#include <math.h></math.h>

#include <stdlib.h></stdlib.h>

#include <time.h></time.h>

#include <gtk gtk.h=""></gtk>

const double pi = 3.141592653589793;

const guint interval_ms = 5;

const guint interval2_ms = 50;

const double G = 1000000;

const double radius = 30;

int width = 700;

int height = 500;

double mx = 0;

double my = 0;

double x = 200;

double y = 200;

double vx = 0;

double vy = 0;

GtkWidget *window;

gboolean motion(GtkWidget *widget, GdkEventMotion *event, gpointer data) {

  mx = event->x;

  my = event->y;

  return TRUE;

}

gboolean timeout(gpointer data) {

  double t = interval_ms/1000.0;

  double dx = mx - x;

  double dy = my - y;

  double d = sqrt(dx*dx + dy*dy);

  double ax = G*dx/(d*d*d);

  double ay = G*dy/(d*d*d);

  if (d < 10.0) ax = ay = 0.0;

  vx += ax*t;

  vy += ay*t;

  x += vx*t;

  y += vy*t;

  if (x < radius)

    { x = radius; vx = fabs(vx); }

  if (x > width - radius)

    { x = width - radius; vx = -fabs(vx); }

  if (y < radius)

    { y = radius; vy = fabs(vy); }

  if (y > height - radius)

    { y = height - radius; vy = -fabs(vy); }

  return TRUE;

}

gboolean timeout2(gpointer data) {

  gtk_widget_queue_draw(window);

  return TRUE;

}

gboolean draw(GtkWidget *widget, cairo_t *cr, gpointer data) {

  cairo_set_source_rgba(cr, 0.9, 0.9, 0.9, 1);

  cairo_paint(cr);

  cairo_set_source_rgba(cr, 0, 0, 0, 1);

  cairo_set_line_width(cr, 1.0);

  cairo_arc(cr, x, y, radius, 0, 2*pi);

  cairo_stroke(cr);

  return TRUE;

}

void activate(GtkApplication* app, gpointer data) {

  window = gtk_application_window_new (app);

  g_signal_connect(window, "draw", G_CALLBACK(draw), NULL);

  g_signal_connect(window, "motion-notify-event", G_CALLBACK(motion), NULL);

  gtk_widget_add_events(window, GDK_POINTER_MOTION_MASK);

  gtk_window_set_default_size(GTK_WINDOW(window), width, height);

  gtk_widget_show_all(window);

  g_timeout_add(interval_ms, timeout, NULL);

  g_timeout_add(interval2_ms, timeout2, NULL);

}

int main(int argc, char **argv) {

  GtkApplication *application = gtk_application_new(NULL, 0);

  g_signal_connect(application, "activate", G_CALLBACK(activate), NULL);

  int status = g_application_run(G_APPLICATION(application), argc, argv);

  g_object_unref(application);

  return status;

}