Magento2: Updating a line of code in core file

To customize Magento core file, you can use Preference or Plugin. More detail here

In your case, you can use preference as the following steps:

1. In the di.xml you can add the reference config:

<preference for="MagentoSalesRuleModelRuleActionDiscountCartFixed" type="TrainingTestModelRuleActionDiscountCartFixed" />


2. Create TrainingTestModelRuleActionDiscountCartFixed.php:

   • @author Bach Lee
     */

   namespace TrainingTestModelRuleActionDiscount;

   use MagentoFrameworkAppObjectManager;
   use MagentoFrameworkPricingPriceCurrencyInterface;
   use MagentoSalesRuleModelDeltaPriceRound;
   use MagentoSalesRuleModelValidator;
   use MagentoSalesRuleModelRuleActionDiscountFactory;

   class CartFixed extends MagentoSalesRuleModelRuleActionDiscountCartFixed
   {
   /**
   * @var string
   */
   private static $discountType = ‘CartFixed’;
   /**
   * @var DeltaPriceRound
   */
   private $deltaPriceRound;

   /**
    * @param Validator $validator
    * @param DataFactory $discountDataFactory
    * @param PriceCurrencyInterface $priceCurrency
    * @param DeltaPriceRound $deltaPriceRound
    */
   public function __construct(
       Validator $validator,
       DataFactory $discountDataFactory,
       PriceCurrencyInterface $priceCurrency,
       DeltaPriceRound $deltaPriceRound = null
   ) {
       $this->deltaPriceRound = $deltaPriceRound ?: ObjectManager::getInstance()->get(DeltaPriceRound::class);
   
       parent::__construct($validator, $discountDataFactory, $priceCurrency, $deltaPriceRound);
   }
   
   /**
    * @param MagentoSalesRuleModelRule $rule
    * @param MagentoQuoteModelQuoteItemAbstractItem $item
    * @param float $qty
    * @return MagentoSalesRuleModelRuleActionDiscountData
    * @throws MagentoFrameworkExceptionLocalizedException
    */
   public function calculate($rule, $item, $qty)
   {
       /** @var MagentoSalesRuleModelRuleActionDiscountData $discountData */
       $discountData = $this->discountFactory->create();
   
       $ruleTotals = $this->validator->getRuleItemTotalsInfo($rule->getId());
   
       $quote = $item->getQuote();
       $address = $item->getAddress();
   
       $itemPrice = $this->validator->getItemPrice($item);
       $baseItemPrice = $this->validator->getItemBasePrice($item);
       $itemOriginalPrice = $this->validator->getItemOriginalPrice($item);
       $baseItemOriginalPrice = $this->validator->getItemBaseOriginalPrice($item);
   
       /**
        * prevent applying whole cart discount for every shipping order, but only for first order
        */
       if ($quote->getIsMultiShipping()) {
           $usedForAddressId = $this->getCartFixedRuleUsedForAddress($rule->getId());
           if ($usedForAddressId && $usedForAddressId != $address->getId()) {
               return $discountData;
           } else {
               $this->setCartFixedRuleUsedForAddress($rule->getId(), $address->getId());
           }
       }
       $cartRules = $address->getCartFixedRules();
       if (!isset($cartRules[$rule->getId()])) {
           $cartRules[$rule->getId()] = $rule->getDiscountAmount();
       }
   
       $availableDiscountAmount = (float)$cartRules[$rule->getId()];
       $discountType = self::$discountType . $rule->getId();
   
       $store = $quote->getStore();
       if ($ruleTotals['items_count'] <= 1) {
           $quoteAmount = $this->priceCurrency->convert($availableDiscountAmount, $store);
           $baseDiscountAmount = min($baseItemPrice * $qty, $availableDiscountAmount);
           $this->deltaPriceRound->reset($discountType);
       } else {
           $ratio = $baseItemPrice * $qty / $ruleTotals['base_items_price'];
           $maximumItemDiscount = $this->deltaPriceRound->round(
               $rule->getDiscountAmount() * $ratio,
               $discountType
           );
   
           $quoteAmount = $this->priceCurrency->convert($maximumItemDiscount, $store);
   
           $baseDiscountAmount = min($baseItemPrice * $qty, $maximumItemDiscount);
           $this->validator->decrementRuleItemTotalsCount($rule->getId());
       }
   
       $baseDiscountAmount = $this->priceCurrency->round($baseDiscountAmount);
   
       $availableDiscountAmount -= $baseDiscountAmount;
       $cartRules[$rule->getId()] = $availableDiscountAmount;
       if ($availableDiscountAmount <= 0) {
           $this->deltaPriceRound->reset($discountType);
       }
   
       $discountData->setAmount($this->priceCurrency->round(min($itemPrice * $qty, $quoteAmount)));
       $discountData->setBaseAmount($baseDiscountAmount);
       $discountData->setOriginalAmount(min($itemOriginalPrice * $qty, $quoteAmount));
       $discountData->setBaseOriginalAmount($this->priceCurrency->round($baseItemOriginalPrice));
       $address->setCartFixedRules($cartRules);
   
       return $discountData;
   }
   

   }

Or you can use cweagans/composer-patches following this question to edit core Magento files

Regards