fix: correctly calculate undersubscription fees

src/Service/GroupsPriceCalculator.php

@@ -244,10 +244,17 @@ class GroupsPriceCalculator
             $shortTermSurcharge = (int) round(($basePrice + $additionalPersonsPrice) * self::SHORT_TERM_FACTORS[$nights] / 100);
         }
 
-        // Rule 4: undersubscription surcharge (only when board is selected)
+        // Board is only billable — and only triggers the catering surcharge — when it costs
+        // something. A free "Selbstversorgung" board is stored as 0 (BoardService::$price is
+        // non-nullable), so a plain null check would charge the surcharge for it.
+        $paidBoardPricePerPersonNight = ($boardServicePricePerPersonNight ?? 0) > 0
+            ? $boardServicePricePerPersonNight
+            : null;
+
+        // Rule 4: undersubscription surcharge (only when a paid board is selected)
         $undersubscriptionSurcharge = 0;
         $undersubscriptionThreshold = null;
-        if ($boardServicePricePerPersonNight !== null) {
+        if ($paidBoardPricePerPersonNight !== null) {
             $surcharge30 = (int) round(('CHF' === $currency ? $this->config['undersubscription30Chf'] : $this->config['undersubscription30Eur']) * 100);
             $surcharge40 = (int) round(('CHF' === $currency ? $this->config['undersubscription40Chf'] : $this->config['undersubscription40Eur']) * 100);
 
@@ -262,8 +269,8 @@ class GroupsPriceCalculator
 
         // Rule 5: board price
         $boardPrice = 0;
-        if ($boardServicePricePerPersonNight !== null) {
-            $boardPrice = $boardServicePricePerPersonNight * $effectivePax * $nights;
+        if ($paidBoardPricePerPersonNight !== null) {
+            $boardPrice = $paidBoardPricePerPersonNight * $effectivePax * $nights;
         }
 
         // Rule 6: additional services

tests/Service/GroupsPriceCalculatorTest.php

@@ -0,0 +1,88 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Tests\Service;
+
+use App\Entity\Groups\AccommodationPrice;
+use App\Service\GroupsPriceCalculator;
+use App\Service\PriceTimelineBuilder;
+use PHPUnit\Framework\TestCase;
+
+class GroupsPriceCalculatorTest extends TestCase
+{
+    private const int NIGHTS = 3;
+
+    public function testFreeBoardDoesNotTriggerUndersubscriptionSurcharge(): void
+    {
+        $result = $this->calculate(30, 0);
+
+        self::assertSame(0, $result['undersubscriptionSurcharge']);
+        self::assertNull($result['undersubscriptionThreshold']);
+        self::assertSame(0, $result['boardPrice']);
+    }
+
+    public function testNoBoardDoesNotTriggerUndersubscriptionSurcharge(): void
+    {
+        $result = $this->calculate(30, null);
+
+        self::assertSame(0, $result['undersubscriptionSurcharge']);
+        self::assertNull($result['undersubscriptionThreshold']);
+        self::assertSame(0, $result['boardPrice']);
+    }
+
+    public function testPaidBoardBelow40TriggersTheLowerThresholdSurcharge(): void
+    {
+        $result = $this->calculate(30, 1500);
+
+        self::assertSame(30 * 500 * self::NIGHTS, $result['undersubscriptionSurcharge']);
+        self::assertSame(40, $result['undersubscriptionThreshold']);
+        self::assertSame(1500 * 30 * self::NIGHTS, $result['boardPrice']);
+    }
+
+    public function testPaidBoardBelow50TriggersTheUpperThresholdSurcharge(): void
+    {
+        $result = $this->calculate(45, 1500);
+
+        self::assertSame(45 * 250 * self::NIGHTS, $result['undersubscriptionSurcharge']);
+        self::assertSame(50, $result['undersubscriptionThreshold']);
+    }
+
+    /**
+     * @return array<string, mixed>
+     */
+    private function calculate(int $paxCount, ?int $boardServicePrice): array
+    {
+        $dateFrom = new \DateTimeImmutable('2026-07-01');
+        $dateTo = $dateFrom->modify(sprintf('+%d days', self::NIGHTS));
+
+        $price = (new AccommodationPrice())
+            ->setDateFrom($dateFrom->modify('-1 month'))
+            ->setDateTo($dateTo->modify('+1 month'))
+            ->setIncludedPax(20)
+            ->setPricePerNight(100_00)
+            ->setPriceAdditionalPerson(10_00)
+            ->setMinNights(1);
+
+        $calculator = new GroupsPriceCalculator(new PriceTimelineBuilder(), [
+            'runningCostsEur' => 0,
+            'runningCostsChf' => 0,
+            'undersubscription30Eur' => 5.0,
+            'undersubscription30Chf' => 6.0,
+            'undersubscription40Eur' => 2.5,
+            'undersubscription40Chf' => 3.0,
+        ]);
+
+        return $calculator->calculateFromSnapshots(
+            $paxCount,
+            0,
+            self::NIGHTS,
+            $dateFrom,
+            $dateTo,
+            [$price],
+            $boardServicePrice,
+            [],
+            'EUR',
+        );
+    }
+}