Boid boid = new Boid(50, 50);
int timer = 0;
void setup(){
  size(400, 400);
  smooth();
  frameRate(24);
  boid.f = 0.1;
  background(250);
}
void draw(){
  boid.tick();
  if(mousePressed){
    boid.heading(mouseX, mouseY, 5.0);
    println(boid.xV + " " + boid.yV);
  }
  if(keyPressed){
    switch(key){
    case 'a':
	boid.turn(-0.1);
	break;
    case 'd':
	boid.turn(0.1);
	break;
    case 'w':
	boid.accelerate(0.5);
	break;
    }
  }
  boid.draw();
}
class Boid{
  float x, y, xV, yV, xA, yA, t, s, f;
  Boid(float x, float y){
    this.x = x;
    this.y = y;
    xV = 0.0f;		// x velocity
    yV = 0.0f;		// y velocity
    xA = 0.0f;		// x acceleration
    yA = 0.0f;		// y acceleration
    t = random(TWO_PI);   // theta - angle of rotation
    s = 0.0f;		 // speed
    f = 0.0f;		 // friction
  }
  void tick(){
    xV += xA;
    yV += yA;
    x += xV;
    y += yV;
    if(xV != 0.0f) xV += xV > 0.0f ? -f : f;
    if(yV != 0.0f) yV += yV > 0.0f ? -f : f;
  }
  void setV(float x, float y){
    xV = x;
    yV = y;
  }
  void turn(float r){
    t += r;
    if(s != 0.0){
	xV = cos(t) * s;
	yV = sin(t) * s;
    }
  }
  void accelerate(float a){
    xA = cos(t) * a;
    yA = sin(t) * a;
  }
  void heading(float x, float y, float speed){
    float xD = x - this.x;
    float yD = y - this.y;
    float vL = sqrt((xD * xD) + (yD * yD));
    s = speed;
    xV = (xD / vL) * s;
    yV = (yD / vL) * s;
    t = theta(x, y);
  }
  void draw(){
    ellipse(x, y, 20, 20);
    float xt = x + cos(t) * 30;
    float yt = y + sin(t) * 30;
    line(x, y, xt, yt);
  }
  float theta(float x, float y){
    return atan2(y - this.y, x - this.x);
  }
}