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


const double pi = 3.141592653589793;

GtkWidget *window;
cairo_surface_t *surface;
cairo_t *context;

gboolean dropping = FALSE;
double x = 0.0;
double y = 0.0;
double r = 0.0;
double speed = 0.0;
double mouse_x = 0.0;
double mouse_y = 0.0;


gboolean timeout(gpointer data) {
  if (dropping) {
    y += speed;
    r += 1.0;
    speed += 1.0;
    
    int iy = (int)y;
    int ix = (int)x;
    if ( ix < 0 || ix >= cairo_image_surface_get_width(surface)
      || iy < 0 || iy >= cairo_image_surface_get_height(surface) )
    {
      dropping = FALSE;
    } else {
      unsigned char *pixels = cairo_image_surface_get_data(surface);
      int stright = cairo_image_surface_get_stride(surface);
      unsigned char *pixel = pixels + iy*stright + ix*4;
      if (pixel[3] > 0) { // alpha channel
        dropping = FALSE;
        cairo_save(context);
        cairo_set_operator(context, CAIRO_OPERATOR_CLEAR);
        cairo_arc(context, x, y, r, 0, 2*pi);
        cairo_fill(context);
        cairo_restore(context);
        cairo_surface_flush(surface);
      }
    }
    gtk_widget_queue_draw(window);
  }
  return TRUE;
}

gboolean motion(GtkWidget *widget, GdkEventMotion *event, gpointer data) {
  if (event->state & GDK_BUTTON1_MASK) {
    cairo_save(context);
    cairo_set_line_cap(context, CAIRO_LINE_CAP_ROUND);
    cairo_set_line_width(context, 50);
    cairo_set_source_rgba(context, 0, 0, 0, 1);
    cairo_move_to(context, mouse_x, mouse_y);
    cairo_line_to(context, event->x, event->y);
    cairo_stroke(context);
    cairo_restore(context);
    cairo_surface_flush(surface);
    gtk_widget_queue_draw(window);
  }
  mouse_x = event->x;
  mouse_y = event->y;
  return TRUE;
}

gboolean button_press(GtkWidget *widget, GdkEventButton *event, gpointer data) {
  if (event->button == 2) {
    cairo_save(context);
    cairo_set_operator(context, CAIRO_OPERATOR_CLEAR);
    cairo_paint(context);
    cairo_restore(context);
    cairo_surface_flush(surface);
    gtk_widget_queue_draw(window);
  } else
  if (event->button == 3) {
    x = event->x;
    y = event->y;
    r = 0;
    speed = 0;
    dropping = TRUE;
    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_surface(cr, surface, 0, 0);
  cairo_paint(cr);
  if (dropping) {
    cairo_set_line_width(cr, 1);
    cairo_arc(cr, x, y, r, 0, 2*pi);
    cairo_set_source_rgba(cr, 1, 1, 1, 1);
    cairo_fill_preserve(cr);
    cairo_set_source_rgba(cr, 0, 0, 0, 1);
    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);
  g_signal_connect(window, "button-press-event", G_CALLBACK(button_press), NULL);
  gtk_widget_add_events(window, GDK_POINTER_MOTION_MASK | GDK_BUTTON_PRESS_MASK);
  gtk_window_set_default_size(
    GTK_WINDOW(window),
    cairo_image_surface_get_width(surface),
    cairo_image_surface_get_height(surface) );
  gtk_widget_show_all(window);

  g_timeout_add(10, timeout, NULL);
}

int main(int argc, char **argv) {
  surface = cairo_image_surface_create(CAIRO_FORMAT_ARGB32, 700, 500);
  context = cairo_create(surface);

  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);

  cairo_destroy(context);
  cairo_surface_destroy(surface);
  return status;
}