#include <libavutil/timestamp.h>
#include <libavformat/avformat.h>

int main(int argc, char **argv)
{
    AVOutputFormat *ofmt = NULL;
    AVFormatContext *ifmt_ctx = NULL, *ofmt_ctx = NULL;
    AVPacket pkt;
    const char *in_filename, *out_filename;
    int ret, i;
    int stream_index = 0;
    int *stream_mapping = NULL;
    int stream_mapping_size = 0;

    if (argc < 3) {
        printf("usage: %s input output\n"
               "API example program to remux a media file with libavformat and libavcodec.\n"
               "The output format is guessed according to the file extension.\n"
               "\n", argv[0]);
        return 1;
    }

    in_filename  = argv[1];
    out_filename = argv[2];

    // 1. 打开输入
    // 1.1 读取文件头，获取封装格式相关信息
    if ((ret = avformat_open_input(&ifmt_ctx, in_filename, 0, 0)) < 0) {
        printf("Could not open input file '%s'", in_filename);
        goto end;
    }
    
    // 1.2 解码一段数据，获取流相关信息
    if ((ret = avformat_find_stream_info(ifmt_ctx, 0)) < 0) {
        printf("Failed to retrieve input stream information");
        goto end;
    }

    av_dump_format(ifmt_ctx, 0, in_filename, 0);

    // 2. 打开输出
    // 2.1 分配输出 ctx
    avformat_alloc_output_context2(&ofmt_ctx, NULL, NULL, out_filename);
    if (!ofmt_ctx) {
        printf("Could not create output context\n");
        ret = AVERROR_UNKNOWN;
        goto end;
    }

    stream_mapping_size = ifmt_ctx->nb_streams;
    stream_mapping = av_mallocz_array(stream_mapping_size, sizeof(*stream_mapping));
    if (!stream_mapping) {
        ret = AVERROR(ENOMEM);
        goto end;
    }

    ofmt = ofmt_ctx->oformat;

    for (i = 0; i < ifmt_ctx->nb_streams; i++) {
        AVStream *out_stream;
        AVStream *in_stream = ifmt_ctx->streams[i];
        AVCodecParameters *in_codecpar = in_stream->codecpar;

        if (in_codecpar->codec_type != AVMEDIA_TYPE_AUDIO &&
            in_codecpar->codec_type != AVMEDIA_TYPE_VIDEO &&
            in_codecpar->codec_type != AVMEDIA_TYPE_SUBTITLE) {
            stream_mapping[i] = -1;
            continue;
        }

        stream_mapping[i] = stream_index++;

        // 2.2 将一个新流(out_stream)添加到输出文件(ofmt_ctx)
        out_stream = avformat_new_stream(ofmt_ctx, NULL);
        if (!out_stream) {
            printf("Failed allocating output stream\n");
            ret = AVERROR_UNKNOWN;
            goto end;
        }

        // 2.3 将当前输入流中的参数拷贝到输出流中
        ret = avcodec_parameters_copy(out_stream->codecpar, in_codecpar);
        if (ret < 0) {
            printf("Failed to copy codec parameters\n");
            goto end;
        }
        out_stream->codecpar->codec_tag = 0;
    }
    av_dump_format(ofmt_ctx, 0, out_filename, 1);

    if (!(ofmt->flags & AVFMT_NOFILE)) {    // TODO: 研究 AVFMT_NOFILE 标志
        // 2.4 创建并初始化一个 AVIOContext，用以访问 URL(out_filename)指定的资源
        ret = avio_open(&ofmt_ctx->pb, out_filename, AVIO_FLAG_WRITE);
        if (ret < 0) {
            printf("Could not open output file '%s'", out_filename);
            goto end;
        }
    }

    // 3. 数据处理
    // 3.1 写输出文件头
    ret = avformat_write_header(ofmt_ctx, NULL);
    if (ret < 0) {
        printf("Error occurred when opening output file\n");
        goto end;
    }

    while (1) {
        AVStream *in_stream, *out_stream;

        // 3.2 从输出流读取一个 packet
        ret = av_read_frame(ifmt_ctx, &pkt);
        if (ret < 0)
            break;

        in_stream  = ifmt_ctx->streams[pkt.stream_index];
        if (pkt.stream_index >= stream_mapping_size ||
            stream_mapping[pkt.stream_index] < 0) {
            av_packet_unref(&pkt);
            continue;
        }

        pkt.stream_index = stream_mapping[pkt.stream_index];
        out_stream = ofmt_ctx->streams[pkt.stream_index];

        /* copy packet */
        // 3.3 更新 packet 中的 pts 和 dts
        // 关于 AVStream.time_base 的说明：
        // 输入：输入流中含有 time_base，在 avformat_find_stream_info()中可取到每个流中的 time_base
        // 输出：avformat_write_header()会根据输出的封装格式确定每个流的 time_base 并写入文件中
        // AVPacket.pts 和 AVPacket.dts 的单位是 AVStream.time_base，不同的封装格式其 AVStream.time_base 不同
        // 所以输出文件中，每个 packet 需要根据输出封装格式重新计算 pts 和 dts
        av_packet_rescale_ts(&pkt, in_stream->time_base, out_stream->time_base);
        pkt.pos = -1;

        // 3.4 将 packet 写入输出
        ret = av_interleaved_write_frame(ofmt_ctx, &pkt);
        if (ret < 0) {
            printf("Error muxing packet\n");
            break;
        }
        av_packet_unref(&pkt);
    }

    // 3.5 写输出文件尾
    av_write_trailer(ofmt_ctx);
end:

    avformat_close_input(&ifmt_ctx);

    /* close output */
    if (ofmt_ctx && !(ofmt->flags & AVFMT_NOFILE))
        avio_closep(&ofmt_ctx->pb);
    avformat_free_context(ofmt_ctx);

    av_freep(&stream_mapping);

    if (ret < 0 && ret != AVERROR_EOF) {
        printf("Error occurred: %s\n", av_err2str(ret));
        return 1;
    }

    return 0;
}

gcc -o remuxing remuxing.c -lavformat -lavcodec -lavutil -g

think@opensuse> ffprobe tnliny.flv 
ffprobe version 4.1 Copyright (c) 2007-2018 the FFmpeg developers
Input #0, flv, from 'tnliny.flv':
  Metadata:
    encoder         : Lavf58.20.100
  Duration: 00:02:26.54, start: 0.000000, bitrate: 446 kb/s
    Stream #0:0: Video: h264 (High), yuv420p(progressive), 800x450, 25 fps, 25 tbr, 1k tbn, 50 tbc
    Stream #0:1: Audio: aac (LC), 44100 Hz, stereo, fltp

./remuxing tnliny.flv tnliny.ts