int n= 3000;
boolean alive= true;
fly[] a= new fly[n];

void setup(){
  size(250, 250);
  stroke(100);
  colorMode(HSB);
  for(int i= 0; i<n; i++){
    a[i]= new fly();
  }
}

void loop(){
  background(125, 80, 255);
  translate(width/2, height/2);

  for(int i= 0; i<n; i++){
    a[i].update();
  }
}

class fly{

  Vec2D f= new Vec2D(random(0, 170), 0);
  float l, xnext, ynext;

  fly(){
    f.rotate(random(-PI, PI));
    l= f.getlength();
  }

  void update(){
    xnext= random(-1, 1);
    ynext= random(-1, 1);

    if(width/2+f.x+xnext<width && width/2+f.x+xnext>0){
      f.x+= xnext;
    }
    else{
      f.x-= xnext;
    }
    if(height/2+f.y+xnext<height && height/2+f.y+ynext>0){
      f.y+= ynext;
    }
    else{
      f.y-= ynext;
    }


    stroke(150);
    point(f.x, f.y);
  }
}

// General vector class for 2D vectors
class Vec2D {
  float x,y;

  Vec2D(float _x,float _y) {
    x=_x;
    y=_y;
  }

  Vec2D(Vec2D v) {
    x=v.x;
    y=v.y;
  }

  void set(float _x,float _y) {
    x=_x;
    y=_y;
  }

  void set(Vec2D v) {
    x=v.x;
    y=v.y;
  }

  void add(float _x,float _y) {
    x+=_x;
    y+=_y;
  }

  void add(Vec2D v) {
    x+=v.x;
    y+=v.y;
  }

  void sub(float _x,float _y) {
    x-=_x;
    y-=_y;
  }

  void sub(Vec2D v) {
    x-=v.x;
    y-=v.y;
  }

  void mult(float m) {
    x*=m;
    y*=m;
  }

  void div(float m) {
    x/=m;
    y/=m;
  }

  float getlength() {
    return sqrt(x*x+y*y);
  }

  float angle() {
    return atan2(y,x);
  }

  void normalise() {
    float l=getlength();
    if(l!=0) {
      x/=l;
      y/=l;
    }
  }

  Vec2D tangent() {
    return new Vec2D(-y,x);
  }

  void rotate(float val) {
    // Due to float not being precise enough, double is used for the calculations
    double cosval=Math.cos(val);
    double sinval=Math.sin(val);
    double tmpx=x*cosval - y*sinval;
    double tmpy=x*sinval + y*cosval;

    x=(float)tmpx;
    y=(float)tmpy;
  }

}