dts and vfr

// When processing VFR video, we need to be sure that long frame
// durations don't cause there to be too few decoded reference
// frames.  So we limit the dts nominal_frame_dur.
// nominal_frame_dur = init_delay if bframes
// nominal_frame_dur = init_delay/2 if bframes && bpyramid
//
// We also want to ensure that there are at most max_delayed_frames
// that have been decoded but not yet displayed by the player.
// max_delayed_frames = 1 if bframes
// max_delayed_frames = 2 if bframes && bpyramid

int64_t dts;
int64_t pts_hist[2];
int min, max;

min = max = 0;
if (max_delayed_frames == 2)
    max = 1;

dts += MIN(nominal_frame_dur, this_frame_dur);

if ( pts > dts )
{
    // pts > dts means we are about to delay another frame
    // unless we do something about it.
    if ( num_delayed_frames == max_delayed_frames )
    {
        // The next dts is less than the pts and would therefore
        // add another delayed frame pushing us above the max.
        // So adjust dts such that we don't delay another frame.
        if ( pts < pts_hist[min] )
        {
            dts = pts;
        }
        else if ( pts > pts_hist[max] )
        {
            dts = pts_hist[min];
            pts_hist[min] = pts_hist[max];
            pts_hist[max] = pts
        }
        else
        {
            dts = pts_hist[min];
            pts_hist[min] = pts;
        }
    }
    else
    {
        num_delayed_frames++;
        if ( pts > pts_hist[max] )
        {
            pts_hist[min] = pts_hist[max];
            pts_hist[max] = pts;
        }
        else
        {
            pts_hist[min] = pts;
        }
    }
}
else if ( pts < dts )
{
    dts = pts;
}