/*
  * RTP parser for H.261 payload format (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 "libavcodec/get_bits.h"

 #include "avformat.h"

 #include "avio_internal.h"

 #include "rtpdec_formats.h"
 

 #define RTP_H261_PAYLOAD_HEADER_SIZE 4

 
 struct PayloadContext {
     AVIOContext *buf;
     uint8_t      endbyte;
     int          endbyte_bits;
     uint32_t     timestamp;
 };
 

 static av_cold void h261_close_context(PayloadContext *pl_ctx)

 {
     /* return if context is invalid */
     if (!pl_ctx)
         return;
 
     /* free buffer if it is valid */

     ffio_free_dyn_buf(&pl_ctx->buf);

 }
 

 static int h261_handle_packet(AVFormatContext *ctx, PayloadContext *rtp_h261_ctx,
                               AVStream *st, AVPacket *pkt, uint32_t *timestamp,
                               const uint8_t *buf, int len, uint16_t seq,
                               int flags)

 {
     int sbit, ebit, gobn, mbap, quant;
     int res;
 

     /* drop data of previous packets in case of non-continuous (lossy) packet stream */
     if (rtp_h261_ctx->buf && rtp_h261_ctx->timestamp != *timestamp) {

         ffio_free_dyn_buf(&rtp_h261_ctx->buf);

         rtp_h261_ctx->endbyte_bits = 0;

     }
 

     /* sanity check for size of input packet: 1 byte payload at least */
     if (len < RTP_H261_PAYLOAD_HEADER_SIZE + 1) {
         av_log(ctx, AV_LOG_ERROR, "Too short RTP/H.261 packet, got %d bytes\n", len);

         return AVERROR_INVALIDDATA;
     }
 
     /*

      * decode 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
      */

     sbit  =  (buf[0] >> 5) & 0x07;
     ebit  =  (buf[0] >> 2) & 0x07;
     gobn  =  (buf[1] >> 4) & 0x0f;

     mbap  = ((buf[1] << 1) & 0x1e) | ((buf[2] >> 7) & 0x01);
     quant =  (buf[2] >> 2) & 0x1f;

 
     /* pass the H.261 payload header and continue with the actual payload */
     buf += RTP_H261_PAYLOAD_HEADER_SIZE;
     len -= RTP_H261_PAYLOAD_HEADER_SIZE;
 
     /* start frame buffering with new dynamic buffer */

     if (!rtp_h261_ctx->buf) {
         /* sanity check: a new frame starts with gobn=0, sbit=0, mbap=0, quant=0 */
         if (!gobn && !sbit && !mbap && !quant) {
             res = avio_open_dyn_buf(&rtp_h261_ctx->buf);

             if (res < 0)
                 return res;
             /* update the timestamp in the frame packet with the one from the RTP packet */

             rtp_h261_ctx->timestamp = *timestamp;

         } else {
             /* frame not started yet, need more packets */
             return AVERROR(EAGAIN);
         }
     }
 
     /* do the "byte merging" at the boundaries of two consecutive frame fragments */

     if (rtp_h261_ctx->endbyte_bits || sbit) {
         if (rtp_h261_ctx->endbyte_bits == sbit) {
             rtp_h261_ctx->endbyte     |= buf[0] & (0xff >> sbit);
             rtp_h261_ctx->endbyte_bits = 0;

             buf++;
             len--;

             avio_w8(rtp_h261_ctx->buf, rtp_h261_ctx->endbyte);

         } else {
             /* ebit/sbit values inconsistent, assuming packet loss */
             GetBitContext gb;
             init_get_bits(&gb, buf, len*8 - ebit);
             skip_bits(&gb, sbit);

             if (rtp_h261_ctx->endbyte_bits) {
                 rtp_h261_ctx->endbyte |= get_bits(&gb, 8 - rtp_h261_ctx->endbyte_bits);
                 avio_w8(rtp_h261_ctx->buf, rtp_h261_ctx->endbyte);

             }
             while (get_bits_left(&gb) >= 8)

                 avio_w8(rtp_h261_ctx->buf, get_bits(&gb, 8));
             rtp_h261_ctx->endbyte_bits = get_bits_left(&gb);
             if (rtp_h261_ctx->endbyte_bits)
                 rtp_h261_ctx->endbyte = get_bits(&gb, rtp_h261_ctx->endbyte_bits) <<
                                         (8 - rtp_h261_ctx->endbyte_bits);

             ebit = 0;

             len  = 0;

         }
     }
     if (ebit) {
         if (len > 0)

             avio_write(rtp_h261_ctx->buf, buf, len - 1);
         rtp_h261_ctx->endbyte_bits = 8 - ebit;
         rtp_h261_ctx->endbyte      = buf[len - 1] & (0xff << ebit);

     } else {

         avio_write(rtp_h261_ctx->buf, buf, len);

     }
 
     /* RTP marker bit means: last fragment of current frame was received;
        otherwise, an additional fragment is needed for the current frame */
     if (!(flags & RTP_FLAG_MARKER))
         return AVERROR(EAGAIN);
 
     /* write the completed last byte from the "byte merging" */

     if (rtp_h261_ctx->endbyte_bits)
         avio_w8(rtp_h261_ctx->buf, rtp_h261_ctx->endbyte);
     rtp_h261_ctx->endbyte_bits = 0;

 
     /* close frame buffering and create resulting A/V packet */

     res = ff_rtp_finalize_packet(pkt, &rtp_h261_ctx->buf, st->index);

     if (res < 0)
         return res;
 
     return 0;
 }
 

 const RTPDynamicProtocolHandler ff_h261_dynamic_handler = {

     .enc_name          = "H261",
     .codec_type        = AVMEDIA_TYPE_VIDEO,
     .codec_id          = AV_CODEC_ID_H261,

     .need_parsing      = AVSTREAM_PARSE_FULL,

     .priv_data_size    = sizeof(PayloadContext),

     .close             = h261_close_context,

     .parse_packet      = h261_handle_packet,

     .static_payload_id = 31,
 };