

		// Initialize data fields
		piecePlacement = matcher.group("piecePlacement");
		activeColor
			= Color.fromFirstChar(matcher.group("activeColor").charAt(0));
		castlingAvailability = matcher.group("castlingAvailability");
		if (!matcher.group("enPassantTargetSquare").equals("-"))
			enPassantTargetSquare
				= Position.fromLAN(matcher.group("enPassantTargetSquare"));
		halfmoveClock = Integer.parseInt(matcher.group("halfmoveClock"));
		fullmoveNumber = Integer.parseInt(matcher.group("fullmoveNumber"));

		// Initialize and clean board
		board = new Board();
		for (int i = 0; i < 8; i++)
			for (int j = 0; j < 8; j++)
				board.getBoardArr()[i][j] = null;

		// Parse individual fields

		// Piece placement
		final String[] rows = piecePlacement.split("/");
		if (rows.length != 8)
			throw new ChessException(
				"FEN string contains invalid piece placement"
			);
		for (int i = 0; i < 8; i++) {
			final char[] cols = rows[i].toCharArray();
			int j = 0;
			for (char c : cols)
				// Empty space
				if (Character.isDigit(c))
					j += Character.getNumericValue(c);
				else {
					Color color
						= Character.isUpperCase(c) ? Color.WHITE : Color.BLACK;
					try {
						Constructor<? extends Piece> pieceConstructor = Piece
							.fromFirstChar(c)
							.getDeclaredConstructor(Color.class, Board.class);
						pieceConstructor.setAccessible(true);
						board.getBoardArr()[j][i]
							= pieceConstructor.newInstance(color, board);
					} catch (
						InstantiationException
						| IllegalAccessException
						| IllegalArgumentException
						| InvocationTargetException
						| NoSuchMethodException
						| SecurityException e
					) {
						e.printStackTrace();
					}
					++j;
				}
		}
		// Active color
		board.getLog().setActiveColor(activeColor);

		// Castling availability
		boolean castlingRights[] = new boolean[4];
		for (char c : castlingAvailability.toCharArray())
			switch (c) {
				case 'K':
					castlingRights[MoveNode.WHITE_KINGSIDE] = true;
					break;
				case 'Q':
					castlingRights[MoveNode.WHITE_QUEENSIDE] = true;
					break;
				case 'k':
					castlingRights[MoveNode.BLACK_KINGSIDE] = true;
					break;
				case 'q':
					castlingRights[MoveNode.BLACK_QUEENSIDE] = true;
					break;
			}
		board.getLog().setCastlingRights(castlingRights);

		// En passant square
		board.getLog().setEnPassant(enPassantTargetSquare);

		// Halfmove clock
		board.getLog().setHalfmoveClock(halfmoveClock);

		// Fullmove number
		board.getLog().setFullmoveNumber(fullmoveNumber);
	}

	/**
	 * Constructs a {@link FENString} form a {@link Board} object.
	 *
	 * @param board the {@link Board} object to encode in this {@link FENString}
	 */
	public FENString(Board board) {
		this.board = board;

		// Serialize individual fields

		// Piece placement
		StringBuilder sb = new StringBuilder();
		for (int i = 0; i < 8; i++) {
			int empty = 0;
			for (int j = 0; j < 8; j++) {
				final Piece piece = board.getBoardArr()[j][i];

				if (piece == null)
					++empty;
				else {

					// Write empty field count
					if (empty > 0) {
						sb.append(empty);
						empty = 0;
					}
					// Write piece character
					char p = piece.firstChar();
					sb.append(
						piece.getColor() == Color.WHITE
							? Character.toUpperCase(p)
							: p
					);
				}
			}
			// Write empty field count
			if (empty > 0) {
				sb.append(empty);
				empty = 0;
			}
			if (i < 7)
				sb.append('/');
		}
		piecePlacement = sb.toString();

		// Active color
		activeColor = board.getLog().getActiveColor();

		// Castling availability
		castlingAvailability = "";
		final char castlingRightsChars[] = new char[] {
			'K', 'Q', 'k', 'q'
		};
		for (int i = 0; i < 4; i++)
			if (board.getLog().getCastlingRights()[i])
				castlingAvailability += castlingRightsChars[i];
		if (castlingAvailability.isEmpty())
			castlingAvailability = "-";

		// En passant availability
		enPassantTargetSquare = board.getLog().getEnPassant();

		// Halfmove clock
		halfmoveClock = board.getLog().getHalfmoveClock();

		// Fullmove counter
		fullmoveNumber = board.getLog().getFullmoveNumber();
	}

	/**
	 * Exports this {@link FENString} object to a FEN string.
	 *
	 * @return a FEN string representing the board
	 */
	@Override
	public String toString() {
		return String.format(
			"%s %c %s %s %d %d",
			piecePlacement,
			activeColor.firstChar(),
			castlingAvailability,
			enPassantTargetSquare == null ? "-" : enPassantTargetSquare.toLAN(),
			halfmoveClock,
			fullmoveNumber
		);
	}

	/**
	 * @return a {@link Board} object corresponding to this {@link FENString}
	 */
	public Board getBoard() { return board; }
}