float planeX, planeY;
float vektorx, vektory;
int mouseX2, mouseY2;
float radius;
int i;

void setup() {
  size(640, 400);

  planeX = 320;
  planeY = 200;

  radius = 50;

  i = 0;
}

void draw() {
  background(150, 150, 220);

  noFill();

  rectMode(CENTER);

  //Kreis um mouse und plane berechnen
  int kreisRadius = getDistance(mouseX, mouseY, planeX, planeY) / 2;
  int kreisMitteX = (mouseX + planeX) / 2;
  int kreisMitteY = (mouseY + planeY) / 2;

  ellipse(mouseX, mouseY, radius * 2, radius * 2);

  //Punkte z1 und z2 berechnen
  //http://stackoverflow.com/questions/3349125/circle-circle-intersection-points
  int d = getDistance(kreisMitteX, kreisMitteY, mouseX, mouseY);
  int a = (pow(radius, 2) - pow(kreisRadius, 2) + pow(d, 2)) / (2 * d);

  int p2x = mouseX + a * (kreisMitteX - mouseX) / d;
  int p2y = mouseY + a * (kreisMitteY - mouseY) / d;

  int h = sqrt(pow(radius, 2) - pow(a, 2));

  int z1x = p2x + h * (mouseY - kreisMitteY) / d;
  int z1y = p2y - h * (mouseX - kreisMitteX) / d;

  int z2x = p2x - h * (mouseY - kreisMitteY) / d;
  int z2y = p2y + h * (mouseX - kreisMitteX) / d;

  fill(255, 255, 255);

  ellipse(mouseX, mouseY, 20, 20);

  rect(planeX,  planeY,  20,  20);

  //Das Flugzeug bewegen
  if(getDistance(planeX, planeY, mouseX, mouseY) > radius + 10) {
    if(abs(mouseX - mouseX2) > 10 || abs(mouseY - mouseY2) > 10) {
      vektorx = planeX - z1x;
      vektory = planeY - z1y;

      float size = sqrt(pow(vektorx, 2) + pow(vektory, 2));

      if (size > 0) {
        vektorx = vektorx / size;
        vektory = vektory / size;
      }

      i = 0;

      mouseX2 = mouseX;
      mouseY2 = mouseY;
    }
    planeX -= vektorx;
    planeY -= vektory;
  }
  else {
    planeX = radius * cos(3.1415926f / 180.0f * i) + mouseX;
    planeY = radius * sin(3.1415926f / 180.0f * i) + mouseY;

    i++;
  }
}

int getDistance(int x1, int y1, int x2, int y2) {
  return sqrt(pow(x1 - x2, 2) + pow(y1 - y2, 2));
}