Reduces the current Cart/Order's total by some percentage of the combined total of the previous 6 orders.

FrequentBuyer.php

<?php
namespace dpdx\plugin\adjusters;

use Craft;
use craft\base\Component;
use craft\helpers\ArrayHelper;

use craft\commerce\Plugin as Commerce;
use craft\commerce\base\AdjusterInterface;
use craft\commerce\elements\Order;
use craft\commerce\models\OrderAdjustment;


use Exception;

/**
 * Frequent Buyer Adjuster
 *
 * @author oof. Studio <hello@oof.studio>
 * @since 0.0.1
 */
class FrequentBuyer extends Component implements AdjusterInterface
{
    // Constants
    // =========================================================================

    /**
     * @inheritdoc
     */
    const ADJUSTMENT_TYPE = 'frequent-buyer';

    /**
     * @var int Interval at which the discount is applied.
     */
    const ADJUSTMENT_ORDER_INTERVAL = 6;

    /**
     * @var int Percentage (as a value between 0 and 1) used as a factor in calculating the discount.
     */
    const ADJUSTMENT_MULTIPLIER = 0.05;

    // Public Methods
    // =========================================================================

    /**
     * @inheritdoc
     */
    public function adjust(Order $order): array
    {
        $orders = Order::find()
            ->customer($order->getCustomer())
            ->isCompleted(true)
            ->orderBy('dateOrdered DESC')
            ->all();

        // Modulo will tell us if the current order is the "Nth + 1" (when this returns `0`, the user has a multiple of `ADJUSTMENT_ORDER_INTERVAL` completed Orders):
        if (count($orders) % self::ADJUSTMENT_ORDER_INTERVAL) {
            // Bail early; nothing to apply!
            return [];
        }

        $mostRecentOrders = array_slice($orders, 0, self::ADJUSTMENT_ORDER_INTERVAL);

        $adjustment = new OrderAdjustment([
            'type' => self::ADJUSTMENT_TYPE,
            'name' => 'Frequent customer discount',
            'description' => 'We appreciate your business!',
            'sourceSnapshot' => [
                'applicableOrderIds' => ArrayHelper::getColumn($orders, 'id')
            ]
        ]);
        
        $adjustment->setOrder($order);

        $adjustment->amount = -1 * self::ADJUSTMENT_MULTIPLIER * array_reduce($mostRecentOrders, function ($total, $order) {
            return $total + $order->getTotal();
        }, 0.0);

        return [$adjustment];
    }
}