// The Nature of Code
// Daniel Shiffman
// http://natureofcode.com

// Pendulum

// A Simple Pendulum Class
// Includes functionality for user can click and drag the pendulum

class Pendulum {

 PVector position; // position of pendulum ball
 PVector origin; // position of arm origin
 float r; // Length of arm
 float angle; // Pendulum arm angle
 float aVelocity; // Angle velocity
 float aAcceleration; // Angle acceleration

 float ballr; // Ball radius
 float damping; // Arbitary damping amount

 boolean dragging = false;

 // This constructor could be improved to allow a greater variety of pendulums
 Pendulum(PVector origin_, float r_) {
 // Fill all variables
 origin = origin_.get();
 position = new PVector();
 r = r_;
 angle = PI/4;

 aVelocity = 0.0;
 aAcceleration = 0.0;
 damping = 0.995; // Arbitrary damping
 ballr = 48.0; // Arbitrary ball radius
 }

 void go() {
 update();
 drag(); //for user interaction
 display();
 }

 // Function to update position
 void update() {
 // As long as we aren't dragging the pendulum, let it swing!
 if (!dragging) {
 float gravity = 0.4; // Arbitrary constant
 aAcceleration = (-1 * gravity / r) * sin(angle); // Calculate acceleration (see: http://www.myphysicslab.com/pendulum1.html)
 aVelocity += aAcceleration; // Increment velocity
 aVelocity *= damping; // Arbitrary damping
 angle += aVelocity; // Increment angle
 }
 }

 void display() {
 position.set(r*sin(angle), r*cos(angle), 0); // Polar to cartesian conversion
 position.add(origin); // Make sure the position is relative to the pendulum's origin

 stroke(0);
 strokeWeight(2);
 // Draw the arm
 line(origin.x, origin.y, position.x, position.y);
 ellipseMode(CENTER);
 fill(175);
 if (dragging) fill(0);
 // Draw the ball
 ellipse(position.x, position.y, ballr, ballr);
 }


 // The methods below are for mouse interaction

 // This checks to see if we clicked on the pendulum ball
 void clicked(int mx, int my) {
 float d = dist(mx, my, position.x, position.y);
 if (d < ballr) {
 dragging = true;
 }
 }

 // This tells us we are not longer clicking on the ball
 void stopDragging() {
 aVelocity = 0; // No velocity once you let go
 dragging = false;
 }

 void drag() {
 // If we are draging the ball, we calculate the angle between the 
 // pendulum origin and mouse position
 // we assign that angle to the pendulum
 if (dragging) {
 PVector diff = PVector.sub(origin, new PVector(mouseX, mouseY)); // Difference between 2 points
 angle = atan2(-1*diff.y, diff.x) - radians(90); // Angle relative to vertical axis
 }
 }
}