How to create custom cart rule in magento 2

As you know magento provides us with four types of cart rules.

But sometimes we need or need to enhance the functionality of the magento cart rule according to the requirement you can have two options either you override the core magento cart rule or create a new cart rule.

Here we create a new custom cart rule

for creating a custom cart rule you need to create the below files

Below the file create just add the option in the dropdown in the rule
Webkul\CartRule\etc\adminhtml\di.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\SalesRule\Model\Rule\Metadata\ValueProvider">
        <plugin name="salesrule-plugin" type="Webkul\CartRule\Plugin\Rule\Metadata\ValueProvider" sortOrder="1" />
    </type>
</config>

<?php

namespace Webkul\CartRule\Plugin\Rule\Metadata;

class ValueProvider {
    public function afterGetMetadataValues(
        \Magento\SalesRule\Model\Rule\Metadata\ValueProvider $subject,
        $result
    ) {


        $applyOptions = [
            'label' => __('Custom'),
            'value' => [
                [
                    'label' => 'Buy X Get next Y with M% discount',
                    'value' => 'buy_x_get_next_y_with_percent',
                ]
            ],
        ];
        array_push($result['actions']['children']['simple_action']['arguments']['data']['config']['options'], $applyOptions);
        return $result;
    }
}

You will see the cart rule in the dropdown

Now pass the class as an argument in class Magento\SalesRule\Model\Rule\Action\Discount\Calculator
in below location
Webkul\CartRule\etc\di.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\SalesRule\Model\Rule\Action\Discount\CalculatorFactory">
        <arguments>
            <argument name="discountRules" xsi:type="array">
                <item name="buy_x_get_next_y_with_percent" xsi:type="string">\Webkul\CartRule\Model\Rule\Action\Discount\BuyXGetNextYWithPercent</item>
            </argument>
        </arguments>
     </type>
</config>

<?php
namespace Webkul\Magento\Model\Rule\Action\Discount;

use Magento\SalesRule\Model\Rule\Action\Discount\AbstractDiscount;

class BuyXGetNextYWithPercent extends AbstractDiscount
{
    /**
     * Calculate Buy X Get next Y with M% discount amount
     *
     * @param \Magento\SalesRule\Model\Rule $rule
     * @param \Magento\Quote\Model\Quote\Item\AbstractItem $item
     * @param float $qty
     * @return \Magento\SalesRule\Model\Rule\Action\Discount\Data
     */
    public function calculate($rule, $item, $qty)
    {
        $rulePercent = min(100, $rule->getDiscountAmount());
        $discountData = $this->_calculate($rule, $item, $qty, $rulePercent);
        
        return $discountData;
    }

    /**
     * @param \Magento\SalesRule\Model\Rule $rule
     * @param \Magento\Quote\Model\Quote\Item\AbstractItem $item
     * @param float $qty
     * @param float $rulePercent
     * @return Data
     */
    protected function _calculate($rule, $item, $qty, $rulePercent)
    {
        /** @var \Magento\SalesRule\Model\Rule\Action\Discount\Data $discountData */
        $discountData = $this->discountFactory->create();

        /**
         * Calculate your logic here
         */
         
        /** Set the discount price in Price **/
        
        $itemPrice = $this->validator->getItemPrice($item);
        $baseItemPrice = $this->validator->getItemBasePrice($item);
        $itemOriginalPrice = $this->validator->getItemOriginalPrice($item);
        $baseItemOriginalPrice = $this->validator->getItemBaseOriginalPrice($item);

        $discountData->setAmount($itemPrice - $discountAmount);
        $discountData->setBaseAmount($baseItemPrice - $discountAmount);
        $discountData->setOriginalAmount($itemOriginalPrice - $discountAmount);
                $discountData->setBaseOriginalAmount($baseItemOriginalPrice - $discountAmount);


        return $discountData;
    }

}

Now Create the rule