Commerce Coupon redeem function

gistfile1.php

/**
 * Apply a coupon to an order and return success or failure.
 *
 * @param $code
 *  Coupon code to reedem.
 * @param $order
 *   The order on which the coupon should be redeemed.
 *
 * @return bool
 */
function commerce_coupon_redeem_coupon_code($code, $order) {
  // Trim trailing spaces.
  $code = trim($code);
  $coupon = commerce_coupon_load_by_code($code);
  $error = &drupal_static('commerce_coupon_error_' . $code);
  $error = '';
  
  if (!$coupon || !$coupon->status) {
    $error = t('Your coupon code is not valid.');
  }

  // The same coupon cannot be added twice.
  $order_wrapper = entity_metadata_wrapper('commerce_order', $order);
  foreach ($order_wrapper->commerce_coupons as $order_coupon_wrapper) {
    if ($order_coupon_wrapper->coupon_id->value() == $coupon->coupon_id) {
      $error = t('The coupon you have entered has already been applied to your order');
      break;
    }
  }

  // Check user permissions.
  if ($coupon) {
    if (!commerce_coupon_access('redeem', $coupon)) {
      $error = t('Unable to redeem this code.');
    }
  
    $coupon_wrapper = entity_metadata_wrapper('commerce_coupon', $coupon);
    
    // Evaluate any inline conditions found on this coupon  
    if (commerce_coupon_evaluate_inline_conditions($coupon_wrapper, 'pre_redeem')) {

      // Invoke the coupon redeem event.
      rules_invoke_event('commerce_coupon_redeem', $coupon, $order);      
    }
  }  
  
  // Return TRUE if the coupon was successfully added.
  if(commerce_coupon_order_has_coupon_code($code, $order)) {
    drupal_set_message(t('Coupon code applied.'));    
    return TRUE;
  }
  else if ($error) {
    drupal_set_message($error, 'error');
  }
}