Add test case for bill dates

tests/integration/Support/Models/BillDateCalculatorTest.php

@@ -0,0 +1,62 @@
+<?php
+/*
+ * BillDateCalculatorTest.php
+ * Copyright (c) 2023 james@firefly-iii.org
+ *
+ * This file is part of Firefly III (https://github.com/firefly-iii).
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+declare(strict_types=1);
+
+namespace Tests\integration\Support\Models;
+
+use Carbon\Carbon;
+use FireflyIII\Support\Models\BillDateCalculator;
+use Tests\integration\TestCase;
+
+/**
+ * Class BillDateCalculatorTest
+ */
+class BillDateCalculatorTest extends TestCase
+{
+    private BillDateCalculator $calculator;
+
+    public function __construct(string $name)
+    {
+        parent::__construct($name);
+        $this->calculator = new BillDateCalculator();
+    }
+
+    public static function provideDates(): iterable
+    {
+        // Carbon $earliest, Carbon $latest, Carbon $billStart, string $period, int $skip, ?Carbon $lastPaid
+        return [
+            // basic monthly bill.
+            '1M' => ['earliest' => Carbon::parse('2023-11-01'), 'latest' => Carbon::parse('2023-11-30'), 'billStart' => Carbon::parse('2023-01-01'), 'period' => 'monthly', 'skip' => 0, 'lastPaid' => null, 'expected' => ['2023-11-01']],
+        ];
+    }
+
+    /**
+     * Stupid long method names I'm not going to do that.
+     *
+     * @dataProvider provideDates
+     */
+    public function testGivenSomeDataItWorks(Carbon $earliest, Carbon $latest, Carbon $billStart, string $period, int $skip, ?Carbon $lastPaid, array $expected): void
+    {
+        $result = $this->calculator->getPayDates($earliest, $latest, $billStart, $period, $skip, $lastPaid);
+        self::assertSame($expected, $result);
+    }
+}