var grids = [

  null,

  null,

];

var grid = null;

var grid_w = null;

var grid_h = null;

var b = 1;

var size = 3;

​

function setup() {

  createCanvas(400, 400);

  grid_w = Math.floor(width / GRID_SIZE);

  grid_h = Math.floor(height / GRID_SIZE);

  grids[0] = (new Array(grid_w * grid_h)).fill(0);

  grids[1] = (new Array(grid_w * grid_h)).fill(0);

  grids[0][(2 * grid_w) + 2] = 1;

  grid = grids[0];

  frameRate(60);

  noStroke();

  colorMode(HSB);

}

​

function draw() {

  background(32);

  drawGrid();

  updateGrid();

  grid = grids[grid == grids[0] ? 1 : 0];

}

​

function mouseDragged() {

  const my = Math.floor(mouseY / GRID_SIZE);

  const mx = Math.floor(mouseX / GRID_SIZE);

  for (let y = my - size; y < my + size; y++) {

    for (let x = mx - size; x < mx + size; x++) {

      grid[(y * grid_w) + x] = b;

    }

  }

  b += 1;

  if (b > 360) {

    b = 1;

  }

}

​

function updateGrid() {

  const newIdx = grid == grids[0] ? 1 : 0;

  for (i = 0; i < grid.length; i++) {

    const y = Math.floor(i / grid_w);

    if (grids[newIdx][i] === 0) {

      if (

        grid[i] !== 0

        && y !== grid_h - 1

        && (

          grid[i + grid_w] === 0

          || grid[i + grid_w - 1] === 0

          || grid[i + grid_w + 1] === 0

        )

      ) {

        if (grid[i + grid_w] === 0) {

          grids[newIdx][i] = 0;

          grids[newIdx][i + grid_w] = grid[i];         

        } else if (grid[i + grid_w - 1] === 0 && grid[i + grid_w + 1] === 0) {

          grids[newIdx][i] = 0;

          grids[newIdx][i + grid_w + random([-1, 1])] = grid[i];

        } else if (grid[i + grid_w - 1] === 0) {

          grids[newIdx][i] = 0;

          grids[newIdx][i + grid_w - 1] = grid[i];

        } else if (grid[i + grid_w + 1] === 0) {

          grids[newIdx][i] = 0;

          grids[newIdx][i + grid_w + 1] = grid[i];

        }

      } else {

        grids[newIdx][i] = grid[i];

      }

    }

    grid[i] = 0;

  }

}

​

function drawGrid() {

  for (i = 0; i < grid_w * grid_h; i++) {

    if (grid[i] !== 0) {

      const x = i % grid_w;

      const y = Math.floor(i / grid_w);

      fill(grid[i], 100, 100);

      rect(x * GRID_SIZE, y * GRID_SIZE, GRID_SIZE, GRID_SIZE);

    }

  }  

}