Prestashop 1.7.X - Better Catalog Price Rule calculation per product

SpecificPrice.php

<?php

/**
 * Fixes the specific Price calculation in default PrestaShop installation.
 * Makes sure that the highest Discount in Catalog Price Rules is applied to the product
 * indipendently if the discount is by % or by amount
 *
 * If you have set up discounts on the customer group level, you may remove these and recreate
 * them as a Catalog Price Rule. This way the fix also applies to these cases.
 *
 * Instruction: Upload to override/classes/, delete var/cache/prod/class_index.php and empty the cache
 *
 * @author   KarmaEDV https://www.karmaedv.ch https://www.karmcraft.com 
 * @license  https://opensource.org/licenses/OSL-3.0 Open Software License (OSL 3.0)
 */

class SpecificPrice extends SpecificPriceCore
{

  public static function getSpecificPrice(
    $id_product,
    $id_shop,
    $id_currency,
    $id_country,
    $id_group,
    $quantity,
    $id_product_attribute = null,
    $id_customer = 0,
    $id_cart = 0,
    $real_quantity = 0
  ) {
    if (!SpecificPrice::isFeatureActive()) {
      return [];
    }
    /*
     * The date is not taken into account for the cache, but this is for the better because it keeps the consistency
     * for the whole script.
     * The price must not change between the top and the bottom of the page
    */

    if (!self::couldHaveSpecificPrice($id_product)) {
      return [];
    }

    if (static::$psQtyDiscountOnCombination === null) {
      static::$psQtyDiscountOnCombination = Configuration::get('PS_QTY_DISCOUNT_ON_COMBINATION');
      // no need to compute the key the first time the function is called, we know the cache has not
      // been computed yet
      $key = null;
    } else {
      $key = self::computeKey(
        $id_product,
        $id_shop,
        $id_currency,
        $id_country,
        $id_group,
        $quantity,
        $id_product_attribute,
        $id_customer,
        $id_cart,
        $real_quantity
      );
    }

    if (!array_key_exists($key, self::$_specificPriceCache)) {

      $price_query = '
        SELECT price FROM `' . _DB_PREFIX_ . 'product`
        WHERE `id_product` = ' . $id_product;        
      $price_orig = Db::getInstance(_PS_USE_SQL_SLAVE_)->getValue($price_query);

      $query_extra = self::computeExtraConditions($id_product, $id_product_attribute, $id_customer, $id_cart);
      if ($key === null) {
        // compute the key after calling computeExtraConditions as it initializes some useful cache
        $key = self::computeKey(
          $id_product,
          $id_shop,
          $id_currency,
          $id_country,
          $id_group,
          $quantity,
          $id_product_attribute,
          $id_customer,
          $id_cart,
          $real_quantity
        );
      }
      $query = '
  SELECT *, ' . SpecificPrice::_getScoreQuery($id_product, $id_shop, $id_currency, $id_country, $id_group, $id_customer) . '
    FROM `' . _DB_PREFIX_ . 'specific_price`
    WHERE
            `id_shop` ' . self::formatIntInQuery(0, $id_shop) . ' AND
            `id_currency` ' . self::formatIntInQuery(0, $id_currency) . ' AND
            `id_country` ' . self::formatIntInQuery(0, $id_country) . ' AND
            `id_group` ' . self::formatIntInQuery(0, $id_group) . ' ' . $query_extra . '
    AND IF(`from_quantity` > 1, `from_quantity`, 0) <= ';

      $query .= (static::$psQtyDiscountOnCombination || !$id_cart || !$real_quantity) ? (int) $quantity : max(1, (int) $real_quantity);
      $query .= " ORDER BY CASE WHEN reduction_type = 'amount' THEN (reduction / " . $price_orig . ") ELSE reduction END DESC";
      self::$_specificPriceCache[$key] = Db::getInstance(_PS_USE_SQL_SLAVE_)->getRow($query);
    }

    return self::$_specificPriceCache[$key];
  }
}