int fieldOver = 999;
int[] fieldsActive = new int[16];

void setup() {
  size(400, 400);
  background(0);

  frameRate(25);
}

void draw() {
  int counter = 0;
  mouseOver(mouseX, mouseY);
  for(int y=0; y<width; y+=100) {
    for(int x=0; x<height; x+=100) {
      if (fieldsActive[counter] == 1 || fieldOver == counter) {
        // Active
        noStroke();
        fill(random(0,100),random(0,100),random(0,100));
        rect(x,y,100,100);
        // Horizontal
        for(int i=0; i<(counter+2)*2; i++) {
          stroke(random(100,255),random(100,255),random(100,255));
          line(x+i*3,y+3*i,x+100,y+3*i);
        }
        // Vertikal
        for(int i=0; i<(counter)*2; i++) {
          stroke(random(100,255),random(100,255),random(100,255));
          line(x,y+3*i,x+3*i,y);
        }          
      } else {
        // Inactive
        noStroke();
        fill(0);
        rect(x,y,100,100);
        stroke(200);
        for(int i=0; i<(counter+1)*2; i++) {
          line(x,y+3*i,x+100,y+3*i);
        }
      }
      counter++;
    }
  }
}

void mouseOver(int x, int y) {
  int col = (x/100) + 1;
  int row = (y/100) + 1;
  fieldOver = (row*4)-(4-col) - 1;
}

void mouseClicked() {
  if (mouseButton == LEFT) {
    int x = mouseX;
    int y = mouseY;
    int col = (x/100) + 1;
    int row = (y/100) + 1;
    int field = (row*4)-(4-col) - 1;
    if (fieldsActive[field] == 1) {
      fieldsActive[field] = 0;
    } else {
      fieldsActive[field] = 1;
    }
  }
}