class SparksLayer extends Layer {
  float gravity = 5.0f;
  float drag = 0.5f;
  float sparkMass = 10.0f;
  int sparkColor = color(0xff, 0xff, 0xff);
  float maxV = 8;
  float lifetimeMin = 5;
  float lifetimeMax = 25;
  ParticleSystem system;
  ArrayList sparks = new ArrayList();
  int maxPositions = 20;
  int nTicks = 2;

  SparksLayer(PApplet parent) {
    super(parent);
  }

  void setup() {
    system = new ParticleSystem(gravity, drag);
    background(0);
  }

  void draw() {
    background(0, 0);
    smooth();
    if (mousePressed) {
      float nSparks = random(1, 20);
      for (int i = 0; i < nSparks; i++) {
        makeSpark();
      }
    }

    advanceTime(nTicks);

    ListIterator it = sparks.listIterator();
    setVisible(false);
    while (it.hasNext()) {
      Spark spark = (Spark) it.next();
      if (spark.lifetime < spark.particle.age()) {
        system.removeParticle(spark.particle);
        it.remove();
      } 
      else {
        setVisible(true);
        drawSpark(spark);
      }
    }
  }

  void advanceTime(int n) {
    for (int i = 0; i < n; i++) {
      system.tick();
      ListIterator it = sparks.listIterator();
      while (it.hasNext()) {
        Spark spark = (Spark) it.next();
        ArrayList positions = spark.pastPositions;
        Vector3D position = spark.particle.position();
        positions.add(new Vector3D(position.x(), position.y(), position.z()));
      }
    }
    ListIterator it = sparks.listIterator();
    while (it.hasNext()) {
      Spark spark = (Spark) it.next();
      int nRemove = spark.pastPositions.size() - maxPositions;
      ListIterator positions = spark.pastPositions.listIterator();
      for (int i = 0; i < nRemove; i++) {
        positions.next();
        positions.remove();
      }
    }
  }

  void drawSpark(Spark spark) {
    stroke(sparkColor);
    int index = spark.pastPositions.size() - 1;
    Vector3D p1 = (Vector3D) spark.pastPositions.get(index--);
    int indexLimit = index - spark.tailLength;
    if (indexLimit < 0) { 
      indexLimit = 0;
    }
    int loopCount = 0;
    while (index > indexLimit) {
      Vector3D p2 = (Vector3D) spark.pastPositions.get(index);
      if (loopCount < 2) {
        strokeWeight(2);
      } 
      else {
        strokeWeight(1);
      }
      line(p1.x(), p1.y(), p2.x(), p2.y());
      p1 = p2;
      loopCount++;
      index--;
    }
  }

  void makeSpark() {
    Particle p = system.makeParticle(sparkMass, mouseX, mouseY, 0);
    float xv = random(-maxV, maxV);
    float yv = random(-maxV, maxV);
    p.velocity().set(xv, yv, 0);
    sparks.add(new Spark(p));
  }

  class Spark {
    Particle particle;

    float lifetime;
    int tailLength;

    ArrayList pastPositions = new ArrayList();

    Spark(Particle p) {
      this.particle = p;
      this.lifetime = random(lifetimeMin, lifetimeMax);
      this.tailLength = (int)random(3, 6);
    }
  }
}