use Time::HiRes qw(time);
sub eta {
    my ($cur, $total, $time) = @_;
    return unless $cur and $time;

    state ($last_progress, $last_time, $last_v, $last_eta);
    state (@v, @eta, $window_size, $window_idx);
    state $init = do {
        ($last_progress, $last_time, $last_v, $last_eta) = (0, 0, 0, -1);
        ($window_size, $window_idx) = (10, 0);
    };
    state $sub_v_weight = sub { 1 + $_[0] };
    state $sub_eta_weight = sub { $_[0] ? 2 * $_[1] : 1 };
    state $sub_weighted_avg = sub {
        my ($sub_weight, $avg, $total_weight, $w) = (shift, 0, 0, 0);
        for my $i (0 .. $#_) {
            # first version messed up the index.
            my $j = ($i + @_ - $window_idx - 1) % @_;
            $w = $sub_weight->($j, $w);
            $avg += $w * $_[$i];
            $total_weight += $w;
        }
        return $avg / $total_weight;
    };

    my $v = ($cur - $last_progress) / (($time - $last_time) || 1);
    $v[$window_idx] = $v;
    $v = $sub_weighted_avg->($sub_v_weight, @v);

    if ($v and $last_v) {
        my ($min_v, $max_v) = $v < $last_v ? ($v, $last_v) : ($last_v, $v);
        $v = $last_v + ($v - $last_v) * $min_v / $max_v;
    }

    my $a = ($v - $last_v) / ($last_time ? ($time - $last_time) : 1);
    my $r = $total - $cur;

    my $eta = $last_eta;
    if ($a and 0 < (my $d = ($v * $v + 2 * $a * $r))) {
        $eta = (sqrt($d) - $v) / $a;
    }
    elsif ($v) { $eta = $r / $v }

    $eta[$window_idx] = $eta;
    $eta = $sub_weighted_avg->($sub_eta_weight, @eta);

    ($last_progress, $last_time, $last_v, $last_eta, $window_idx)
        = ($cur, $time, $v, $eta, ($window_idx + 1) % $window_size);

    return $eta > 0 ? $eta : 0;
}