GameOfLife.java

package interview;

import java.awt.Color;
import java.awt.Graphics;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class GameOfLife {

    public static void main(String[] args) throws InterruptedException {
        String pattern = """
                ........................*...........
                ......................*.*...........
                ............**......**............**
                ...........*...*....**............**
                **........*.....*...**..............
                **........*...*.**....*.*...........
                ..........*.....*.......*...........
                ...........*...*....................
                ............**......................
                """;

        /*
        String pattern = """
                .***......*.....*......***.
                *..*.....***...***.....*..*
                ...*....**.*...*.**....*...
                ...*...................*...
                ...*..*.............*..*...
                ...*..**...........**..*...
                ..*...**...........**...*..
                """;
         */

        String[] lines = pattern.split("\n");
        int patternRows = lines.length;
        int patternCols = lines[0].length();
        int left = 5;
        int top = 5;
        int right = 10;
        int bottom = 20;

        int period = 25;
        int rows = patternRows + top + bottom;
        int cols = patternCols + left + right;
        int width = cols * 20;
        int height = rows * 20;
        Board board = new Board(rows, cols);

        for (int r=0; r<lines.length; r++) {
            for (int c=0; c<lines[r].length(); c++) {
                if (lines[r].charAt(c) == '*') {
                    board.grid[top + r][left + c] = true;
                }
            }
        }

        BoardCanvas boardCanvas = new BoardCanvas(board);
        JFrame frame = new JFrame("Conway's Game of Life");
        frame.add(boardCanvas);
        frame.setSize(width, height);
        frame.setVisible(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        while (true) {
            boardCanvas.repaint();
            Thread.sleep(period);
            board.nextGeneration();
        }
    }

    public static class Board {
        final int rows;
        final int cols;

        boolean[][] grid; // [row][col], grid[0][1] = grid[row=0][col=1]

        Board(int rows, int cols) {
            this.rows = rows;
            this.cols = cols;
            grid = new boolean[rows][cols];
        }

        boolean isValid(int row, int col) {
            return row >= 0 && row < rows && col >= 0 && col < cols;
        }

        int numberOfLiveNeighbors(int row, int col) {
            int count = 0;
            for (int r = row - 1; r <= row + 1; r++) {
                for (int c = col - 1; c <= col + 1; c++) {
                    if (isValid(r, c)) {
                        if (r != row || c != col) {
                            if (grid[r][c]) {
                                count++;
                            }
                        }
                    }
                }
            }
            return count;
        }

        boolean isAliveInNextGen(int row, int col) {
            int numLiveNeighbors = numberOfLiveNeighbors(row, col);
            if (grid[row][col]) {
                return numLiveNeighbors == 2 || numLiveNeighbors == 3;
            }
            return numLiveNeighbors == 3;
        }

        void nextGeneration() {
            boolean[][] next = new boolean[rows][cols];
            for (int r = 0; r < rows; r++) {
                for (int c = 0; c < cols; c++){
                    if (isAliveInNextGen(r, c)) {
                        next[r][c] = true;
                    }
                }
            }
            grid = next;
        }
    }

    public static class BoardCanvas extends JPanel {
        final Board board;

        BoardCanvas(Board board) {
            this.board = board;
        }

        @Override
        public void paint(Graphics g) {
            super.paint(g);
            int windowWidth = getWidth();
            int windowHeight = getHeight();
            g.setColor(Color.WHITE);
            g.fillRect(0, 0, windowWidth, windowHeight);
            g.setColor(Color.BLACK);
            int cellWidth = windowWidth / board.cols; // 300 / 3 = 100
            int cellHeight = windowHeight / board.rows;
            for (int col = 1; col <= board.cols - 1; col++) {
                g.drawLine(col * cellWidth, 0, col * cellWidth, windowHeight);
            }
            for (int row = 1; row <= board.rows - 1; row++) {
                g.drawLine(0, row * cellHeight, windowWidth, row * cellHeight);
            }
            for (int row = 0; row < board.rows; row++) {
                for (int col = 0; col < board.cols; col++) {
                    if (board.grid[row][col]) {
                        g.fillRect(col * cellWidth, row * cellHeight, cellWidth, cellHeight);
                    }
                }
            }
        }
    }

}