/*
 * @name Forces
 * @arialabel 9 grey balls drop from the top of the window and slow down as they reach the bottom half of the screen which is dark grey in color. Their change in speed mimics objects slowing down due to water resistance
 * @description Demonstration of multiple force acting on bodies
 * (<a href="http://natureofcode.com">natureofcode.com</a>)
 */
// Demonstration of multiple force acting on
// bodies (Mover class)
// Bodies experience gravity continuously
// Bodies experience fluid resistance when in "water"

// Nine moving bodies
let movers = [];

// Liquid
let liquid;

function setup() {
  // createCanvas(640, 360);
  createCanvas(windowWidth, windowHeight);
  reset();
  // Create liquid object
  liquid = new Liquid(0, height / 2, width, height / 2, 0.1);
}

function draw() {
  background(127);

  // Draw water
  liquid.display();

  for (let i = 0; i < movers.length; i++) {
    // Is the Mover in the liquid?
    if (liquid.contains(movers[i])) {
      // Calculate drag force
      let dragForce = liquid.calculateDrag(movers[i]);
      // Apply drag force to Mover
      movers[i].applyForce(dragForce);
    }

    // Gravity is scaled by mass here!
    let gravity = createVector(0, 0.1 * movers[i].mass);
    // Apply gravity
    movers[i].applyForce(gravity);

    // Update and display
    movers[i].update();
    movers[i].display();
    movers[i].checkEdges();
  }
}

function mousePressed() {
  reset();
}

// Restart all the Mover objects randomly
function reset() {
  for (let i = 0; i < 9; i++) {
    movers[i] = new Mover(random(0.5, 3), 40 + i * 70, 0);
  }
}

let Liquid = function(x, y, w, h, c) {
  this.x = x;
  this.y = y;
  this.w = w;
  this.h = h;
  this.c = c;
};

// Is the Mover in the Liquid?
Liquid.prototype.contains = function(m) {
  let l = m.position;
  return (
    l.x > this.x &&
    l.x < this.x + this.w &&
    l.y > this.y &&
    l.y < this.y + this.h
  );
};

// Calculate drag force
Liquid.prototype.calculateDrag = function(m) {
  // Magnitude is coefficient * speed squared
  let speed = m.velocity.mag();
  let dragMagnitude = this.c * speed * speed;

  // Direction is inverse of velocity
  let dragForce = m.velocity.copy();
  dragForce.mult(-1);

  // Scale according to magnitude
  // dragForce.setMag(dragMagnitude);
  dragForce.normalize();
  dragForce.mult(dragMagnitude);
  return dragForce;
};

Liquid.prototype.display = function() {
  noStroke();
  fill(50);
  rect(this.x, this.y, this.w, this.h);
};

function Mover(m, x, y) {
  this.mass = m;
  this.position = createVector(x, y);
  this.velocity = createVector(0, 0);
  this.acceleration = createVector(0, 0);
}

// Newton's 2nd law: F = M * A
// or A = F / M
Mover.prototype.applyForce = function(force) {
  let f = p5.Vector.div(force, this.mass);
  this.acceleration.add(f);
};

Mover.prototype.update = function() {
  // Velocity changes according to acceleration
  this.velocity.add(this.acceleration);
  // position changes by velocity
  this.position.add(this.velocity);
  // We must clear acceleration each frame
  this.acceleration.mult(0);
};

Mover.prototype.display = function() {
  stroke(0);
  strokeWeight(2);
  fill(255, 127);
  ellipse(this.position.x, this.position.y, this.mass * 16, this.mass * 16);
};

// Bounce off bottom of window
Mover.prototype.checkEdges = function() {
  if (this.position.y > height - this.mass * 8) {
    // A little dampening when hitting the bottom
    this.velocity.y *= -0.9;
    this.position.y = height - this.mass * 8;
  }
};