#include <gtk/gtk.h>

#define MAX_LEN 50


GtkWidget *window;
int pos = 0;
int len = 0;
double px[MAX_LEN];
double py[MAX_LEN];


gboolean timeout(gpointer data) {
  --len;
  if (len < 0) len = 0;
  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, 1.0);
  cairo_paint(cr);

  cairo_set_source_rgba(cr, 0.0, 0.0, 0.0, 1.0);
  cairo_set_line_cap(cr, CAIRO_LINE_CAP_ROUND);
  cairo_move_to(cr, px[pos], py[pos]);
  for(int i = 1; i < len; ++i) {
    int p = (MAX_LEN + pos - i) % MAX_LEN;
    cairo_set_line_width(cr, len - i);
    cairo_line_to(cr, px[p], py[p]);
    cairo_stroke(cr);
    cairo_move_to(cr, px[p], py[p]);
  }

  return TRUE;
}

gboolean motion(GtkWidget *widget, GdkEventMotion *event, gpointer data) {
  ++pos;
  ++len;
  if (pos >= MAX_LEN) pos = 0;
  if (len > MAX_LEN) len = MAX_LEN;
  px[pos] = event->x;
  py[pos] = event->y;
  gtk_widget_queue_draw(window);
  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), 700, 500);
  gtk_widget_show_all(window);
  g_timeout_add(100, timeout, 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;
}