feat: fold purchase and promo voucher fields into one code field

src/Service/VoucherCodeClassifier.php

@@ -0,0 +1,69 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Service;
+
+use App\Enum\VoucherCodeKind;
+
+/**
+ * Tells promo codes and purchase codes apart, so the booking form needs only one voucher field.
+ *
+ * Promo codes are prefixed "R-". Purchase codes (Kauf and Kulanz) consist of uppercase letters only
+ * and never contain a hyphen, so the prefix cannot collide with them. Promo codes issued before the
+ * prefix was introduced are recognised from `legacy_promo_voucher_codes` until their date has passed.
+ */
+class VoucherCodeClassifier
+{
+    public const PROMO_PREFIX = 'R-';
+
+    private const MAX_LENGTH = 50;
+
+    /** @var array<string, \DateTimeImmutable> Normalised code => last day the code is recognised */
+    private readonly array $legacyPromoCodes;
+
+    /**
+     * @param array<string, string> $legacyPromoCodes Code => last day it is accepted (Y-m-d)
+     */
+    public function __construct(array $legacyPromoCodes = [])
+    {
+        $codes = [];
+        foreach ($legacyPromoCodes as $code => $lastDay) {
+            $codes[$this->normalize((string) $code)] = new \DateTimeImmutable($lastDay);
+        }
+
+        $this->legacyPromoCodes = $codes;
+    }
+
+    /**
+     * Codes are matched case-insensitively; BusPro receives them in uppercase.
+     */
+    public function normalize(string $code): string
+    {
+        return mb_strtoupper(trim($code));
+    }
+
+    public function classify(string $code, \DateTimeImmutable $today = new \DateTimeImmutable('today')): VoucherCodeKind
+    {
+        $code = $this->normalize($code);
+
+        if ('' === $code || mb_strlen($code) > self::MAX_LENGTH || 1 !== preg_match('/^[A-Z0-9\-]+$/', $code)) {
+            return VoucherCodeKind::Malformed;
+        }
+
+        if (str_starts_with($code, self::PROMO_PREFIX)) {
+            return VoucherCodeKind::Promo;
+        }
+
+        $lastDay = $this->legacyPromoCodes[$code] ?? null;
+        if (null !== $lastDay && $lastDay >= $today) {
+            return VoucherCodeKind::Promo;
+        }
+
+        if (str_contains($code, '-')) {
+            return VoucherCodeKind::Malformed;
+        }
+
+        return VoucherCodeKind::Purchase;
+    }
+}