int ALPH = 0x20000000;

int NUM_PARTICLES  = 20;
int NUM_ATTRACTORS = 8;

float damp = 0.9;
float accel = 10;

color[] colours = {  ALPH | 0x00A3D39C, 
                     ALPH | 0x0086AD80, 
                     ALPH | 0x00698764, 
                     ALPH | 0x00C9D49D, 
                     ALPH | 0x00A4AD80, 
                     ALPH | 0x00808764, 
                     ALPH | 0x002B2B2B, 
                     ALPH | 0x00D4D4D4, 
                     ALPH | 0x00000000 };

Particle[] particle;
Attractor[] attractor;

class Attractor {
  float x,y;
  Attractor() {
    x = random(width);
    y = random(height);
  }
}

class Particle {
  float x,y,vx,vy,px,py;
  Particle() {
    x = px = random(width);
    y = py = random(height);
//    vx = random(1) > 0.5 ? random(-15,-12) : random(12,15);
//    vy = random(1) > 0.5 ? random(-15,-12) : random(12,15);
    vx = random(-accel/2,accel/2);
    vy = random(-accel/2,accel/2);
  }
  void step() {
    px = x;
    py = y;
    for (int i = 0; i < attractor.length; i++) {
      float d2 = sq(attractor[i].x-x) + sq(attractor[i].y-y);
      if (d2 > 0.1) {
        vx += accel * (attractor[i].x-x) / d2;
        vy += accel * (attractor[i].y-y) / d2;
      }
    }
    x += vx;
    y += vy;
    vx *= damp;
    vy *= damp;
  }
}

void setup() {
  size(400,400);
  background(255);
  stroke(255);
  smooth();
  noFill();
  attractor = new Attractor[NUM_ATTRACTORS];
  for (int i = 0; i < attractor.length; i++) {
    attractor[i] = new Attractor();
  }
  particle = new Particle[NUM_PARTICLES];
  for (int i = 0; i < particle.length; i++) {
    particle[i] = new Particle();
  }
}

void loop() {
  for (int j = 0; j < particle.length; j++) {
    stroke(colours[j%colours.length]);
    line(particle[j].x,particle[j].y,particle[j].px,particle[j].py);
    particle[j].step();
  }
}

void mousePressed() {
  background(255);  
  for (int i = 0; i < attractor.length; i++) {
    attractor[i] = new Attractor();
  }
}