public static function checkCartItemsInventory($mes, $cart) {
        global $usces;

        // just quit if none of the cart items are combo-sets
        $found = false;
        foreach ($cart as $crow) {
            $sels = unserialize($crow['serial']);
            if (!empty($sels['comboSetId']) && !empty($sels['comboSetItems'])) {
                $found = true;
                break;
            }
        }

        if ($found === false) {
            return $mes;
        }

        $mes = '';
        $stocks = [];
        foreach ($cart as $i => $cart_row) {
            $rowmes = '';
            $sels = unserialize($cart_row['serial']);
            $cart_row = $cart[$i];
            $post_id = $cart_row['post_id'];
            $sku = $cart_row['sku'];
            $sku_code = urldecode($cart_row['sku']);

            $quant = isset($_POST['quant'][$i][$post_id][$sku]) ? (int)trim($_POST['quant'][$i][$post_id][$sku]) : $cart_row['quantity'];
            $stock = $usces->getItemZaikoNum($post_id, $sku_code);
            if (!isset($stocks[$post_id][$sku])) {
                if (!\WCUtils::is_blank($stock)) {
                    $stocks[$post_id][$sku] = $stock;
                } else {
                    $stocks[$post_id][$sku] = null;
                }
            }
            $checkstock = $stocks[$post_id][$sku];
            $stocks[$post_id][$sku] = $stocks[$post_id][$sku] - $quant;
            $product = wel_get_product($post_id);
            $itemRestriction = isset($product['itemRestriction']) ? $product['itemRestriction'] : '';
            $itemOrderAcceptable = $usces->getItemOrderAcceptable($post_id);
            $post_status = get_post_status($post_id);

            if (1 > (int)$quant) {
                $rowmes .= sprintf(__('Enter the correct amount for the No.%d item.', 'usces'), ($i + 1)) . '<br />';
            } elseif (!$usces->is_item_zaiko($post_id, $sku_code) || ($itemOrderAcceptable != 1 && \WCUtils::is_zero($stock)) || 'publish' != $post_status) {
                $rowmes .= sprintf(__('Sorry, No.%d item is sold out.', 'usces'), ($i + 1)) . '<br />';
            } elseif ($quant > (int)$itemRestriction && !\WCUtils::is_blank($itemRestriction) && !\WCUtils::is_zero($itemRestriction)) {
                $rowmes .= sprintf(__('This article is limited by %1$d at a time for the No.%2$d item.', 'usces'), $itemRestriction, ($i + 1)) . '<br />';
            } elseif ($itemOrderAcceptable != 1 && 0 > $stocks[$post_id][$sku] && !\WCUtils::is_blank($stock)) {
                $rowmes .= sprintf(__('Stock of No.%1$d item is remainder %2$d.', 'usces'), ($i + 1), $checkstock) . '<br />';
            }

            $mes .= $rowmes;
            if ((int)$quant > 0 && empty($rowmes)) {
                $gierrors = [];
                if (!empty($sels['comboSetId']) && !empty($sels['comboSetItems'])) {
                    $mapifiedselections = [];
                    foreach ($sels['comboSetItems'] as $giserial => $gitem) {
                        $data = unserialize($giserial);
                        $mapifiedselections[$data['groupId']][] = $data['itemId'];
                    }
                    $result = self::comboSetSelectionsAreValid($sels['comboSetId'], $mapifiedselections);
                    if ($result instanceof GenericError) {
                        $message = $result->message;
                        if ($result->errorcode === ErrorStore::COMBO_GROUP_IS_REQUIRED) {
                            $message = __('Selections for this combo-set item have been updated. Please refresh the page and try again.', 'wcexics');
                        }
                        $gierrors[] = $message;
                    } else {
                        $giindex = 0;
                        foreach ($sels['comboSetItems'] as $giserial => $gitem) {
                            $gitemquant = $sels['comboSetItems'][$giserial]['quant'] * $quant;

                            $gicrow = (new \usces_cart())->key_unserialize($giserial);
                            $gipost_id = $gicrow['post_id'];
                            $gisku = $gicrow['sku'];
                            $gisku_code = urldecode($gicrow['sku']);

                            $stock = $usces->getItemZaikoNum($gipost_id, $gisku_code);
                            if (!isset($stocks[$gipost_id][$gisku])) {
                                if (!\WCUtils::is_blank($stock)) {
                                    $stocks[$gipost_id][$gisku] = $stock;
                                } else {
                                    $stocks[$gipost_id][$gisku] = null;
                                }
                            }
                            $checkstock = $stocks[$gipost_id][$gisku];
                            $stocks[$gipost_id][$gisku] = $stocks[$gipost_id][$gisku] - $gitemquant;
                            $giproduct = wel_get_product($gipost_id);
                            $itemRestriction = isset($giproduct['itemRestriction']) ? $giproduct['itemRestriction'] : '';
                            $itemOrderAcceptable = $usces->getItemOrderAcceptable($gipost_id);
                            $post_status = get_post_status($gipost_id);

                            if (!$usces->is_item_zaiko($gipost_id, $gisku_code) || ($itemOrderAcceptable != 1 && \WCUtils::is_zero($stock)) || 'publish' != $post_status) {
                                $gierrors[] = sprintf(__('Sorry, No.%d item is sold out.', 'usces'), ($giindex + 1));
                            } elseif ($gitemquant > (int)$itemRestriction && !\WCUtils::is_blank($itemRestriction) && !\WCUtils::is_zero($itemRestriction)) {
                                $gierrors[] = sprintf(__('This article is limited by %1$d at a time for the No.%2$d item.', 'usces'), $itemRestriction, ($giindex + 1));
                            } elseif ($itemOrderAcceptable != 1 && 0 > $stocks[$gipost_id][$gisku] && !\WCUtils::is_blank($stock)) {
                                $gierrors[] = sprintf(__('Stock of No.%1$d item is remainder %2$d.', 'usces'), ($giindex + 1), $checkstock);
                            }

                            $giindex++;
                        }
                    }
                }

                $mes .= self::constructIndexedErrorMessage($i, $gierrors);
            }
        }

        return $mes;
    }