#include <stdlib.h></stdlib.h>

#include <stdio.h></stdio.h>

#include <string.h></string.h>

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

void changed(GtkEditable *editable, gpointer data) {

  const char *text = gtk_entry_get_text(GTK_ENTRY(editable));

  double result = 0.0;

  for(const char *c = text; c; ++c) {

    if (*c == '+' || *c == '-' || *c == '*' || *c == '/') {

      char *end;

      double a = strtof(text, &end);

      double b = strtof(c + 1, &end);

      if (*c == '+') result = a + b;

      if (*c == '-') result = a - b;

      if (*c == '*') result = a * b;

      if (*c == '/') result = a / b;

    } else

    if (*c == '=') {

      int size = c - text + 1;

      char *new_text = malloc(size + 20);

      memcpy(new_text, text, size);

      sprintf(new_text + size, "%g", result);

      gtk_entry_set_text(GTK_ENTRY(editable), new_text);

      free(new_text);

      break;

    }

  }

}

void activate(GtkApplication* app, gpointer data) {

  GtkWidget *window = gtk_application_window_new(app);

  gtk_container_set_border_width(GTK_CONTAINER(window), 10);

  GtkWidget *vbox = gtk_box_new(GTK_ORIENTATION_VERTICAL, 5);

  gtk_container_add(GTK_CONTAINER(window), vbox);

  GtkWidget *label = gtk_label_new("Enter simple expression like: 2+3=");

  gtk_container_add(GTK_CONTAINER(vbox), label);

  GtkWidget *entry = gtk_entry_new();

  g_signal_connect(entry, "changed", G_CALLBACK(changed), NULL);

  gtk_container_add(GTK_CONTAINER(vbox), entry);

  gtk_widget_show_all(window);

}

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;

}