Fixed the SimpleTax adjusters' calculation logic when the tax is included in the price

Author: fulopattila122

Changelog.md

@@ -35,6 +35,7 @@
 - Added the polymorphic `source` relation/fields to the `Adjustment` model
 - Added the `getSource` and `setSource` methods to the `Adjustment` model
 - Changed the `setAdjustable` method to return the itself (was `void` before)
+- Changed (fixed, in fact) the `SimpleTax`/`SimpleTaxDeduction` adjusters to properly calculate the tax amount when it is included in the price
 - Changed the default-generated video hash to a base 58 ULID instead of nano id
 - Changed the minimum Laravel version requirements to v10.48, v11.44 and v12.2 respectively
 

src/Adjustments/Adjusters/SimpleTax.php

@@ -61,7 +61,10 @@ public function isIncluded(): bool
 
     private function calculateTaxAmount(Adjustable $adjustable): float
     {
-        return $adjustable->preAdjustmentTotal() * $this->rate / 100;
+        return match ($this->isIncluded) {
+            true => $adjustable->preAdjustmentTotal() / (100 +  $this->rate) *  $this->rate,
+            false => $adjustable->preAdjustmentTotal() *  $this->rate / 100,
+        };
     }
 
     private function getModelAttributes(Adjustable $adjustable): array

src/Adjustments/Adjusters/SimpleTaxDeduction.php

@@ -61,7 +61,10 @@ public function isIncluded(): bool
 
     private function calculateTaxAmount(Adjustable $adjustable): float
     {
-        return -1 * $adjustable->preAdjustmentTotal() * $this->rate / 100;
+        return -1 * match ($this->isIncluded) {
+            true => $adjustable->preAdjustmentTotal() / (100 +  $this->rate) *  $this->rate,
+            false => $adjustable->preAdjustmentTotal() *  $this->rate / 100,
+        };
     }
 
     private function getModelAttributes(Adjustable $adjustable): array

src/Adjustments/Changelog.md

@@ -9,6 +9,7 @@
 - Added the `getSource` and `setSource` methods to the `Adjustment` model
 - Changed the `setAdjustable` method to return the itself (was `void` before)
 - Changed the minimum Laravel version requirements to v10.48, v11.44 and v12.2 respectively
+- Changed (fixed, in fact) the `SimpleTax`/`SimpleTaxDeduction` adjusters to properly calculate the tax amount when it is included in the price
 
 ## 5.0.0
 ##### 2025-09-03

src/Adjustments/Tests/Feature/SimpleTaxTest.php

@@ -0,0 +1,56 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Feature;
+
+use PHPUnit\Framework\Attributes\Test;
+use Vanilo\Adjustments\Adjusters\SimpleTax;
+use Vanilo\Adjustments\Adjusters\SimpleTaxDeduction;
+use Vanilo\Adjustments\Tests\Examples\Order;
+use Vanilo\Adjustments\Tests\TestCase;
+
+class SimpleTaxTest extends TestCase
+{
+    #[Test] public function a_simple_non_included_tax_can_be_added_to_an_adjustable_order()
+    {
+        $order = Order::create(['items_total' => 100]);
+        $order->adjustments()->create(new SimpleTax(20, false));
+
+        $this->assertCount(1, $order->adjustments());
+        $this->assertEquals(20, $order->adjustments()->total());
+        $this->assertEquals(120, $order->total());
+    }
+
+    #[Test] public function an_included_tax_gets_calculated_correctly_based_on_the_rate_and_order_items_total()
+    {
+        $order = Order::create(['items_total' => 119]);
+        $order->adjustments()->create(new SimpleTax(19, true));
+
+        $this->assertEquals(119, $order->total());
+        $this->assertTrue($order->adjustments()->first()->isCharge());
+        $this->assertCount(1, $order->adjustments());
+        $this->assertEquals(19, $order->adjustments()->first()->getAmount());
+    }
+
+    #[Test] public function a_simple_non_included_tax_deductaion_can_be_added_to_an_adjustable_order()
+    {
+        $order = Order::create(['items_total' => 100]);
+        $order->adjustments()->create(new SimpleTaxDeduction(20, false));
+
+        $this->assertCount(1, $order->adjustments());
+        $this->assertEquals(-20, $order->adjustments()->total());
+        $this->assertEquals(80, $order->total());
+    }
+
+    #[Test] public function an_included_tax_deducation_gets_calculated_correctly_based_on_the_rate_and_order_items_total()
+    {
+        $order = Order::create(['items_total' => 119]);
+        $order->adjustments()->create(new SimpleTaxDeduction(19, true));
+
+        $this->assertEquals(119, $order->total());
+        $this->assertTrue($order->adjustments()->first()->isCredit());
+        $this->assertCount(1, $order->adjustments());
+        $this->assertEquals(-19, $order->adjustments()->first()->getAmount());
+    }
+}