<?php
namespace Gene\SampleProductsCartRule\Model\Rule\Action;

/**
 * This plugs in to \Magento\SalesRule\Model\Rule\Action\Discount\CalculatorFactory
 * and implements the Magento\SalesRule\Model\Rule\Action\Discount\DiscountInterface
 */
class SampleItems extends \Magento\SalesRule\Model\Rule\Action\Discount\AbstractDiscount
{
    public function __construct(
        \Magento\SalesRule\Model\Validator $validator,
        \Magento\SalesRule\Model\Rule\Action\Discount\DataFactory $discountDataFactory,
        \Magento\Framework\Pricing\PriceCurrencyInterface $priceCurrency
    ) {
        parent::__construct($validator, $discountDataFactory, $priceCurrency);
    }

    public function getSampleSkus($rule)
    {
        return explode(",", $rule->getProductIds());
    }

    /**
     * The below is run for each line item in the cart once
     * it meets the salesrule criteria as set in the admin.
     *
     * The problem is that it is not run when a change is made in the cart
     * and the salesrule is removed from the quote, so there is no
     * clean place to reset the custom price.
     */
    public function calculate($rule, $item, $qty)
    {
        $discountData = $this->discountFactory->create();

        if (!in_array($item->getSku(), $this->getSampleSkus($rule))) {
            return $discountData;
        }

        $fixedPrice = $rule->getDiscountAmount();
        $item->setCustomPrice($fixedPrice);

        return $discountData;
    }
}