/*
  * RTP packetization for H.261 video (RFC 4587)
  * Copyright (c) 2014 Thomas Volkert <thomas@homer-conferencing.com>
  *
  * This file is part of FFmpeg.
  *
  * FFmpeg is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
  * License as published by the Free Software Foundation; either
  * version 2.1 of the License, or (at your option) any later version.
  *
  * FFmpeg is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Lesser General Public License for more details.
  *
  * You should have received a copy of the GNU Lesser General Public
  * License along with FFmpeg; if not, write to the Free Software
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
 #include "avformat.h"
 #include "rtpenc.h"
 

 #define RTP_H261_HEADER_SIZE 4
 

 static const uint8_t *find_resync_marker_reverse(const uint8_t *av_restrict start,
                                                  const uint8_t *av_restrict end)

 {
     const uint8_t *p = end - 1;
     start += 1; /* Make sure we never return the original start. */
     for (; p > start; p--) {
         if (p[0] == 0 && p[1] == 1)
             return p;
     }
     return end;
 }
 
 void ff_rtp_send_h261(AVFormatContext *ctx, const uint8_t *frame_buf, int frame_size)

 {

     int cur_frame_size;

     int last_packet_of_frame;

     RTPMuxContext *rtp_ctx = ctx->priv_data;

 
     /* use the default 90 KHz time stamp */
     rtp_ctx->timestamp = rtp_ctx->cur_timestamp;
 
     /* continue as long as not all frame data is processed */
     while (frame_size > 0) {

         /*
          * encode the H.261 payload header according to section 4.1 of RFC 4587:
          * (uses 4 bytes between RTP header and H.261 stream per packet)
          *
          *    0                   1                   2                   3
          *    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
          *   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          *   |SBIT |EBIT |I|V| GOBN  |   MBAP  |  QUANT  |  HMVD   |  VMVD   |
          *   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          *
          *      Start bit position (SBIT): 3 bits
          *      End bit position (EBIT): 3 bits
          *      INTRA-frame encoded data (I): 1 bit
          *      Motion Vector flag (V): 1 bit
          *      GOB number (GOBN): 4 bits
          *      Macroblock address predictor (MBAP): 5 bits
          *      Quantizer (QUANT): 5 bits
          *      Horizontal motion vector data (HMVD): 5 bits
          *      Vertical motion vector data (VMVD): 5 bits
          */
         rtp_ctx->buf[0] = 1; /* sbit=0, ebit=0, i=0, v=1 */
         rtp_ctx->buf[1] = 0; /* gobn=0, mbap=0 */
         rtp_ctx->buf[2] = 0; /* quant=0, hmvd=5 */
         rtp_ctx->buf[3] = 0; /* vmvd=0 */
         if (frame_size < 2 || frame_buf[0] != 0 || frame_buf[1] != 1) {

             /* A full, correct fix for this would be to make the H.261 encoder

              * support inserting extra GOB headers (triggered by setting e.g.
              * "-ps 1"), and including information about macroblock boundaries
              * (such as for h263_rfc2190). */
             av_log(ctx, AV_LOG_WARNING,

                    "RTP/H.261 packet not cut at a GOB boundary, not signaled correctly\n");

         }

         cur_frame_size = FFMIN(rtp_ctx->max_payload_size - RTP_H261_HEADER_SIZE, frame_size);

         /* look for a better place to split the frame into packets */
         if (cur_frame_size < frame_size) {
             const uint8_t *packet_end = find_resync_marker_reverse(frame_buf,
                                                                    frame_buf + cur_frame_size);
             cur_frame_size = packet_end - frame_buf;
         }

         /* calculate the "marker" bit for the RTP header */
         last_packet_of_frame = cur_frame_size == frame_size;

         /* complete and send RTP packet */
         memcpy(&rtp_ctx->buf[RTP_H261_HEADER_SIZE], frame_buf, cur_frame_size);
         ff_rtp_send_data(ctx, rtp_ctx->buf, RTP_H261_HEADER_SIZE + cur_frame_size, last_packet_of_frame);

         frame_buf  += cur_frame_size;
         frame_size -= cur_frame_size;

     }
 }