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


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

double mouse_x = 0.0;
double mouse_y = 0.0;


void line(double x1, double y1, double x2, double y2) {
  cairo_set_source_rgba(context, 0, 0, 0, 1);
  cairo_set_line_width(context, 1.0);
  cairo_set_line_cap(context, CAIRO_LINE_CAP_ROUND);
  for(int col = -10; col <= 10; ++col) {
    for(int row = -10; row <= 10; ++row) {
      cairo_move_to(context, x1 + col*200.0, y1 + row*200.0);
      cairo_line_to(context, x2 + col*200.0, y2 + row*200.0);
    }
  }
  cairo_stroke(context);
}

gboolean motion(GtkWidget *widget, GdkEventMotion *event, gpointer data) {
  if (event->state & GDK_BUTTON1_MASK) {
    line(mouse_x, mouse_y, event->x, event->y);
    cairo_surface_flush(surface);
    gtk_widget_queue_draw(window);
  }

  if (event->state & GDK_BUTTON3_MASK) {
    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);
  }

  mouse_x = event->x;
  mouse_y = event->y;
  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);
  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),
    cairo_image_surface_get_width(surface),
    cairo_image_surface_get_height(surface) );
  gtk_widget_show_all(window);
}

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;
}