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

#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;

}