AVCodec ff_h264_decoder = {
    .name                  = "h264",
    .long_name             = NULL_IF_CONFIG_SMALL("H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10"),
    .type                  = AVMEDIA_TYPE_VIDEO,
    .id                    = AV_CODEC_ID_H264,
    .priv_data_size        = sizeof(H264Context),
    .init                  = h264_decode_init,
    .close                 = h264_decode_end,
    .decode                = h264_decode_frame,
    .capabilities          = /*AV_CODEC_CAP_DRAW_HORIZ_BAND |*/ AV_CODEC_CAP_DR1 |
                             AV_CODEC_CAP_DELAY | AV_CODEC_CAP_SLICE_THREADS |
                             AV_CODEC_CAP_FRAME_THREADS,
    .hw_configs            = (const AVCodecHWConfigInternal*[]) {
#if CONFIG_H264_DXVA2_HWACCEL
                               HWACCEL_DXVA2(h264),
#endif
#if CONFIG_H264_D3D11VA_HWACCEL
                               HWACCEL_D3D11VA(h264),
#endif
#if CONFIG_H264_D3D11VA2_HWACCEL
                               HWACCEL_D3D11VA2(h264),
#endif
#if CONFIG_H264_NVDEC_HWACCEL
                               HWACCEL_NVDEC(h264),
#endif
#if CONFIG_H264_VAAPI_HWACCEL
                               HWACCEL_VAAPI(h264),
#endif
#if CONFIG_H264_VDPAU_HWACCEL
                               HWACCEL_VDPAU(h264),
#endif
#if CONFIG_H264_VIDEOTOOLBOX_HWACCEL
                               HWACCEL_VIDEOTOOLBOX(h264),
#endif
                               NULL
                           },
    .caps_internal         = FF_CODEC_CAP_INIT_THREADSAFE | FF_CODEC_CAP_EXPORTS_CROPPING,
    .flush                 = flush_dpb,
    .init_thread_copy      = ONLY_IF_THREADS_ENABLED(decode_init_thread_copy),
    .update_thread_context = ONLY_IF_THREADS_ENABLED(ff_h264_update_thread_context),
    .profiles              = NULL_IF_CONFIG_SMALL(ff_h264_profiles),
    .priv_class            = &h264_class,
};

static av_cold int h264_decode_init(AVCodecContext *avctx)
{
    H264Context *h = avctx->priv_data;
    int ret;

    // 基本参数初始化
    ret = h264_init_context(avctx, h);
    if (ret < 0)
        return ret;

    // 初始化熵编码器
    ret = ff_thread_once(&h264_vlc_init, ff_h264_decode_init_vlc);
    if (ret != 0) {
        av_log(avctx, AV_LOG_ERROR, "pthread_once has failed.");
        return AVERROR_UNKNOWN;
    }

    if (avctx->ticks_per_frame == 1) {
        if(h->avctx->time_base.den < INT_MAX/2) {
            h->avctx->time_base.den *= 2;
        } else
            h->avctx->time_base.num /= 2;
    }
    avctx->ticks_per_frame = 2;

    // 初始化extradata
    if (avctx->extradata_size > 0 && avctx->extradata) {
        ret = ff_h264_decode_extradata(avctx->extradata, avctx->extradata_size,
                                       &h->ps, &h->is_avc, &h->nal_length_size,
                                       avctx->err_recognition, avctx);
        if (ret < 0) { 
            h264_decode_end(avctx);
            return ret;
        }
    }

    if (h->ps.sps && h->ps.sps->bitstream_restriction_flag &&
        h->avctx->has_b_frames < h->ps.sps->num_reorder_frames) {
        h->avctx->has_b_frames = h->ps.sps->num_reorder_frames;
    }

    avctx->internal->allocate_progress = 1;

    ff_h264_flush_change(h);

    if (h->enable_er < 0 && (avctx->active_thread_type & FF_THREAD_SLICE))
        h->enable_er = 0;

    if (h->enable_er && (avctx->active_thread_type & FF_THREAD_SLICE)) {
        av_log(avctx, AV_LOG_WARNING,
               "Error resilience with slice threads is enabled. It is unsafe and unsupported and may crash. "
               "Use it at your own risk\n");
    }

    return 0;
}

static int h264_decode_frame(AVCodecContext *avctx, void *data,
                             int *got_frame, AVPacket *avpkt)
{
    const uint8_t *buf = avpkt->data;
    int buf_size       = avpkt->size;
    H264Context *h     = avctx->priv_data;
    AVFrame *pict      = data;
    int buf_index;
    int ret;

    h->flags = avctx->flags;
    h->setup_finished = 0;
    h->nb_slice_ctx_queued = 0;

    ff_h264_unref_picture(h, &h->last_pic_for_ec);

    /* end of stream, output what is still in the buffers */
    if (buf_size == 0)
        return send_next_delayed_frame(h, pict, got_frame, 0);

    if (h->is_avc && av_packet_get_side_data(avpkt, AV_PKT_DATA_NEW_EXTRADATA, NULL)) {
        int side_size;
        uint8_t *side = av_packet_get_side_data(avpkt, AV_PKT_DATA_NEW_EXTRADATA, &side_size);
        if (is_extra(side, side_size))
            ff_h264_decode_extradata(side, side_size,
                                     &h->ps, &h->is_avc, &h->nal_length_size,
                                     avctx->err_recognition, avctx);
    }
    if (h->is_avc && buf_size >= 9 && buf[0]==1 && buf[2]==0 && (buf[4]&0xFC)==0xFC) {
        if (is_extra(buf, buf_size))
            return ff_h264_decode_extradata(buf, buf_size,
                                            &h->ps, &h->is_avc, &h->nal_length_size,
                                            avctx->err_recognition, avctx);
    }

    // 解析nalu
    buf_index = decode_nal_units(h, buf, buf_size);
    if (buf_index < 0)
        return AVERROR_INVALIDDATA;

    if (!h->cur_pic_ptr && h->nal_unit_type == H264_NAL_END_SEQUENCE) {
        av_assert0(buf_index <= buf_size);
        return send_next_delayed_frame(h, pict, got_frame, buf_index);
    }

    if (!(avctx->flags2 & AV_CODEC_FLAG2_CHUNKS) && (!h->cur_pic_ptr || !h->has_slice)) {
        if (avctx->skip_frame >= AVDISCARD_NONREF ||
            buf_size >= 4 && !memcmp("Q264", buf, 4))
            return buf_size;
        av_log(avctx, AV_LOG_ERROR, "no frame!\n");
        return AVERROR_INVALIDDATA;
    }

    if (!(avctx->flags2 & AV_CODEC_FLAG2_CHUNKS) ||
        (h->mb_y >= h->mb_height && h->mb_height)) {
        if ((ret = ff_h264_field_end(h, &h->slice_ctx[0], 0)) < 0)
            return ret;

        /* Wait for second field. */
        // 输出到pict
        if (h->next_output_pic) {
            ret = finalize_frame(h, pict, h->next_output_pic, got_frame);
            if (ret < 0)
                return ret;
        }
    }

    av_assert0(pict->buf[0] || !*got_frame);

    ff_h264_unref_picture(h, &h->last_pic_for_ec);

    return get_consumed_bytes(buf_index, buf_size);
}

static int decode_nal_units(H264Context *h, const uint8_t *buf, int buf_size)
{
    AVCodecContext *const avctx = h->avctx;
    int nals_needed = 0; ///< number of NALs that need decoding before the next frame thread starts
    int idr_cleared=0;
    int i, ret = 0;

    h->has_slice = 0;
    h->nal_unit_type= 0;

    if (!(avctx->flags2 & AV_CODEC_FLAG2_CHUNKS)) {
        h->current_slice = 0;
        if (!h->first_field) {
            h->cur_pic_ptr = NULL;
            ff_h264_sei_uninit(&h->sei);
        }
    }

    if (h->nal_length_size == 4) {
        if (buf_size > 8 && AV_RB32(buf) == 1 && AV_RB32(buf+5) > (unsigned)buf_size) {
            h->is_avc = 0;
        }else if(buf_size > 3 && AV_RB32(buf) > 1 && AV_RB32(buf) <= (unsigned)buf_size)
            h->is_avc = 1;
    }

    ret = ff_h2645_packet_split(&h->pkt, buf, buf_size, avctx, h->is_avc, h->nal_length_size,
                                avctx->codec_id, avctx->flags2 & AV_CODEC_FLAG2_FAST, 0);
    if (ret < 0) {
        av_log(avctx, AV_LOG_ERROR,
               "Error splitting the input into NAL units.\n");
        return ret;
    }

    if (avctx->active_thread_type & FF_THREAD_FRAME)
        nals_needed = get_last_needed_nal(h);
    if (nals_needed < 0)
        return nals_needed;

    for (i = 0; i < h->pkt.nb_nals; i++) {
        H2645NAL *nal = &h->pkt.nals[i];
        int max_slice_ctx, err;

        if (avctx->skip_frame >= AVDISCARD_NONREF &&
            nal->ref_idc == 0 && nal->type != H264_NAL_SEI)
            continue;

        // FIXME these should stop being context-global variables
        h->nal_ref_idc   = nal->ref_idc;
        h->nal_unit_type = nal->type;

        err = 0;
        switch (nal->type) {
        case H264_NAL_IDR_SLICE:
            if ((nal->data[1] & 0xFC) == 0x98) {
                av_log(h->avctx, AV_LOG_ERROR, "Invalid inter IDR frame\n");
                h->next_outputed_poc = INT_MIN;
                ret = -1;
                goto end;
            }
            if(!idr_cleared) {
                idr(h); // FIXME ensure we don‘t lose some frames if there is reordering
            }
            idr_cleared = 1;
            h->has_recovery_point = 1;
        case H264_NAL_SLICE:
            h->has_slice = 1;

            if ((err = ff_h264_queue_decode_slice(h, nal))) {
                H264SliceContext *sl = h->slice_ctx + h->nb_slice_ctx_queued;
                sl->ref_count[0] = sl->ref_count[1] = 0;
                break;
            }

            if (h->current_slice == 1) {
                if (avctx->active_thread_type & FF_THREAD_FRAME &&
                    i >= nals_needed && !h->setup_finished && h->cur_pic_ptr) {
                    ff_thread_finish_setup(avctx);
                    h->setup_finished = 1;
                }

                if (h->avctx->hwaccel &&
                    (ret = h->avctx->hwaccel->start_frame(h->avctx, buf, buf_size)) < 0)
                    goto end;
            }

            max_slice_ctx = avctx->hwaccel ? 1 : h->nb_slice_ctx;
            if (h->nb_slice_ctx_queued == max_slice_ctx) {
                if (h->avctx->hwaccel) {
                    ret = avctx->hwaccel->decode_slice(avctx, nal->raw_data, nal->raw_size);
                    h->nb_slice_ctx_queued = 0;
                } else
                    ret = ff_h264_execute_decode_slices(h);
                if (ret < 0 && (h->avctx->err_recognition & AV_EF_EXPLODE))
                    goto end;
            }
            break;
        case H264_NAL_DPA:
        case H264_NAL_DPB:
        case H264_NAL_DPC:
            avpriv_request_sample(avctx, "data partitioning");
            break;
        case H264_NAL_SEI:
            ret = ff_h264_sei_decode(&h->sei, &nal->gb, &h->ps, avctx);
            h->has_recovery_point = h->has_recovery_point || h->sei.recovery_point.recovery_frame_cnt != -1;
            if (avctx->debug & FF_DEBUG_GREEN_MD)
                debug_green_metadata(&h->sei.green_metadata, h->avctx);
            if (ret < 0 && (h->avctx->err_recognition & AV_EF_EXPLODE))
                goto end;
            break;
        case H264_NAL_SPS: {
            GetBitContext tmp_gb = nal->gb;
            if (avctx->hwaccel && avctx->hwaccel->decode_params) {
                ret = avctx->hwaccel->decode_params(avctx,
                                                    nal->type,
                                                    nal->raw_data,
                                                    nal->raw_size);
                if (ret < 0)
                    goto end;
            }
            if (ff_h264_decode_seq_parameter_set(&tmp_gb, avctx, &h->ps, 0) >= 0)
                break;
            av_log(h->avctx, AV_LOG_DEBUG,
                   "SPS decoding failure, trying again with the complete NAL\n");
            init_get_bits8(&tmp_gb, nal->raw_data + 1, nal->raw_size - 1);
            if (ff_h264_decode_seq_parameter_set(&tmp_gb, avctx, &h->ps, 0) >= 0)
                break;
            ff_h264_decode_seq_parameter_set(&nal->gb, avctx, &h->ps, 1);
            break;
        }
        case H264_NAL_PPS:
            if (avctx->hwaccel && avctx->hwaccel->decode_params) {
                ret = avctx->hwaccel->decode_params(avctx,
                                                    nal->type,
                                                    nal->raw_data,
                                                    nal->raw_size);
                if (ret < 0)
                    goto end;
            }
            ret = ff_h264_decode_picture_parameter_set(&nal->gb, avctx, &h->ps,
                                                       nal->size_bits);
            if (ret < 0 && (h->avctx->err_recognition & AV_EF_EXPLODE))
                goto end;
            break;
        case H264_NAL_AUD:
        case H264_NAL_END_SEQUENCE:
        case H264_NAL_END_STREAM:
        case H264_NAL_FILLER_DATA:
        case H264_NAL_SPS_EXT:
        case H264_NAL_AUXILIARY_SLICE:
            break;
        default:
            av_log(avctx, AV_LOG_DEBUG, "Unknown NAL code: %d (%d bits)\n",
                   nal->type, nal->size_bits);
        }

        if (err < 0) {
            av_log(h->avctx, AV_LOG_ERROR, "decode_slice_header error\n");
        }
    }

    ret = ff_h264_execute_decode_slices(h);
    if (ret < 0 && (h->avctx->err_recognition & AV_EF_EXPLODE))
        goto end;

    // set decode_error_flags to allow users to detect concealed decoding errors
    if ((ret < 0 || h->slice_ctx->er.error_occurred) && h->cur_pic_ptr) {
        h->cur_pic_ptr->f->decode_error_flags |= FF_DECODE_ERROR_DECODE_SLICES;
    }

    ret = 0;
end:

#if CONFIG_ERROR_RESILIENCE
    /*
     * FIXME: Error handling code does not seem to support interlaced
     * when slices span multiple rows
     * The ff_er_add_slice calls don‘t work right for bottom
     * fields; they cause massive erroneous error concealing
     * Error marking covers both fields (top and bottom).
     * This causes a mismatched s->error_count
     * and a bad error table. Further, the error count goes to
     * INT_MAX when called for bottom field, because mb_y is
     * past end by one (callers fault) and resync_mb_y != 0
     * causes problems for the first MB line, too.
     */
    if (!FIELD_PICTURE(h) && h->current_slice &&
        h->ps.sps == (const SPS*)h->ps.sps_list[h->ps.pps->sps_id]->data &&
        h->enable_er) {

        H264SliceContext *sl = h->slice_ctx;
        int use_last_pic = h->last_pic_for_ec.f->buf[0] && !sl->ref_count[0];

        ff_h264_set_erpic(&sl->er.cur_pic, h->cur_pic_ptr);

        if (use_last_pic) {
            ff_h264_set_erpic(&sl->er.last_pic, &h->last_pic_for_ec);
            sl->ref_list[0][0].parent = &h->last_pic_for_ec;
            memcpy(sl->ref_list[0][0].data, h->last_pic_for_ec.f->data, sizeof(sl->ref_list[0][0].data));
            memcpy(sl->ref_list[0][0].linesize, h->last_pic_for_ec.f->linesize, sizeof(sl->ref_list[0][0].linesize));
            sl->ref_list[0][0].reference = h->last_pic_for_ec.reference;
        } else if (sl->ref_count[0]) {
            ff_h264_set_erpic(&sl->er.last_pic, sl->ref_list[0][0].parent);
        } else
            ff_h264_set_erpic(&sl->er.last_pic, NULL);

        if (sl->ref_count[1])
            ff_h264_set_erpic(&sl->er.next_pic, sl->ref_list[1][0].parent);

        sl->er.ref_count = sl->ref_count[0];

        ff_er_frame_end(&sl->er);
        if (use_last_pic)
            memset(&sl->ref_list[0][0], 0, sizeof(sl->ref_list[0][0]));
    }
#endif /* CONFIG_ERROR_RESILIENCE */
    /* clean up */
    if (h->cur_pic_ptr && !h->droppable && h->has_slice) {
        ff_thread_report_progress(&h->cur_pic_ptr->tf, INT_MAX,
                                  h->picture_structure == PICT_BOTTOM_FIELD);
    }

    return (ret < 0) ? ret : buf_size;
}

static av_cold int h264_decode_end(AVCodecContext *avctx)
{
    H264Context *h = avctx->priv_data;
    int i;

    ff_h264_remove_all_refs(h);
    ff_h264_free_tables(h);

    for (i = 0; i < H264_MAX_PICTURE_COUNT; i++) {
        ff_h264_unref_picture(h, &h->DPB[i]);
        av_frame_free(&h->DPB[i].f);
    }
    memset(h->delayed_pic, 0, sizeof(h->delayed_pic));

    h->cur_pic_ptr = NULL;

    av_freep(&h->slice_ctx);
    h->nb_slice_ctx = 0;

    ff_h264_sei_uninit(&h->sei);
    ff_h264_ps_uninit(&h->ps);

    ff_h2645_packet_uninit(&h->pkt);

    ff_h264_unref_picture(h, &h->cur_pic);
    av_frame_free(&h->cur_pic.f);
    ff_h264_unref_picture(h, &h->last_pic_for_ec);
    av_frame_free(&h->last_pic_for_ec.f);

    return 0;
}