text: /*
   * Bitstream filter for unpacking DivX-style packed B-frames in MPEG-4 (divx_packed)
   * Copyright (c) 2015 Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.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 "avcodec.h"
  #include "bsf.h"
  #include "mpeg4video.h"
  
  typedef struct UnpackBFramesBSFContext {
      uint8_t *b_frame_buf;
      int      b_frame_buf_size;
  } UnpackBFramesBSFContext;
  
  /* search next start code */
  static unsigned int find_startcode(const uint8_t *buf, int buf_size, int *pos)
  {
      unsigned int startcode = 0xFF;
  
      for (; *pos < buf_size;) {
          startcode = ((startcode << 8) | buf[*pos]) & 0xFFFFFFFF;
          *pos +=1;
          if ((startcode & 0xFFFFFF00) != 0x100)
              continue;  /* no startcode */
          return startcode;
      }
  
      return 0;
  }
  
  /* determine the position of the packed marker in the userdata,
   * the number of VOPs and the position of the second VOP */
  static void scan_buffer(const uint8_t *buf, int buf_size,
                          int *pos_p, int *nb_vop, int *pos_vop2) {
      unsigned int startcode;
      int pos, i;
  
      for (pos = 0; pos < buf_size;) {
          startcode = find_startcode(buf, buf_size, &pos);
  
          if (startcode == USER_DATA_STARTCODE && pos_p) {
              /* check if the (DivX) userdata string ends with 'p' (packed) */
              for (i = 0; i < 255 && pos + i + 1 < buf_size; i++) {
                  if (buf[pos + i] == 'p' && buf[pos + i + 1] == '\0') {
                      *pos_p = pos + i;
                      break;
                  }
              }
          } else if (startcode == VOP_STARTCODE && nb_vop) {
              *nb_vop += 1;
              if (*nb_vop == 2 && pos_vop2) {
                  *pos_vop2 = pos - 4; /* subtract 4 bytes startcode */
              }
          }
      }
  }
  
  /* allocate new buffer and copy size bytes from src */
  static uint8_t *create_new_buffer(const uint8_t *src, int size) {
      uint8_t *dst = av_malloc(size + AV_INPUT_BUFFER_PADDING_SIZE);
  
      if (dst) {
          memcpy(dst, src, size);
          memset(dst + size, 0, AV_INPUT_BUFFER_PADDING_SIZE);
      }
  
      return dst;
  }
  
  static int mpeg4_unpack_bframes_filter(AVBSFContext *ctx, AVPacket *out)
  {
      UnpackBFramesBSFContext *s = ctx->priv_data;
      int pos_p = -1, nb_vop = 0, pos_vop2 = -1, ret = 0;
      AVPacket *in;
  
      ret = ff_bsf_get_packet(ctx, &in);
      if (ret < 0)
          return ret;
  
      scan_buffer(in->data, in->size, &pos_p, &nb_vop, &pos_vop2);
      av_log(ctx, AV_LOG_DEBUG, "Found %d VOP startcode(s) in this packet.\n", nb_vop);
  
      if (pos_vop2 >= 0) {
          if (s->b_frame_buf) {
              av_log(ctx, AV_LOG_WARNING,
                     "Missing one N-VOP packet, discarding one B-frame.\n");
              av_freep(&s->b_frame_buf);
              s->b_frame_buf_size = 0;
          }
          /* store the packed B-frame in the BSFContext */
          s->b_frame_buf_size = in->size - pos_vop2;
          s->b_frame_buf      = create_new_buffer(in->data + pos_vop2, s->b_frame_buf_size);
          if (!s->b_frame_buf) {
              s->b_frame_buf_size = 0;
              av_packet_free(&in);
              return AVERROR(ENOMEM);
          }
      }
  
      if (nb_vop > 2) {
          av_log(ctx, AV_LOG_WARNING,
         "Found %d VOP headers in one packet, only unpacking one.\n", nb_vop);
      }
  
      if (nb_vop == 1 && s->b_frame_buf) {
          /* use frame from BSFContext */
          av_packet_from_data(out, s->b_frame_buf, s->b_frame_buf_size);
          if (in->size <= MAX_NVOP_SIZE) {
              /* N-VOP */
              av_log(ctx, AV_LOG_DEBUG, "Skipping N-VOP.\n");
              s->b_frame_buf      = NULL;
              s->b_frame_buf_size = 0;
          } else {
              /* copy packet into BSFContext */
              s->b_frame_buf_size = in->size;
              s->b_frame_buf      = create_new_buffer(in->data, in->size);
              if (!s->b_frame_buf) {
                  s->b_frame_buf_size = 0;
                  av_packet_unref(out);
                  av_packet_free(&in);
                  return AVERROR(ENOMEM);
              }
          }
      } else if (nb_vop >= 2) {
          /* use first frame of the packet */
          av_packet_move_ref(out, in);
          out->size = pos_vop2;
      } else if (pos_p >= 0) {
          av_log(ctx, AV_LOG_DEBUG, "Updating DivX userdata (remove trailing 'p').\n");
          av_packet_move_ref(out, in);
          /* remove 'p' (packed) from the end of the (DivX) userdata string */
          out->data[pos_p] = '\0';
      } else {
          /* copy packet */
          av_packet_move_ref(out, in);
      }
  
      av_packet_free(&in);
  
      return 0;
  }
  
  static int mpeg4_unpack_bframes_init(AVBSFContext *ctx)
  {
      if (ctx->par_in->extradata) {
          int pos_p_ext = -1;
          scan_buffer(ctx->par_in->extradata, ctx->par_in->extradata_size, &pos_p_ext, NULL, NULL);
          if (pos_p_ext >= 0) {
              av_log(ctx, AV_LOG_DEBUG,
                     "Updating DivX userdata (remove trailing 'p') in extradata.\n");
              ctx->par_out->extradata[pos_p_ext] = '\0';
          }
      }
  
      return 0;
  }
  
  static void mpeg4_unpack_bframes_close(AVBSFContext *bsfc)
  {
      UnpackBFramesBSFContext *ctx = bsfc->priv_data;
      av_freep(&ctx->b_frame_buf);
  }
  
  static const enum AVCodecID codec_ids[] = {
      AV_CODEC_ID_MPEG4, AV_CODEC_ID_NONE,
  };
  
  const AVBitStreamFilter ff_mpeg4_unpack_bframes_bsf = {
      .name           = "mpeg4_unpack_bframes",
      .priv_data_size = sizeof(UnpackBFramesBSFContext),
      .init           = mpeg4_unpack_bframes_init,
      .filter         = mpeg4_unpack_bframes_filter,
      .close          = mpeg4_unpack_bframes_close,
      .codec_ids      = codec_ids,
  };
  