VLC MP4_GetInterleaving, annotated source code

mp4.c

/* Analyzes chunks to find max interleave length
 * sets flat flag if no interleaving is in use */
static void MP4_GetInterleaving( demux_t *p_demux, vlc_tick_t *pi_max_contiguous, bool *pb_flat )
{
    demux_sys_t *p_sys = p_demux->p_sys;
    *pi_max_contiguous = 0;
    *pb_flat = true;

    /* Find first recorded chunk */
    mp4_track_t *tk = NULL;
    uint64_t i_duration = 0;

    /// 遍历所有 track，找到位置最前的一个
    for( unsigned i=0; i < p_sys->i_tracks; i++ )
    {
        mp4_track_t *cur = &p_sys->track[i];
        if( !cur->i_chunk_count )
            continue;

        if( tk == NULL || cur->chunk[0].i_offset < tk->chunk[0].i_offset )
            tk = cur;
    }

    /// `i_chunk`: 每个 track 当前 chunk 的游标
    /// `i_offset`: chunk 在文件中的绝对位置
    /// `i_virtual_run_number`: chunks interleaving sequence (WTF is it?)
    for( ; tk != NULL; )
    {
        i_duration += tk->chunk[tk->i_chunk].i_duration;
        tk->i_chunk++;

        /// 遍历所有 track，找到未处理 chunk 位置最靠前的 track
        /* Find next chunk in data order */
        mp4_track_t *nexttk = NULL;
        for( unsigned i=0; i < p_sys->i_tracks; i++ )
        {
            mp4_track_t *cur = &p_sys->track[i];
            /// 已经到结尾，略过
            if( cur->i_chunk == cur->i_chunk_count )
                continue;

            if( nexttk == NULL ||
                cur->chunk[cur->i_chunk].i_offset < nexttk->chunk[nexttk->i_chunk].i_offset )
                nexttk = cur;
        }

        /* copy previous run */
        if( nexttk && nexttk->i_chunk > 0 )
            nexttk->chunk[nexttk->i_chunk].i_virtual_run_number =
                    nexttk->chunk[nexttk->i_chunk - 1].i_virtual_run_number;

        if( tk != nexttk )
        {
            vlc_tick_t i_dur = MP4_rescale_mtime( i_duration, tk->i_timescale );
            if( i_dur > *pi_max_contiguous )
                *pi_max_contiguous = i_dur;
            i_duration = 0;

            /// 当前的 track 尚未结束，但是另一 track 的 chunk 出现，所以是交织的排布
            if( tk->i_chunk != tk->i_chunk_count )
                *pb_flat = false;

            if( nexttk && nexttk->i_chunk > 0 ) /* new run number */
                nexttk->chunk[nexttk->i_chunk].i_virtual_run_number++;
        }

        tk = nexttk;
    }

    /* reset */
    for( unsigned i=0; i < p_sys->i_tracks; i++ )
        p_sys->track[i].i_chunk = 0;
}