#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;

}