<?php
/**
 * @file
 * Force anonymous users to login before being able to checkout.
 */

/**
 * Implements hook_commerce_checkout_router().
 */
function commerce_checkout_redirect_commerce_checkout_router($order, $checkout_page) {
  if (user_is_anonymous()) {
    $_SESSION['commerce_checkout_redirect_anonymous'] = TRUE;
    drupal_set_message(t('You need to be logged in to be able to checkout.'));
    return drupal_goto('user/login');
  }
}

/**
 * Implements hook_form_alter().
 */
function commerce_checkout_redirect_form_alter(&$form, &$form_state, $form_id) {
  if (in_array($form_id, array('user_login', 'user_register_form'))) {
    // Append the checkout redirect function on user's forms.
    $form['#submit'][] = 'commerce_checkout_redirect_redirect_anonymous_submit';
  }
}

/**
 * Submit callback for the user forms that will perform the redirection.
 */
function commerce_checkout_redirect_redirect_anonymous_submit($form, &$form_state) {
  // Because the user in the order may have been updated (from uid 0 to the real
  // uid for example), clear static cache before trying to get the order.
  drupal_static_reset('commerce_cart_order_id');
  $order_id = commerce_cart_order_id($GLOBALS['user']->uid);
  if (!empty($_SESSION['commerce_checkout_redirect_anonymous']) && user_is_logged_in() && $order_id) {
    unset($_SESSION['commerce_checkout_redirect_anonymous']);
    $form_state['redirect'] = 'checkout/' . $order_id;
  }
}