/**
   * Updates the particle view. This should be called on each draw cycle in order to update the
   * positions of all nodes and edges in the viewer. If you need to update the positions of
   * particles without drawing it (e.g. to speed up movement, call updateParticles() instead.
   */
  public void draw() {
    parent.pushStyle();
    parent.pushMatrix();
    zoomer.transform();
    updateCentroid();
    centroid.tick();

    parent.translate(width / 2, height / 2);
    parent.scale(centroid.getZ());
    parent.translate(-centroid.getX(), -centroid.getY());

    if (!isPaused) {
      updateParticles();
    }

    // Ensure that any selected element is positioned at the mouse location.
    if (selectedNode != null) {
      Particle p = nodes.get(selectedNode);
      p.makeFixed();
      float mX = (zoomer.getMouseCoord().x - (width / 2)) / centroid.getZ() + centroid.getX();
      float mY = (zoomer.getMouseCoord().y - (height / 2)) / centroid.getZ() + centroid.getY();
      p.position().set(mX, mY, 0);
    }

    // Draw edges if we have positive stroke weight.
    if (parent.g.strokeWeight > 0) {
      parent.stroke(0, 180);
      parent.noFill();

      for (Map.Entry<E, Spring> row : edges.entrySet()) {
        E edge = row.getKey();
        Spring spring = row.getValue();
        Vector3D p1 = spring.getOneEnd().position();
        Vector3D p2 = spring.getTheOtherEnd().position();
        edge.draw(parent, p1.x(), p1.y(), p2.x(), p2.y());
      }
    }

    // Draw nodes.
    parent.noStroke();
    parent.fill(120, 50, 50, 180);

    for (Map.Entry<N, Particle> row : nodes.entrySet()) {
      N node = row.getKey();
      Vector3D p = row.getValue().position();
      node.draw(parent, p.x(), p.y());
    }

    parent.popMatrix();
    parent.popStyle();
  }
  /**
   * Tethers the given node to its location with the given strength.
   *
   * @param node The node to be tethered.
   * @param strength Strength of the tether.
   * @return True if the viewer contains the given node and it was tethered successfully.
   */
  public boolean tether(N node, float strength) {
    Particle p1 = nodes.get(node);
    if (p1 == null) {
      return false;
    }

    // Grab the tethering stake if it has already been created, otherwise create a new one.
    Particle stake = stakes.get(node);
    if (stake == null) {
      stake = physics.makeParticle(1, node.getLocation().x, node.getLocation().y, 0);
      stake.makeFixed();
      stakes.put(node, stake);
    }

    // Grab the tether if it has already been created, otherwise create a new one.
    Spring tether = tethers.get(stake);
    if (tether == null) {
      tether = physics.makeSpring(stake, p1, strength, DAMPING, Float.MIN_VALUE);
      tethers.put(stake, tether);
    } else {
      tether.setStrength(strength);
    }
    return true;
  }