// Nature of Code 2011
// Daniel Shiffman
// Chapter 3: Oscillation

// Class to describe an anchor point that can connect to "Bob" objects via a spring
// Thank you: http://www.myphysicslab.com/spring2d.html

class Spring { 

 // position
 PVector anchor;

 // Rest length and spring constant
 float len;
 float k = 0.2;

 // Constructor
 Spring(float x, float y, int l) {
 anchor = new PVector(x, y);
 len = l;
 } 

 // Calculate spring force
 void connect(Bob b) {
 // Vector pointing from anchor to bob position
 PVector force = PVector.sub(b.position, anchor);
 // What is distance
 float d = force.mag();
 // Stretch is difference between current distance and rest length
 float stretch = d - len;

 // Calculate force according to Hooke's Law
 // F = k * stretch
 force.normalize();
 force.mult(-1 * k * stretch);
 b.applyForce(force);
 }

 // Constrain the distance between bob and anchor between min and max
 void constrainLength(Bob b, float minlen, float maxlen) {
 PVector dir = PVector.sub(b.position, anchor);
 float d = dir.mag();
 // Is it too short?
 if (d < minlen) {
 dir.normalize();
 dir.mult(minlen);
 // Reset position and stop from moving (not realistic physics)
 b.position = PVector.add(anchor, dir);
 b.velocity.mult(0);
 // Is it too long?
 } 
 else if (d > maxlen) {
 dir.normalize();
 dir.mult(maxlen);
 // Reset position and stop from moving (not realistic physics)
 b.position = PVector.add(anchor, dir);
 b.velocity.mult(0);
 }
 }

 void display() { 
 stroke(0);
 fill(175);
 strokeWeight(2);
 rectMode(CENTER);
 rect(anchor.x, anchor.y, 10, 10);
 }

 void displayLine(Bob b) {
 strokeWeight(2);
 stroke(0);
 line(b.position.x, b.position.y, anchor.x, anchor.y);
 }
}