function flags(array $a)
{

    $size  = count($a);
    $peaks = [false];
    $next  = [];
    for ($i = 1; $i < $size; $i++) {
        $peaks[$i] = $a[$i - 1] < $a[$i] && $a[$i] > ($a[$i + 1] ?? 0);
    }

    $next[$size - 1] = -1;
    for ($i = $size - 2; $i >= 0; $i--) {
        $next[$i] = $peaks[$i] ? $i : $next[$i + 1];
    }
    $i      = 1;
    $result = 0;
    while ($i * ($i - 1) <= $size) {
        $pos = 0;
        $num = 0;
        while ($pos < $size && $num < $i) {
            $pos = $next[$pos];
            if ($pos === -1) {
                break;
            }
            ++$num;
            $pos += $i;
        }
        $i++;
        $result = max($result, $num);
    }

    return $result;
}