let velocityGrid;

let w = 10;

let cols, rows;

let hueValue = 255;

let cnt = 0;

let gravity = 0.1;

let panna;

​

function make2DArray(cols, rows) {

  let arr = new Array(cols);

  for (let i = 0; i < arr.length; i++) {

    arr[i] = new Array(rows);

    // Fill the array with 0s

    for (let j = 0; j < arr[i].length; j++) {

      arr[i][j] = 0;

    }

  }

  return arr;

}

// Check if a row is within the bounds

function withinCols(i) {

  return i >= 0 && i <= cols - 1;

}

​

// Check if a column is within the bounds

function withinRows(j) {

  return j >= 0 && j <= rows - 1;

}

​

function setup() {

  createCanvas(500, 500);

  colorMode(HSB, 360, 255, 255);

  cols = (width/2-50)/w;

  rows = (height/2-50)/w;

  grid = make2DArray(cols, rows);

  velocityGrid = make2DArray(cols, rows, 1);

  panna = new Panna(width/2-100,height/2-100,200);

}

​

function draw() {

  background(0);

    let mcol = floor((width/2-150)/w);

    let mrow = floor((height/2-250)/w);

​

    // Randomly add an area of sand particles

    let matrix = 5;

    let extent = floor(matrix / 2);

    for (let i = -extent; i <= extent; i++) {

      for (let j = -extent; j <= extent; j++) {

        if (random(1) < 0.75) {

          let col = mcol + i;

          let roww = mrow + j;

​

          if (withinCols(col) && withinRows(roww)) {

            grid[col][roww] = hueValue;

            velocityGrid[col][roww] = 1;

          }

        }

      }

    }

​

  // Draw the sand

  for (let i = 0; i < cols; i++) {

    for (let j = 0; j < rows; j++) {

      noStroke();

      if (grid[i][j] > 0) {

        fill(grid[i][j]);

        square(width/2-(cols*w)/2+i*w,height/2-(rows*w)/2+j*w,w);

      }

    }

  }

​

  // Create a 2D array for the next frame of animation

  let nextGrid = make2DArray(cols, rows);

  let nextVelocityGrid = make2DArray(cols, rows);

​

  // Check every cell

  for (let i = 0; i < cols; i++) {

    for (let j = 0; j < rows; j++) {

      // What is the state?

      let state = grid[i][j];

      let velocity = velocityGrid[i][j];

      let moved = false;

      if (state > 0) {

        let newPos = int(j + velocity);

        for (let y = newPos; y > j; y--) {

          let below = grid[i][y];

          let dir = 1;

          if (random(1) < 0.5) {

            dir *= -1;

          }

          let belowA = -1;

          let belowB = -1;

          if (withinCols(i + dir)) belowA = grid[i + dir][y];

          if (withinCols(i - dir)) belowB = grid[i - dir][y];

​

          if (below === 0) {

            nextGrid[i][y] = state;

            nextVelocityGrid[i][y] = velocity + gravity;

            moved = true;

            break;

          } else if (belowA === 0) {

            nextGrid[i + dir][y] = state;

            nextVelocityGrid[i + dir][y] = velocity + gravity;

            moved = true;

            break;

          } else if (belowB === 0) {

            nextGrid[i - dir][y] = state;

            nextVelocityGrid[i - dir][y] = velocity + gravity;

            moved = true;

            break;

          }

        }

      }

​

      if (state > 0 && !moved) {

        nextGrid[i][j] = grid[i][j];

        nextVelocityGrid[i][j] = velocityGrid[i][j] + gravity;

      }

    }

  }

  grid = nextGrid;

  velocityGrid = nextVelocityGrid;

  panna.show();

  cnt++;

  if(cnt>=200){

    fill('black');

    stroke('black');

    square(width/2-20,height/2-20,10);

    square(width/2+20,height/2-20,10);

    square(width/2,height/2,10);

  }

}

​