/**
 * collision_detection2.pde
 *
 * This program demonstrates collision detection in one dimension.
 * Two objects move toward each other and stop when they collide.
 * Collision is detected by comparing the distance between the objects' centers.
 * This only works for circular objects!
 *
 * created: 4-oct-2011/sklar
 *
 */

int     d = 20;  // diameter of objects
PVector v1, v2;  // velocities of objects
PVector p1, p2;  // positions of objects
boolean running; // flag indicating that simulation is running

/**
  * setup()
  */
void setup() {
  size( 800,600 );
  v1 = new PVector( 1, 0, 0 );
  p1 = new PVector( 0, (d+height)/2, 0 );
  v2 = new PVector( -1, 0, 0 );
  p2 = new PVector( width-d, (d+height)/2, 0 );
  ellipseMode( CORNER );
  running = true;
} // end of setup()

/**
  * draw()
  */
void draw() {
  if ( running ) {
    // clear screen
    background( #ffffff );
    // draw two objects
    stroke( #333333 );
    fill( #000066 );
    ellipse( (int)p1.x, (int)p1.y, d, d );
    fill( #006600 );
    ellipse( (int)p2.x, (int)p2.y, d, d );
    // update positions of objects
    p1.x += v1.x;
    p1.y += v1.y;
    p2.x += v2.x;
    p2.y += v2.y;
    // did we collide?
    if ( isCollision() ) {
      running = false;
      restart();
    }
  }
} // end of draw()

/**
  * keyPressed()
  */
void keyPressed() {
  if (( key == 'q' ) || ( key == 'Q' )) {
    exit();
  }
  else if (( key == 'r' ) || ( key == 'R' )) {
    restart();
  }
  else if (( key == 's' ) || ( key == 'S' )) {
    running = ! ( running );
  }
} // end of keyPressed()

/**
  * restart()
  */
void restart() {
  v1 = new PVector( 1, 0, 0 );
  p1 = new PVector( 0, (d+height)/2, 0 );
  v2 = new PVector( 1, 0, 0 );
  p2 = new PVector( width-d, (d+height)/2, 0 );
} // end of restart()

/**
 * isCollision()
 * this function returns true if the two objects have collided; false otherwise
 */
boolean isCollision() {
  float r = d/2;  // radius of objects
  PVector p1center = new PVector( p1.x + r, p1.y + r ); // center of first object
  PVector p2center = new PVector( p2.x + r, p2.y + r ); // center of second object
  float d = sqrt( sq(p1center.x - p2center.x) + sq(p1center.y - p2center.y )); // distance between the centers of the two objects
  if ( d < 2 * r ) {
    return( true );
  }
  else {
    return( false );
  }
} // end of isCollision()