chore: code cleanup

src/Enum/RecurrenceRule.php

@@ -0,0 +1,52 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Enum;
+
+/**
+ * How often a series repeats.
+ *
+ * Every rule is a whole number of days, which is what lets BookingSeries::occurrences() be a
+ * plain stride from the start date. Every stride that is a multiple of 7 therefore keeps the
+ * weekday fixed by construction -- that is why a series needs no weekday field at all.
+ *
+ * There is deliberately no calendar-monthly rule: "the 31st of each month" has no counterpart in
+ * five months of the year, and it would not hold the weekday. FOUR_WEEKLY is the nearest thing
+ * that behaves, at the cost of drifting against the calendar -- 13 occurrences a year, not 12,
+ * which is why it is not labelled "Monatlich".
+ */
+enum RecurrenceRule: string
+{
+    case DAILY = 'daily';
+    case WEEKLY = 'weekly';
+    case TWO_WEEKLY = 'two_weekly';
+    case FOUR_WEEKLY = 'four_weekly';
+
+    public function label(): string
+    {
+        return match ($this) {
+            self::DAILY => 'Täglich',
+            self::WEEKLY => 'Wöchentlich',
+            self::TWO_WEEKLY => 'Alle zwei Wochen',
+            self::FOUR_WEEKLY => 'Alle vier Wochen',
+        };
+    }
+
+    /** Days between two occurrences. */
+    public function intervalDays(): int
+    {
+        return match ($this) {
+            self::DAILY => 1,
+            self::WEEKLY => 7,
+            self::TWO_WEEKLY => 14,
+            self::FOUR_WEEKLY => 28,
+        };
+    }
+
+    /** Whether the rule lands on the same weekday every time, i.e. whether naming it adds anything. */
+    public function keepsWeekday(): bool
+    {
+        return 0 === $this->intervalDays() % 7;
+    }
+}