/**
 * This is the fundamental unit of time (in seconds) in terms
 * of which frame timestamps are represented.
 * 这是表示帧时间戳的基本时间单位(以秒为单位)。
**/
typedef struct AVRational{
    int num; ///< Numerator 分子
    int den; ///< Denominator 分母
} AVRational;

static inline double av_q2d(AVRational a){
    return a.num / (double) a.den;
}

//计算一桢在整个视频中的时间位置
timestamp(秒) = pts * av_q2d(st->time_base);

if (pkt->pts != AV_NOPTS_VALUE) {
    state->audio_clock = av_q2d(state->audio_st->time_base) * pkt->pts;
}

// 每秒钟音频播放的字节数 采样率 * 通道数 * 采样位数 (一个sample占用的字节数)
n = 2 * state->audio_ctx->channels;
state->audio_clock += (double) data_size /
                   (double) (n * state->audio_ctx->sample_rate);

double get_audio_clock(VideoState *state) {
    double pts;
    int buf_size, bytes_per_sec;

    //上一步获取的PTS
    pts = state->audio_clock;
    // 音频缓冲区还没有播放的数据
    buf_size = state->audio_buf_size - state->audio_buf_index; 
    // 每秒钟音频播放的字节数
    bytes_per_sec = state->audio_ctx->sample_rate * state->audio_ctx->channels * 2;
    pts -= (double) buf_size / bytes_per_sec;
    return pts;
}

avcodec_send_packet(state->video_ctx, packet);
while (avcodec_receive_frame(state->video_ctx, pFrame) == 0) {
    if ((pts = pFrame->best_effort_timestamp) != AV_NOPTS_VALUE) {
    } else {
        pts = 0;
    }
    pts *= av_q2d(state->video_st->time_base); // 时间基换算，单位为秒

    pts = synchronize_video(state, pFrame, pts);
    
    av_packet_unref(packet);
}

double synchronize_video(VideoState *state, AVFrame *src_frame, double pts) {

    double frame_delay;

    if (pts != 0) {
        state->video_clock = pts;
    } else {
        pts = state->video_clock;// PTS错误，使用上一次的PTS值
    }
    //根据时间基，计算每一帧的间隔时间
    frame_delay = av_q2d(state->video_ctx->time_base);
    //解码后的帧要延时的时间
    frame_delay += src_frame->repeat_pict * (frame_delay * 0.5);
    state->video_clock += frame_delay;//得到video_clock,实际上也是预测的下一帧视频的时间
    return pts;
}