add stopsGrowing discount modifier

docs/formula-and-modifiers.md

@@ -71,9 +71,10 @@ discount.since('08.2018').grows('1%').every('month').min('10 USD')
 discount.since('08.2000').grows('30%').every('year').max('100%')
 discount.since('08.2018').grows('20 USD').every('2 months').min('15 USD').max('80 USD')
 discount.since('08.2018').till('12.2018').grows('10pp').every('month')
+discount.since('08.2018').grows('10 USD').every('month').stopsGrowing('10.2018')
 ```
 
-Supports absolute (`USD`), relative (`%`), and percentage point (`pp`) steps. `min`/`max` cap the accumulated discount.
+Supports absolute (`USD`), relative (`%`), and percentage point (`pp`) steps. `min`/`max` cap the accumulated discount. `stopsGrowing()` caps the growth calculation at the given month while the discount continues to apply.
 
 ### Increase
 
@@ -159,6 +160,7 @@ Modifiers use a composable addon system for configuration:
 | `MonthPeriod` / `YearPeriod` / `DayPeriod` | Time periods for `every()` and `lasts()` |
 | `Discount` | Discount value (absolute, relative %, or percentage point) |
 | `Step` | Growth step for GrowingDiscount |
+| `StopsGrowing` | Last month when GrowingDiscount grows |
 | `Reason` | Human-readable comment attached to modifier charges |
 | `Minimum` / `Maximum` | Bounds for accumulated discount values |
 

src/charge/modifiers/GrowingDiscount.php

@@ -19,6 +19,7 @@
 use hiqdev\php\billing\charge\modifiers\addons\Period;
 use hiqdev\php\billing\charge\modifiers\addons\Since;
 use hiqdev\php\billing\charge\modifiers\addons\Step;
+use hiqdev\php\billing\charge\modifiers\addons\StopsGrowing;
 use Money\Money;
 
 /**
@@ -32,6 +33,7 @@ class GrowingDiscount extends FixedDiscount
     public const STEP = 'step';
     public const MIN = 'min';
     public const MAX = 'max';
+    public const STOPS_GROWING = 'stopsGrowing';
 
     public function __construct($step, $min = null, array $addons = [])
     {
@@ -90,6 +92,16 @@ public function every($string = 1)
         return $this->addAddon(self::PERIOD, Period::fromString($string));
     }
 
+    public function stopsGrowing($month)
+    {
+        return $this->addAddon(self::STOPS_GROWING, new StopsGrowing($month));
+    }
+
+    public function getStopsGrowing(): ?StopsGrowing
+    {
+        return $this->getAddon(self::STOPS_GROWING);
+    }
+
     #[\Override]
     public function calculateSum(?ChargeInterface $charge = null): Money
     {
@@ -126,6 +138,15 @@ protected function countPeriodsPassed(DateTimeImmutable $time)
             throw new \Exception('no period given for growing discount');
         }
 
-        return $period->countPeriodsPassed($since->getValue(), $time);
+        $sinceTime = $since->getValue();
+        $stopsGrowing = $this->getStopsGrowing();
+        if ($stopsGrowing instanceof StopsGrowing && $stopsGrowing->getValue() < $time) {
+            $time = $stopsGrowing->getValue();
+        }
+        if ($time < $sinceTime) {
+            $time = $sinceTime;
+        }
+
+        return $period->countPeriodsPassed($sinceTime, $time);
     }
 }

src/charge/modifiers/addons/StopsGrowing.php

@@ -0,0 +1,25 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * PHP Billing Library
+ *
+ * @link      https://github.com/hiqdev/php-billing
+ * @package   php-billing
+ * @license   BSD-3-Clause
+ * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
+ */
+
+namespace hiqdev\php\billing\charge\modifiers\addons;
+
+use hiqdev\php\billing\charge\modifiers\AddonInterface;
+
+/**
+ * Stops growing addon.
+ *
+ * @author Andrii Vasyliev <sol@hiqdev.com>
+ */
+class StopsGrowing extends Date implements AddonInterface
+{
+}

tests/behat/GrowingDiscount.feature

@@ -73,6 +73,40 @@ Feature: Growing discount
             | 2019-01-01 | monthly 100 USD | discount -80 USD |
             | 2028-11-01 | monthly 100 USD | discount -80 USD |
 
+    Scenario Outline: absolute discount stops growing but keeps applying
+        Given formula is discount.since('08.2018').grows('20 USD').every('2 months').min('15 USD').max('80 USD').stopsGrowing('10.2018')
+         When action date is <date>
+         Then first charge is <first>
+          And second charge is <second>
+        Examples:
+            | date       | first           | second           |
+            | 2018-07-31 | monthly 100 USD |                  |
+            | 2018-08-01 | monthly 100 USD | discount -30 USD |
+            | 2018-09-01 | monthly 100 USD | discount -30 USD |
+            | 2018-10-01 | monthly 100 USD | discount -70 USD |
+            | 2018-11-01 | monthly 100 USD | discount -70 USD |
+            | 2018-12-01 | monthly 100 USD | discount -70 USD |
+            | 2019-01-01 | monthly 100 USD | discount -70 USD |
+            | 2028-11-01 | monthly 100 USD | discount -70 USD |
+
+    Scenario Outline: relative discount stops growing but keeps applying
+        Given formula is discount.since('08.2000').grows('30%').every('year').max('100%').stopsGrowing('08.2003')
+         When action date is <date>
+         Then first charge is <first>
+          And second charge is <second>
+        Examples:
+            | date       | first           | second              |
+            | 2000-07-31 | monthly 100 USD |                     |
+            | 2000-08-01 | monthly 100 USD | discount -30.00 USD |
+            | 2001-08-01 | monthly 100 USD | discount -51.00 USD |
+            | 2002-08-01 | monthly 100 USD | discount -65.70 USD |
+            | 2003-01-01 | monthly 100 USD | discount -65.70 USD |
+            | 2003-08-01 | monthly 100 USD | discount -75.99 USD |
+            | 2004-08-01 | monthly 100 USD | discount -75.99 USD |
+            | 2005-08-01 | monthly 100 USD | discount -75.99 USD |
+            | 2011-08-01 | monthly 100 USD | discount -75.99 USD |
+            | 2024-08-01 | monthly 100 USD | discount -75.99 USD |
+
     Scenario Outline: discount maximum may not match step: relative step but absolute max
         Given formula is discount.since('08.2018').grows('20pp').every('month').min('30%').max('77 USD')
          When action date is <date>