private ArrayList<Ball> balls = new ArrayList<Ball>();
private boolean clear = false;

void setup() {
  size(1280, 900);
  background(0);

  ellipseMode(CENTER);

  int[] init = {100, 100};
  noStroke();


}

void draw() {
  // clears the sceren before starting
  if (clear) {
    background(0);
  }

  for (int i = 0; i < balls.size(); i++) {
    int radius = balls.get(i).getRadius();
    int x = balls.get(i).getPosition()[0];
    int y = balls.get(i).getPosition()[1];

    float vel_x = balls.get(i).getVelocity()[0];
    float vel_y = balls.get(i).getVelocity()[1];

    int[] ball_color = balls.get(i).getColor();

    fill(ball_color[0], ball_color[1], ball_color[2]);

    ellipse(x, y, radius * 2, radius * 2);

    balls.get(i).updatePosition();
  }

  balls.add(new Ball((int) random(500, 750), (int) random(300, 500)));

 ellipse(width / 2, height / 2, 50, 50);

}

void mouseClicked() {
  balls.add(new Ball(mouseX, mouseY));
}

void keyPressed() {
   if (key == CODED) {
     if (keyCode == UP) {
       if (clear) {
         clear = false;
       } else {
         clear = true;
       }
     }
   }
}

public static int shitcolor = 0;

class Ball {

  private int radius; //random later
  private int[] colour = new int[3]; // random later

  // where the ball is
  private int x;
  private int y;

  // where the ball is moving
  private float velocity_x;
  private float velocity_y;

  // acceleration
  private float acceleration_x = 0;
  private float acceleration_y = 0;

  // relative to gravity
  private int delta_x;
  private int delta_y;
  private float delta_d;
  private float acceleration_angle;

  // how much it bounces
  private float elasticity; // random later

  // how much it is affected by acceleration
  private float friction;


  private int add = 1;

  Ball(int initial_x, int initial_y) {
    // setup position
    this.x = initial_x;
    this.y = initial_y;

    // setup random velocity
    //this.velocity_x = random(1, 10);
    //this.velocity_y = random(1, 10);

    this.velocity_x = 5;
    this.velocity_y = 0;

    // properties
    this.radius = 2;
    this.colour[0] = shitcolor;
    this.colour[1] = shitcolor;
    this.colour[2] = shitcolor;

    //this.radius = (int) random(2, 20);
    //this.colour[0] = (int) random(25, 255);
    //this.colour[1] = (int) random(25, 255);
    //this.colour[2] = (int) random(25, 255);

    this.elasticity = random(0.2, 0.95);
    this.friction = random(0.05, 0.4);



    if (shitcolor < 255) {
      shitcolor++;
    }
  }

  // METHODS
  void updatePosition() {
    // moves the object with velocity
    this.x += this.velocity_x;
    this.y += this.velocity_y;

    this.velocity_x += this.acceleration_x;
    this.velocity_y += this.acceleration_y;

    /*// check for boundaries
    // right
    if ((this.x + radius) >= width) {
      velocity_x *= -1;
      this.x = width - radius;
    }
    // left
    if ((this.x - radius) <= 0) {
      velocity_x *= -1;
      this.x = radius;
    }
    // bottom
    if ((this.y + radius) >= height) {
      velocity_y *= -1 * this.elasticity;
      this.y = height - radius;
    }
    // top
    if ((this.y - radius) <= 0) {
      velocity_y *= -1;
      this.y = radius;
    }*/

    // gravity
    // find where acceleration should be
    this.delta_x = (width / 2) - this.x;
    this.delta_y = (height / 2) - this.y;
    this.delta_d = sqrt(sq(this.delta_x) + sq(this.delta_y));

    /*if (this.delta_x == 0 && this.delta_y > 0) { //above
      this.acceleration_angle = 90;
    } else if (this.delta_x == 0 && this.delta_y < 0) { //below
      this.acceleration_angle = 270;
    } else if (this.delta_y == 0 && this.delta_x > 0) { //left
      this.acceleration_angle = 180;
    } else if (this.delta_y == 0 && this.delta_x < 0) { //right
      this.acceleration_angle = 0;
    } else {
      this.acceleration_angle = atan(this.delta_x / this.delta_y);
    }*/

    /*/ execute acceleration
    if (0 < this.acceleration_angle && this.acceleration_angle < 90) {
      this.acceleration_x = -1 * sin(this.acceleration_angle) * 0.2;
      this.acceleration_y = -1 * cos(this.acceleration_angle) * 0.2;
    } else if (90 < this.acceleration_angle && this.acceleration_angle < 180) {
      this.acceleration_x = sin(this.acceleration_angle) * 0.2;
      this.acceleration_y = -1 * cos(this.acceleration_angle) * 0.2;
    } else if (180 < this.acceleration_angle && this.acceleration_angle < 270) {
      this.acceleration_x = sin(this.acceleration_angle) * 0.2;
      this.acceleration_y = cos(this.acceleration_angle) * 0.2;
    } else if (270 < this.acceleration_angle) {
      this.acceleration_x = -1 * sin(this.acceleration_angle) * 0.2;
      this.acceleration_y = cos(this.acceleration_angle) * 0.2;
    }*/
    this.acceleration_x = this.delta_x * 0.01;
    this.acceleration_y = this.delta_y * 0.01;

  }

  // SETTERS
  void setPosition(int x, int y) {
    this.x = x;
    this.y = y;
  }

  void setVelocity(int[] velocity) {
    this.velocity_x = velocity[0];
    this.velocity_y = velocity[1];
  }

  // GETTERS
  int getRadius() {
    return this.radius;
  }

  int[] getColor() {
    return this.colour;
  }

  int[] getPosition() {
    int[] return_pos = {x, y};
    return return_pos;
  }

  float[] getVelocity() {
    float[] return_vel = {this.velocity_x, this.velocity_y};
    return return_vel;
  }
}