basic model

app/src/main/java/chess/model/ChessBoard.java

@@ -0,0 +1,107 @@
+package chess.model;
+
+import chess.model.visitor.KingIdentifier;
+import chess.model.visitor.PiecePathChecker;
+
+public class ChessBoard {
+	public static class Cell {
+		private Piece piece;
+
+		public Cell() {
+			this.piece = null;
+		}
+
+		public Piece getPiece() {
+			return piece;
+		}
+
+		public void setPiece(Piece piece) {
+			this.piece = piece;
+		}
+
+	}
+
+	private final Cell[][] cells;
+
+	public ChessBoard() {
+		this.cells = new Cell[Coordinate.VALUE_MAX][Coordinate.VALUE_MAX];
+	}
+
+	public void applyMove(Move move) {
+		assert (move.isValid());
+		Piece deadPiece = pieceAt(move.getFinish());
+		if (deadPiece != null) {
+			deadPiece.eject();
+		}
+		Piece movingPiece = pieceAt(move.getStart());
+		pieceComes(movingPiece, move.getFinish());
+		pieceLeaves(move.getStart());
+	}
+
+	public void undoMove(Move move) {
+		// for later
+	}
+
+	public boolean isCellEmpty(Coordinate coordinate) {
+		return pieceAt(coordinate) == null;
+	}
+
+	public Piece pieceAt(Coordinate coordinate) {
+		assert (coordinate.isValid());
+		return cellAt(coordinate).getPiece();
+	}
+
+	public void clearBoard() {
+		for (int i = 0; i < Coordinate.VALUE_MAX; i++) {
+			for (int j = 0; j < Coordinate.VALUE_MAX; j++) {
+				pieceLeaves(new Coordinate(i, j));
+			}
+		}
+	}
+
+	private Cell cellAt(Coordinate coordinate) {
+		return this.cells[coordinate.getX()][coordinate.getY()];
+	}
+
+	private void pieceComes(Piece piece, Coordinate coordinate) {
+		cellAt(coordinate).setPiece(piece);
+	}
+
+	private void pieceLeaves(Coordinate coordinate) {
+		cellAt(coordinate).setPiece(null);
+	}
+
+	public Coordinate findKing(Color color) {
+		KingIdentifier kingIdentifier = new KingIdentifier(color);
+		for (int i = 0; i < Coordinate.VALUE_MAX; i++) {
+			for (int j = 0; j < Coordinate.VALUE_MAX; j++) {
+				Coordinate coordinate = new Coordinate(i, j);
+				Piece piece = pieceAt(coordinate);
+				if (piece != null && kingIdentifier.visit(piece)) {
+					return coordinate;
+				}
+			}
+		}
+		assert false : "No king found ?!";
+		return null;
+	}
+
+	public boolean isKingInCheck(Color color) {
+		Coordinate kingPos = findKing(color);
+		assert kingPos.isValid() : "King position is invalid!";
+
+		for (int i = 0; i < Coordinate.VALUE_MAX; i++) {
+			for (int j = 0; j < Coordinate.VALUE_MAX; j++) {
+				Coordinate attackCoords = new Coordinate(i, j);
+				Piece attackPiece = pieceAt(attackCoords);
+				if (attackPiece == null)
+					continue;
+
+				PiecePathChecker checker = new PiecePathChecker(this, new Move(attackCoords, kingPos));
+				if (checker.isValidForPiece(attackPiece))
+					return true;
+			}
+		}
+		return false;
+	}
+}

app/src/main/java/chess/model/Color.java

@@ -0,0 +1,6 @@
+package chess.model;
+
+public enum Color {
+	White,
+	Black
+}

app/src/main/java/chess/model/Coordinate.java

@@ -0,0 +1,41 @@
+package chess.model;
+
+public class Coordinate {
+	private final int x;
+	private final int y;
+
+	public static int VALUE_MAX = 8;
+
+	public Coordinate(int x, int y) {
+		this.x = x;
+		this.y = y;
+	}
+
+	public int getX() {
+		return x;
+	}
+
+	public int getY() {
+		return y;
+	}
+
+	public boolean isValid() {
+		return 0 <= this.x && this.x < VALUE_MAX && 0 <= this.y && this.y < VALUE_MAX;
+	}
+
+	public static Coordinate fromIndex(int index) {
+		return new Coordinate(index % VALUE_MAX, index / VALUE_MAX);
+	}
+
+	public int toIndex() {
+		return this.y * VALUE_MAX + this.x;
+	}
+
+	@Override
+	public boolean equals(Object obj) {
+		if (obj instanceof Coordinate coo) {
+			return this.x == coo.x && this.y == coo.y;
+		}
+		return false;
+	}
+}

app/src/main/java/chess/model/Direction.java

@@ -0,0 +1,46 @@
+package chess.model;
+
+public enum Direction {
+
+	Unset(65),
+	Front(8), Back(-8), Left(-1), Right(1),
+	FrontLeft(7), FrontRight(9), BackLeft(-9), BackRight(-7);
+
+	private final int indexOffset;
+
+	Direction(int indexOffset) {
+		this.indexOffset = indexOffset;
+	}
+
+	public int getIndexOffset() {
+		return indexOffset;
+	}
+
+	public static Direction findDirection(Move move) {
+		assert move.isValid() : "Move is invalid!";
+		int diffX = move.getFinish().getX() - move.getStart().getX();
+		int diffY = move.getFinish().getY() - move.getStart().getY();
+
+		if (diffX == 0 && diffY < 0)
+			return Direction.Back;
+		if (diffX == 0 && diffY > 0)
+			return Direction.Front;
+
+		if (diffX < 0 && diffY == 0)
+			return Direction.Left;
+		if (diffX > 0 && diffY == 0)
+			return Direction.Right;
+
+		if (diffX < 0 && -diffX == diffY)
+			return Direction.FrontLeft;
+		if (diffX > 0 && diffX == diffY)
+			return Direction.FrontRight;
+
+		if (diffY < 0 && diffX == diffY)
+			return Direction.BackLeft;
+		if (diffY > 0 && diffX == -diffY)
+			return Direction.BackRight;
+
+		return Direction.Unset;
+	}
+}

app/src/main/java/chess/model/Move.java

@@ -0,0 +1,42 @@
+package chess.model;
+
+public class Move {
+	private final Coordinate start;
+	private final Coordinate finish;
+
+	public Move(Coordinate start, Coordinate finish) {
+		this.start = start;
+		this.finish = finish;
+	}
+
+	public boolean isValid() {
+		return this.start.isValid() && this.finish.isValid() && !this.start.equals(this.finish);
+	}
+
+	public Coordinate getStart() {
+		return start;
+	}
+
+	public Coordinate getFinish() {
+		return finish;
+	}
+
+	public int traversedCells() {
+		assert isValid() : "Move is invalid!";
+
+		int diffX = getFinish().getX() - getStart().getX();
+		int diffY = getFinish().getY() - getStart().getY();
+
+		assert Math.abs(diffX) < Coordinate.VALUE_MAX : "Move is too big!";
+		assert Math.abs(diffX) < Coordinate.VALUE_MAX : "Move is too big!";
+
+		if (diffX == 0)
+			return Math.abs(diffY);
+		if (diffY == 0)
+			return Math.abs(diffX);
+		if (Math.abs(diffX) == Math.abs(diffY))
+			return Math.abs(diffX);
+		return 0;
+	}
+
+}

app/src/main/java/chess/model/Piece.java

@@ -0,0 +1,31 @@
+package chess.model;
+
+public abstract class Piece {
+
+	private final Color color;
+	private boolean moved;
+
+	public Piece(Color color) {
+		this.color = color;
+		this.moved = false;
+	}
+
+	public void move() {
+		this.moved = true;
+	}
+
+	public Color getColor() {
+		return color;
+	}
+
+	public boolean hasMoved() {
+		return moved;
+	}
+
+	public void eject() {
+		// for later
+	}
+
+	public abstract <T> T accept(PieceVisitor<T> visitor);
+
+}

app/src/main/java/chess/model/PieceVisitor.java

@@ -0,0 +1,28 @@
+package chess.model;
+
+import chess.model.pieces.Bishop;
+import chess.model.pieces.King;
+import chess.model.pieces.Knight;
+import chess.model.pieces.Pawn;
+import chess.model.pieces.Queen;
+import chess.model.pieces.Rook;
+
+public interface PieceVisitor<T> {
+
+	default T visit(Piece piece) {
+		return piece.accept(this);
+	}
+
+	T visitPiece(Bishop bishop);
+
+	T visitPiece(King king);
+
+	T visitPiece(Knight knight);
+
+	T visitPiece(Pawn pawn);
+
+	T visitPiece(Queen queen);
+
+	T visitPiece(Rook rook);
+
+}

app/src/main/java/chess/model/pieces/Bishop.java

@@ -0,0 +1,18 @@
+package chess.model.pieces;
+
+import chess.model.Color;
+import chess.model.Piece;
+import chess.model.PieceVisitor;
+
+public class Bishop extends Piece {
+
+	public Bishop(Color color) {
+		super(color);
+	}
+
+	@Override
+	public <T> T accept(PieceVisitor<T> visitor) {
+		return visitor.visitPiece(this);
+	}
+
+}

app/src/main/java/chess/model/pieces/King.java

@@ -0,0 +1,17 @@
+package chess.model.pieces;
+
+import chess.model.Color;
+import chess.model.Piece;
+import chess.model.PieceVisitor;
+
+public class King extends Piece {
+
+	public King(Color color) {
+		super(color);
+	}
+
+	@Override
+	public <T> T accept(PieceVisitor<T> visitor) {
+		return visitor.visitPiece(this);
+	}
+}

app/src/main/java/chess/model/pieces/Knight.java

@@ -0,0 +1,17 @@
+package chess.model.pieces;
+
+import chess.model.Color;
+import chess.model.Piece;
+import chess.model.PieceVisitor;
+
+public class Knight extends Piece {
+
+	public Knight(Color color) {
+		super(color);
+	}
+
+	@Override
+	public <T> T accept(PieceVisitor<T> visitor) {
+		return visitor.visitPiece(this);
+	}
+}

app/src/main/java/chess/model/pieces/Pawn.java

@@ -0,0 +1,21 @@
+package chess.model.pieces;
+
+import chess.model.Color;
+import chess.model.Piece;
+import chess.model.PieceVisitor;
+
+public class Pawn extends Piece {
+
+	public Pawn(Color color) {
+		super(color);
+	}
+
+	@Override
+	public <T> T accept(PieceVisitor<T> visitor) {
+		return visitor.visitPiece(this);
+	}
+
+	public int multiplier() {
+		return getColor() == Color.White ? 1 : -1;
+	}
+}

app/src/main/java/chess/model/pieces/Queen.java

@@ -0,0 +1,17 @@
+package chess.model.pieces;
+
+import chess.model.Color;
+import chess.model.Piece;
+import chess.model.PieceVisitor;
+
+public class Queen extends Piece {
+
+	public Queen(Color color) {
+		super(color);
+	}
+
+	@Override
+	public <T> T accept(PieceVisitor<T> visitor) {
+		return visitor.visitPiece(this);
+	}
+}

app/src/main/java/chess/model/pieces/Rook.java

@@ -0,0 +1,17 @@
+package chess.model.pieces;
+
+import chess.model.Color;
+import chess.model.Piece;
+import chess.model.PieceVisitor;
+
+public class Rook extends Piece {
+
+	public Rook(Color color) {
+		super(color);
+	}
+
+	@Override
+	public <T> T accept(PieceVisitor<T> visitor) {
+		return visitor.visitPiece(this);
+	}
+}

app/src/main/java/chess/model/visitor/KingIdentifier.java

@@ -0,0 +1,45 @@
+package chess.model.visitor;
+
+import chess.model.Color;
+import chess.model.PieceVisitor;
+import chess.model.pieces.*;
+
+public class KingIdentifier implements PieceVisitor<Boolean> {
+
+	private final Color color;
+
+	public KingIdentifier(Color color) {
+		this.color = color;
+	}
+
+	@Override
+	public Boolean visitPiece(Bishop bishop) {
+		return false;
+	}
+
+	@Override
+	public Boolean visitPiece(King king) {
+		return king.getColor() == color;
+	}
+
+	@Override
+	public Boolean visitPiece(Knight knight) {
+		return false;
+	}
+
+	@Override
+	public Boolean visitPiece(Pawn pawn) {
+		return false;
+	}
+
+	@Override
+	public Boolean visitPiece(Queen queen) {
+		return false;
+	}
+
+	@Override
+	public Boolean visitPiece(Rook rook) {
+		return false;
+	}
+
+}

app/src/main/java/chess/model/visitor/PermissiveRuleChecker.java

@@ -0,0 +1,119 @@
+package chess.model.visitor;
+
+import chess.model.Coordinate;
+import chess.model.Direction;
+import chess.model.Move;
+import chess.model.Piece;
+import chess.model.PieceVisitor;
+import chess.model.pieces.Bishop;
+import chess.model.pieces.King;
+import chess.model.pieces.Knight;
+import chess.model.pieces.Pawn;
+import chess.model.pieces.Queen;
+import chess.model.pieces.Rook;
+
+public class PermissiveRuleChecker implements PieceVisitor<Boolean> {
+
+	private final Move move;
+
+	public PermissiveRuleChecker(Move move) {
+		this.move = move;
+		assert move.isValid() : "Move is invalid!";
+	}
+
+	public boolean isValidFor(Piece piece) {
+		return visit(piece);
+	}
+
+	@Override
+	public Boolean visitPiece(Bishop bishop) {
+		Direction moveDirection = Direction.findDirection(this.move);
+
+		switch (moveDirection) {
+			case FrontLeft:
+			case BackLeft:
+			case FrontRight:
+			case BackRight:
+				return true;
+
+			default:
+				return false;
+		}
+	}
+
+	@Override
+	public Boolean visitPiece(King king) {
+		return this.move.traversedCells() == 1;
+	}
+
+	@Override
+	public Boolean visitPiece(Knight knight) {
+		Coordinate piecePos = move.getStart();
+		final Coordinate[] positions = {
+				new Coordinate(piecePos.getX() - 1, piecePos.getY() - 2),
+				new Coordinate(piecePos.getX() - 1, piecePos.getY() + 2),
+				new Coordinate(piecePos.getX() + 1, piecePos.getY() - 2),
+				new Coordinate(piecePos.getX() + 1, piecePos.getY() + 2),
+				new Coordinate(piecePos.getX() + 2, piecePos.getY() - 1),
+				new Coordinate(piecePos.getX() + 2, piecePos.getY() + 1),
+				new Coordinate(piecePos.getX() - 2, piecePos.getY() - 1),
+				new Coordinate(piecePos.getX() - 2, piecePos.getY() + 1),
+		};
+
+		for (int i = 0; i < positions.length; i++) {
+			if (this.move.getFinish().equals(positions[i]))
+				return true;
+		}
+
+		return false;
+	}
+
+	@Override
+	public Boolean visitPiece(Pawn pawn) {
+		Direction moveDirection = Direction.findDirection(this.move);
+		int directionIndexOffset = moveDirection.getIndexOffset();
+		int distance = this.move.traversedCells();
+
+		// Revoke moving backwards
+		if (directionIndexOffset * pawn.multiplier() < 0)
+			return false;
+
+		// Allowing straight moves
+		if (Math.abs(directionIndexOffset) == Direction.Front.getIndexOffset()) {
+			if (pawn.hasMoved())
+				return distance == 1;
+			return distance == 1 || distance == 2;
+		}
+
+		// Allowing small diagonal moves
+		if (Math.abs(directionIndexOffset) == Direction.FrontLeft.getIndexOffset()
+				|| Math.abs(directionIndexOffset) == Direction.FrontRight.getIndexOffset()) {
+			return distance == 1;
+		}
+
+		return false;
+	}
+
+	@Override
+	public Boolean visitPiece(Queen queen) {
+		Direction moveDirection = Direction.findDirection(this.move);
+		return moveDirection != Direction.Unset;
+	}
+
+	@Override
+	public Boolean visitPiece(Rook rook) {
+		Direction moveDirection = Direction.findDirection(this.move);
+
+		switch (moveDirection) {
+			case Front:
+			case Back:
+			case Left:
+			case Right:
+				return true;
+
+			default:
+				return false;
+		}
+	}
+
+}

app/src/main/java/chess/model/visitor/PiecePathChecker.java

@@ -0,0 +1,116 @@
+package chess.model.visitor;
+
+import chess.model.ChessBoard;
+import chess.model.Color;
+import chess.model.Coordinate;
+import chess.model.Direction;
+import chess.model.Move;
+import chess.model.Piece;
+import chess.model.PieceVisitor;
+import chess.model.pieces.Bishop;
+import chess.model.pieces.King;
+import chess.model.pieces.Knight;
+import chess.model.pieces.Pawn;
+import chess.model.pieces.Queen;
+import chess.model.pieces.Rook;
+
+public class PiecePathChecker implements PieceVisitor<Boolean> {
+
+	private final ChessBoard board;
+	private final Move move;
+
+	public PiecePathChecker(ChessBoard board, Move move) {
+		this.move = move;
+		this.board = board;
+	}
+
+	public boolean isValidForPiece(Piece piece) {
+		return visit(piece);
+	}
+
+	@Override
+	public Boolean visitPiece(Bishop bishop) {
+		if (!new PermissiveRuleChecker(this.move).isValidFor(bishop))
+			return false;
+		return testPath(bishop.getColor());
+	}
+
+	@Override
+	public Boolean visitPiece(King king) {
+		if (!new PermissiveRuleChecker(this.move).isValidFor(king))
+			return false;
+
+		Piece destPiece = board.pieceAt(this.move.getFinish());
+		if (destPiece == null)
+			return true;
+		return destPiece.getColor() != king.getColor();
+	}
+
+	@Override
+	public Boolean visitPiece(Knight knight) {
+		if (!new PermissiveRuleChecker(this.move).isValidFor(knight))
+			return false;
+
+		Piece destPiece = board.pieceAt(this.move.getFinish());
+		if (destPiece == null)
+			return true;
+		return destPiece.getColor() != knight.getColor();
+	}
+
+	@Override
+	public Boolean visitPiece(Pawn pawn) {
+		if (!new PermissiveRuleChecker(this.move).isValidFor(pawn))
+			return false;
+
+		Direction moveDirection = Direction.findDirection(this.move);
+		// ...
+		// moveDirection = Directions(int(findDirection(move)) * pawn.multiplier())
+
+		if (moveDirection == Direction.Front)
+			return testPath(pawn.getColor());
+
+		assert moveDirection == Direction.FrontLeft || moveDirection == Direction.FrontRight;
+
+		Piece destPiece = this.board.pieceAt(this.move.getFinish());
+		if (destPiece == null)
+			return true;
+
+		return destPiece.getColor() != pawn.getColor();
+	}
+
+	@Override
+	public Boolean visitPiece(Queen queen) {
+		if (!new PermissiveRuleChecker(this.move).isValidFor(queen))
+			return false;
+		return testPath(queen.getColor());
+	}
+
+	@Override
+	public Boolean visitPiece(Rook rook) {
+		if (!new PermissiveRuleChecker(this.move).isValidFor(rook))
+			return false;
+		return testPath(rook.getColor());
+	}
+
+	private boolean testPath(Color color) {
+		Direction moveDirection = Direction.findDirection(this.move);
+		int distance = this.move.traversedCells();
+		int stepIndex = move.getStart().toIndex();
+
+		for (int step = 0; step < distance; step++) {
+			stepIndex += moveDirection.getIndexOffset();
+
+			if (Coordinate.fromIndex(stepIndex).equals(move.getFinish())) {
+				Piece pieceDest = this.board.pieceAt(move.getFinish());
+				if (pieceDest == null)
+					return true;
+				return pieceDest.getColor() != color;
+			}
+
+			if (!this.board.isCellEmpty(Coordinate.fromIndex(stepIndex)))
+				return false;
+		}
+
+		return false;
+	}
+}