<?php

namespace App\Services\Calendar;

use Carbon\CarbonInterface;
use DateTimeZone;
use RuntimeException;
use Sabre\VObject\Component\VCalendar;
use Sabre\VObject\Component\VEvent;
use Sabre\VObject\Reader;

class CalendarOccurrenceExpander
{
    public function expand(string $ical, CarbonInterface $from, CarbonInterface $to): string
    {
        if (! preg_match('/^(RRULE|RDATE|RECURRENCE-ID)[;:]/mi', $ical)) {
            return $ical;
        }

        $calendar = Reader::read($ical);
        if (! $calendar instanceof VCalendar) {
            throw new RuntimeException('La source agenda ne contient pas un calendrier iCalendar.');
        }

        $additionalDates = clone $calendar;
        $additionalUids = [];

        foreach ($calendar->select('VEVENT') as $event) {
            // A cancelled detached occurrence may contain only UID and RECURRENCE-ID.
            // It must still suppress the original occurrence during expansion.
            if (! isset($event->DTSTART) && isset($event->{'RECURRENCE-ID'})) {
                $start = clone $event->{'RECURRENCE-ID'};
                $start->name = 'DTSTART';
                $event->add($start);
            }

            // VObject chooses RDATE instead of RRULE when both are present.
            // Expand their union separately, then deduplicate by occurrence identity.
            if (isset($event->RRULE, $event->RDATE)) {
                $additionalUids[] = (string) $event->UID;
                unset($event->RDATE);
            }
        }

        $timezone = new DateTimeZone((string) config('app.timezone', 'UTC'));
        $expanded = $calendar->expand($from, $to, $timezone);

        if ($additionalUids !== []) {
            foreach ($additionalDates->select('VEVENT') as $event) {
                if (! in_array((string) $event->UID, $additionalUids, true)) {
                    $additionalDates->remove($event);
                } else {
                    unset($event->RRULE);
                    if (! isset($event->DTSTART) && isset($event->{'RECURRENCE-ID'})) {
                        $start = clone $event->{'RECURRENCE-ID'};
                        $start->name = 'DTSTART';
                        $event->add($start);
                    }
                }
            }

            $seen = [];
            foreach ($expanded->select('VEVENT') as $event) {
                $seen[$this->identity($event)] = true;
            }
            foreach ($additionalDates->expand($from, $to, $timezone)->select('VEVENT') as $event) {
                $key = $this->identity($event);
                if (! isset($seen[$key])) {
                    $expanded->add(clone $event);
                    $seen[$key] = true;
                }
            }
        }

        return $expanded->serialize();
    }

    private function identity(VEvent $event): string
    {
        return (string) $event->UID.'|'.(string) $event->{'RECURRENCE-ID'};
    }
}