stemcell/app/code/Stemcell/Checkout/Model/Cart.php

<?php

namespace Stemcell\Checkout\Model;

use Magento\Catalog\Model\Product;
use Magento\Framework\App\ObjectManager;
use Magento\Framework\DataObject;
use Magento\Framework\Exception\LocalizedException;
use Psr\Log\LoggerInterface;

class Cart extends \Magento\Checkout\Model\Cart
{
    /**
     * Update cart items information
     *
     * @param  array $data
     * @return $this
     * @throws \Magento\Framework\Exception\LocalizedException
     * @SuppressWarnings(PHPMD.CyclomaticComplexity)
     * @SuppressWarnings(PHPMD.NPathComplexity)
     */
    public function updateItems($data)
    {
        $infoDataObject = new \Magento\Framework\DataObject($data);
        $this->_eventManager->dispatch(
            'checkout_cart_update_items_before',
            ['cart' => $this, 'info' => $infoDataObject]
        );

        $qtyRecalculatedFlag = false;
        foreach ($data as $itemId => $itemInfo) {
            $item = $this->getQuote()->getItemById($itemId);
            if (!$item) {
                continue;
            }

            if (!empty($itemInfo['remove']) || isset($itemInfo['qty']) && $itemInfo['qty'] == '0') {
                $this->removeItem($itemId);
                continue;
            }

            $qty = isset($itemInfo['qty']) ? (double)$itemInfo['qty'] : false;
            if ($qty > 0) {
                $item->clearMessage();
                $item->setHasError(false);
                $item->setQty($qty);

                if ($item->getHasError()) {
                    throw new \Magento\Framework\Exception\LocalizedException(__($item->getMessage()));
                }

                if (isset($itemInfo['before_suggest_qty']) && $itemInfo['before_suggest_qty'] != $qty) {
                    $qtyRecalculatedFlag = true;
                    $this->messageManager->addNoticeMessage(
                        __('Quantity was recalculated from %1 to %2', $itemInfo['before_suggest_qty'], $qty),
                        'quote_item' . $item->getId()
                    );
                }
            }

            if (isset($itemInfo['extension_attributes']) && is_array($itemInfo['extension_attributes'])) {
                foreach ($itemInfo['extension_attributes'] as $attribute => $val) {
                    if ($val!='' || $val!=null) {
                        $item->setData($attribute, $val);
                    }
                }
            }
            if (!empty($itemInfo['discount_percent'])) {
                $item->setDiscountAmount($item->getRowTotal()*$itemInfo['discount_percent']);
                $item->setDiscountPercent($itemInfo['discount_percent']);
            }

//            $item->setRowTotal($item->getRowTotal());
//            $item->setDiscountPercent($itemInfo['discount_percent']);
        }

        isset($data['salesperson_id']) && $this->getQuote()->setData('salesperson_id', $data['salesperson_id']);
        isset($data['customer_note']) && $this->getQuote()->setCustomerNote($data['customer_note']);

        if ($qtyRecalculatedFlag) {
            $this->messageManager->addNoticeMessage(
                __('We adjusted product quantities to fit the required increments.')
            );
        }

        $this->_eventManager->dispatch(
            'checkout_cart_update_items_after',
            ['cart' => $this, 'info' => $infoDataObject]
        );

        return $this;
    }

    /**
     * Get request quantity
     *
     * @param Product $product
     * @param \Magento\Framework\DataObject|int|array $request
     * @return int|DataObject
     * @throws LocalizedException
     */
    protected function getQtyRequest($product, $request = 0)
    {
        $request = $this->_getProductRequest($request);
        $stockItem = $this->stockRegistry->getStockItem($product->getId(), $product->getStore()->getWebsiteId());
        $minimumQty = $stockItem->getMinSaleQty();
        //If product quantity is not specified in request and there is set minimal qty for it
        if ($minimumQty
            && $minimumQty > 0
            && !$request->getQty()
        ) {
            $request->setQty($minimumQty);
        }

        return $request;
    }

    /**
     * Add product to shopping cart (quote)
     *
     * @param int|Product $productInfo
     * @param \Magento\Framework\DataObject|int|array $requestInfo
     * @return $this
     * @throws \Magento\Framework\Exception\LocalizedException
     * @SuppressWarnings(PHPMD.CyclomaticComplexity)
     */
    public function addProduct($productInfo, $requestInfo = null)
    {
        $product = $this->_getProduct($productInfo);
        $productId = $product->getId();

        if ($productId) {
            $request = $this->getQtyRequest($product, $requestInfo);
            try {
                $this->_eventManager->dispatch(
                    'checkout_cart_product_add_before',
                    ['info' => $requestInfo, 'product' => $product]
                );
                $result = $this->getQuote()->addProduct($product, $request);
            } catch (\Magento\Framework\Exception\LocalizedException $e) {
                $this->_checkoutSession->setUseNotice(false);
                $result = $e->getMessage();
            }
            /**
             * String we can get if prepare process has error
             */
            if (is_string($result)) {
                if ($product->hasOptionsValidationFail()) {
                    $redirectUrl = $product->getUrlModel()->getUrl(
                        $product,
                        ['_query' => ['startcustomization' => 1]]
                    );
                } else {
                    $redirectUrl = $product->getProductUrl();
                }
                $this->_checkoutSession->setRedirectUrl($redirectUrl);
                if ($this->_checkoutSession->getUseNotice() === null) {
                    $this->_checkoutSession->setUseNotice(true);
                }
                throw new \Magento\Framework\Exception\LocalizedException(__($result));
            }
        } else {
            throw new \Magento\Framework\Exception\LocalizedException(__('The product does not exist.'));
        }

        $this->_eventManager->dispatch(
            'checkout_cart_product_add_after',
            ['quote_item' => $result, 'product' => $product]
        );
        $this->_checkoutSession->setLastAddedProductId($productId);
        return $this;
    }

    public function truncate()
    {
        $this->getQuote()->removeAllItems();
        $this->getQuote()->setCustomerNote(null)->setData('salesperson_id', 0);
        return $this;
    }
}