int linkCount;

boolean dragLink; //state of dragging a link
int dragOriginLet = 0; //dragging from inlet: -1. dragging from outlet: 1.
int dragOriginId = -1; //which nodes out- / inlet did user click on?

void Link(int parentNode, int targetNode) {
  /**
   * Check if user is adding or removing a link
   */
  int exists = -1;
  for (int i = 0; i < linkCount; i++) {
    if (links[i].parentNode == parentNode && links[i].targetNode == targetNode) {
      exists = i;
      break;
    }
  }
  if (exists != -1) {
    /**
     * !active state is pretty much the same as deleted state that the node-class comes with
     */
    links[exists].active = !links[exists].active;
  } else {
    links[linkCount] = new Link(parentNode, targetNode);
    linkCount++;
  }
}

class Link {
  int parentNode, targetNode;
  boolean active;
  PVector link = new PVector(0, 0);

  Link(int parentNode_, int targetNode_) {
    parentNode = parentNode_;
    targetNode = targetNode_;
    active = true;
  }

  /**
   * This could be an event that only gets called when nodes are moved or resized.
   */
  void update() {
    link.x = nodes[targetNode].inletCenter.x - nodes[parentNode].outletCenter.x;
    link.y = nodes[targetNode].inletCenter.y - nodes[parentNode].outletCenter.y;
  }

  void display() {
    stroke(darkMode? 255 : 0);
    strokeWeight(2);
    pushMatrix();
    translate(nodes[parentNode].outletCenter.x, nodes[parentNode].outletCenter.y);
    line(0, 0, link.x, link.y);
    translate(-nodes[parentNode].outletCenter.x+(nodes[parentNode].outletCenter.x+nodes[targetNode].inletCenter.x)/2, -nodes[parentNode].outletCenter.y+(nodes[parentNode].outletCenter.y+nodes[targetNode].inletCenter.y)/2);
    rotate(link.heading());
    fill(darkMode? 0 : 255);
    triangle(0, textSize/2, textSize, 0, 0, -textSize/2);
    popMatrix();
  }
}