<?php
function get_next_payer_username($conn, $meeting_id)
{
    $current_year = (int)date('Y');

    // Alle zugesagten Nutzer mit paid_count
    $sql = "
        SELECT 
            u.id,
            u.username,
            u.birthday,
            u.last_birthday_year,
            (SELECT COUNT(*) FROM meeting_teilnehmer WHERE user_id = u.id AND paid = 1) AS paid_count
        FROM meeting_teilnehmer mt
        JOIN users u ON mt.user_id = u.id
        WHERE mt.meeting_id = ? AND mt.rsvp_status = 'accepted'
        ORDER BY u.username
    ";
    $stmt = mysqli_prepare($conn, $sql);
    mysqli_stmt_bind_param($stmt, "i", $meeting_id);
    mysqli_stmt_execute($stmt);
    $candidates = mysqli_fetch_all(mysqli_stmt_get_result($stmt), MYSQLI_ASSOC);
    mysqli_stmt_close($stmt);

    if (empty($candidates)) return null;

    // Finde minimale paid_count
    $min_paid = min(array_column($candidates, 'paid_count'));
    $eligible = array_filter($candidates, fn($c) => $c['paid_count'] == $min_paid);

    $birthday_eligible = [];
    $others = [];

    foreach ($eligible as $c) {
        $is_birthday_payer_eligible = (
            !empty($c['birthday'])
            && $c['birthday'] !== '0000-00-00'
            && $c['last_birthday_year'] != $current_year
        );

        if ($is_birthday_payer_eligible) {
            $birthday_eligible[] = $c;
        } else {
            $others[] = $c;
        }
    }

    // Sortiere alphabetisch
    $sort = fn($a, $b) => strcmp($a['username'], $b['username']);

    if (!empty($birthday_eligible)) {
        usort($birthday_eligible, $sort);
        return $birthday_eligible[0]['username'];
    }

    $all = array_merge($birthday_eligible, $others);
    usort($all, $sort);
    return $all[0]['username'];
}