GameOfLife based on a general cell distribution function, #gdcr13

Game.java

package gameoflife;

public class Game {
	public interface Distribution {
		boolean cellExists(int x, int y);
	}

	final private Distribution distribution;

	public Game(Distribution initialDistribution) {
		this.distribution = initialDistribution;
	}

	public Game next() {
		return new Game(nextDistribution());
	}

	public Distribution nextDistribution() {
		return new Distribution() {
			
			@Override
			public boolean cellExists(int x, int y) {
				return livingCellHasTwoNeighbours(x, y) || cellHasTreeNeighbours(x, y);
			}

			private boolean cellHasTreeNeighbours(int x, int y) {
				return  countNeighbours(distribution, x, y) == 3;
			}

			public boolean livingCellHasTwoNeighbours(int x, int y) {
				return distribution.cellExists(x, y) && countNeighbours(distribution, x, y) == 2;
			}

			private int countNeighbours(Distribution distribution, int x, int y) {
				return cellNumber(x+1, y+1) +
						cellNumber(x+1, y) +
						cellNumber(x+1, y-1) +
						cellNumber(x, y+1) +
						cellNumber(x, y-1) +
						cellNumber(x-1, y+1) +
						cellNumber(x-1, y) +
						cellNumber(x-1, y-1);
			}

			private int cellNumber(int x, int y) {
				return distribution.cellExists(x, y) ? 1 : 0;
			}
		};
	}

	public Distribution distribution() {
		return distribution;
	}
}

GameTest.java

package gameoflife;

import static org.hamcrest.CoreMatchers.equalTo;
import static org.junit.Assert.assertThat;
import gameoflife.Game.Distribution;

import org.junit.Test;

public class GameTest {
	final int someY = 0;
	final int someX = 0;
	@Test
	public void emptyDistribution_generatesNoCellsForx0_y0() {
		Distribution emptyDistribution = new Distribution() {
			@Override
			public boolean cellExists(int x, int y) {
				return false;
			}
		};

		final Game game = new Game(emptyDistribution);
		final Game nextBoard = game.next();
		Distribution nextDistribution = nextBoard.distribution();
		assertThat(false, equalTo(nextDistribution.cellExists(someX, someY)));
	}

	@Test
	public void singlePointDistribution_generatesNoCellsForx0_y0() {
		Distribution singlePointDistribution = new Distribution() {
			@Override
			public boolean cellExists(int x, int y) {
				return someX == x && someY == y;
			}
		};
		final Game game = new Game(singlePointDistribution);
		final Game nextBoard = game.next();
		Distribution nextDistribution = nextBoard.distribution();
		assertThat(false, equalTo(nextDistribution.cellExists(someX, someY)));
	}

	@Test
	public void verticalThreePointsDistribution() {
		Distribution distribution = new Distribution() {
			@Override
			public boolean cellExists(int x, int y) {
				return someX == x && (someY == y-1 || someY == y || someY == y+1);
			}
		};
		final Game game = new Game(distribution);
		final Game nextBoard = game.next();
		Distribution nextDistribution = nextBoard.distribution();
		assertThat(true, equalTo(nextDistribution.cellExists(someX, someY)));
	}

	@Test
	public void angularThreePointsDistribution() {
		Distribution distribution = new Distribution() {
			@Override
			public boolean cellExists(int x, int y) {
				return someX == x && someY == y+1 
						|| someX == x + 1 && someY == y+1 
						|| someX == x + 1 && someY == y;
			}
		};
		final Game game = new Game(distribution);
		final Game nextBoard = game.next();
		Distribution nextDistribution = nextBoard.distribution();
		assertThat(true, equalTo(nextDistribution.cellExists(someX, someY)));
	}
}