<?php

function calendar_next_events(int $n = 5): array {
    $url   = 'https://alpinux.org/public-calendars/n5BWPYsxw7FCYozM';
    $cache = sys_get_temp_dir() . '/alpinux_calendar.ics';
    $ttl   = 3600;

    if (!file_exists($cache) || (time() - filemtime($cache)) >= $ttl) {
        $data = @file_get_contents($url);
        if ($data !== false) file_put_contents($cache, $data);
    }

    if (!file_exists($cache)) return [];

    return _ical_upcoming(file_get_contents($cache), $n);
}

function _ical_upcoming(string $ical, int $n): array {
    $events = [];
    $now    = time();

    // Unfold continuation lines before parsing
    $ical = preg_replace('/\r?\n[ \t]/', '', $ical);

    preg_match_all('/BEGIN:VEVENT(.+?)END:VEVENT/s', $ical, $blocks);

    foreach ($blocks[1] as $b) {
        $e = [];

        if (preg_match('/^SUMMARY[^:]*:(.+)$/m', $b, $m))
            $e['title'] = _ical_unescape(trim($m[1]));

        if (preg_match('/^LOCATION[^:]*:(.+)$/m', $b, $m))
            $e['location'] = _ical_unescape(trim($m[1]));

        if (preg_match('/^DESCRIPTION[^:]*:(.+)$/m', $b, $m))
            $e['description'] = _ical_unescape(trim($m[1]));

        if (preg_match('/^URL[^:]*:(.+)$/m', $b, $m))
            $e['url'] = trim($m[1]);

        // Handles: DTSTART:YYYYMMDD, DTSTART:YYYYMMDDTHHmmss[Z], DTSTART;TZID=...:YYYYMMDDTHHmmss
        foreach (['start' => 'DTSTART', 'end' => 'DTEND'] as $key => $prop) {
            if (preg_match('/^' . $prop . '(?:;[^:]+)?:([\dTZ]+)/m', $b, $m)) {
                $raw = $m[1];
                if (strlen($raw) === 8) {
                    $e[$key] = mktime(0, 0, 0, (int)substr($raw, 4, 2), (int)substr($raw, 6, 2), (int)substr($raw, 0, 4));
                } else {
                    $dt = DateTime::createFromFormat('Ymd\THis\Z', $raw, new DateTimeZone('UTC'))
                       ?: DateTime::createFromFormat('Ymd\THis',  $raw, new DateTimeZone('Europe/Paris'));
                    if ($dt) $e[$key] = $dt->getTimestamp();
                }
            }
        }

        if (isset($e['start'], $e['title']) && $e['start'] >= $now)
            $events[] = $e;
    }

    usort($events, fn($a, $b) => $a['start'] - $b['start']);

    return array_slice($events, 0, $n);
}

function _ical_unescape(string $s): string {
    return str_replace(['\\n', '\\,', '\\;', '\\\\'], ["\n", ',', ';', '\\'], $s);
}