x264 - x264_reference_hierarchy_reset

static inline void x264_reference_hierarchy_reset( x264_t *h )
{
    int ref;
    int b_hasdelayframe = 0;

    // 非参考B帧一般是IS_DISPOSABLE
    /* look for delay frames -- chain must only contain frames that are disposable */
    for( int i = 0; h->frames.current[i] && IS_DISPOSABLE( h->frames.current[i]->i_type ); i++ )
        b_hasdelayframe |= h->frames.current[i]->i_coded
                        != h->frames.current[i]->i_frame + h->sps->vui.i_num_reorder_frames;

    /* This function must handle b-pyramid and clear frames for open-gop */
    if( h->param.i_bframe_pyramid != X264_B_PYRAMID_STRICT && !b_hasdelayframe && h->frames.i_poc_last_open_gop == -1 )
        return;

    /* Remove last BREF. There will never be old BREFs in the
     * dpb during a BREF decode when pyramid == STRICT */
    // 标记要清除的参考帧， BREF 帧 或
    // h->frames.reference[ref]->i_poc < h->frames.i_poc_last_open_gop
    // 参考帧
    // 并设置list0重序标志
    for( ref = 0; h->frames.reference[ref]; ref++ )
    {
        if( ( h->param.i_bframe_pyramid == X264_B_PYRAMID_STRICT
            && h->frames.reference[ref]->i_type == X264_TYPE_BREF )
            || ( h->frames.reference[ref]->i_poc < h->frames.i_poc_last_open_gop
            && h->sh.i_type != SLICE_TYPE_B ) )
        {
            int diff = h->i_frame_num - h->frames.reference[ref]->i_frame_num;
            h->sh.mmco[h->sh.i_mmco_command_count].i_difference_of_pic_nums = diff;
            h->sh.mmco[h->sh.i_mmco_command_count++].i_poc = h->frames.reference[ref]->i_poc;
            x264_frame_push_unused( h, x264_frame_shift( &h->frames.reference[ref] ) );
            h->b_ref_reorder[0] = 1;
            ref--;
        }
    }

    // 计算list0 清除参考帧的数目（从距离远者开始清除）
    /* Prepare room in the dpb for the delayed display time of the later b-frame's */
    if( h->param.i_bframe_pyramid )
        h->sh.i_mmco_remove_from_end = X264_MAX( ref + 2 - h->frames.i_max_dpb, 0 );
}