#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;

}