vorbisfile.c

/********************************************************************
 *                                                                  *
 * THIS FILE IS PART OF THE OggVorbis SOFTWARE CODEC SOURCE CODE.   *
 * USE, DISTRIBUTION AND REPRODUCTION OF THIS LIBRARY SOURCE IS     *
 * GOVERNED BY A BSD-STYLE SOURCE LICENSE INCLUDED WITH THIS SOURCE *
 * IN 'COPYING'. PLEASE READ THESE TERMS BEFORE DISTRIBUTING.       *
 *                                                                  *
 * THE OggVorbis SOURCE CODE IS (C) COPYRIGHT 1994-2009             *
 * by the Xiph.Org Foundation http://www.xiph.org/                  *
 *                                                                  *
 ********************************************************************

 function: stdio-based convenience library for opening/seeking/decoding
 last mod: $Id$

 ********************************************************************/

#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <math.h>

#include "vorbis/codec.h"

/* we don't need or want the static callback symbols here */
#define OV_EXCLUDE_STATIC_CALLBACKS
#include "vorbis/vorbisfile.h"

#include "os.h"
#include "misc.h"

/* A 'chained bitstream' is a Vorbis bitstream that contains more than
   one logical bitstream arranged end to end (the only form of Ogg
   multiplexing allowed in a Vorbis bitstream; grouping [parallel
   multiplexing] is not allowed in Vorbis) */

/* A Vorbis file can be played beginning to end (streamed) without
   worrying ahead of time about chaining (see decoder_example.c).  If
   we have the whole file, however, and want random access
   (seeking/scrubbing) or desire to know the total length/time of a
   file, we need to account for the possibility of chaining. */

/* We can handle things a number of ways; we can determine the entire
   bitstream structure right off the bat, or find pieces on demand.
   This example determines and caches structure for the entire
   bitstream, but builds a virtual decoder on the fly when moving
   between links in the chain. */

/* There are also different ways to implement seeking.  Enough
   information exists in an Ogg bitstream to seek to
   sample-granularity positions in the output.  Or, one can seek by
   picking some portion of the stream roughly in the desired area if
   we only want coarse navigation through the stream. */

/*************************************************************************
 * Many, many internal helpers.  The intention is not to be confusing;
 * rampant duplication and monolithic function implementation would be
 * harder to understand anyway.  The high level functions are last.  Begin
 * grokking near the end of the file */

/* read a little more data from the file/pipe into the ogg_sync framer
*/
#define CHUNKSIZE 65536

static long _get_data(OggVorbis_File *vf){
  errno=0;
  if(!(vf->callbacks.read_func))return(-1);
  if(vf->datasource){
    char *buffer=ogg_sync_buffer(&vf->oy,CHUNKSIZE);
    long bytes=(vf->callbacks.read_func)(buffer,1,CHUNKSIZE,vf->datasource);
    if(bytes>0)ogg_sync_wrote(&vf->oy,bytes);
    if(bytes==0 && errno)return(-1);
    return(bytes);
  }else
    return(0);
}

/* save a tiny smidge of verbosity to make the code more readable */
static int _seek_helper(OggVorbis_File *vf,ogg_int64_t offset){
  if(vf->datasource){
    if(!(vf->callbacks.seek_func)||
       (vf->callbacks.seek_func)(vf->datasource, offset, SEEK_SET) == -1)
      return OV_EREAD;
    vf->offset=offset;
    ogg_sync_reset(&vf->oy);
  }else{
    /* shouldn't happen unless someone writes a broken callback */
    return OV_EFAULT;
  }
  return 0;
}

/* The read/seek functions track absolute position within the stream */

/* from the head of the stream, get the next page.  boundary specifies
   if the function is allowed to fetch more data from the stream (and
   how much) or only use internally buffered data.

   boundary: -1) unbounded search
              0) read no additional data; use cached only
              n) search for a new page beginning for n bytes

   return:   <0) did not find a page (OV_FALSE, OV_EOF, OV_EREAD)
              n) found a page at absolute offset n */

static ogg_int64_t _get_next_page(OggVorbis_File *vf,ogg_page *og,
                                  ogg_int64_t boundary){
  if(boundary>0)boundary+=vf->offset;
  while(1){
    long more;

    if(boundary>0 && vf->offset>=boundary)return(OV_FALSE);
    more=ogg_sync_pageseek(&vf->oy,og);

    if(more<0){
      /* skipped n bytes */
      vf->offset-=more;
    }else{
      if(more==0){
        /* send more paramedics */
        if(!boundary)return(OV_FALSE);
        {
          long ret=_get_data(vf);
          if(ret==0)return(OV_EOF);
          if(ret<0)return(OV_EREAD);
        }
      }else{
        /* got a page.  Return the offset at the page beginning,
           advance the internal offset past the page end */
        ogg_int64_t ret=vf->offset;
        vf->offset+=more;
        return(ret);

      }
    }
  }
}

/* find the latest page beginning before the current stream cursor
   position. Much dirtier than the above as Ogg doesn't have any
   backward search linkage.  no 'readp' as it will certainly have to
   read. */
/* returns offset or OV_EREAD, OV_FAULT */
static ogg_int64_t _get_prev_page(OggVorbis_File *vf,ogg_page *og){
  ogg_int64_t begin=vf->offset;
  ogg_int64_t end=begin;
  ogg_int64_t ret;
  ogg_int64_t offset=-1;

  while(offset==-1){
    begin-=CHUNKSIZE;
    if(begin<0)
      begin=0;

    ret=_seek_helper(vf,begin);
    if(ret)return(ret);

    while(vf->offset<end){
      memset(og,0,sizeof(*og));
      ret=_get_next_page(vf,og,end-vf->offset);
      if(ret==OV_EREAD)return(OV_EREAD);
      if(ret<0){
        break;
      }else{
        offset=ret;
      }
    }
  }

  /* In a fully compliant, non-multiplexed stream, we'll still be
     holding the last page.  In multiplexed (or noncompliant streams),
     we will probably have to re-read the last page we saw */
  if(og->header_len==0){
    ret=_seek_helper(vf,offset);
    if(ret)return(ret);

    ret=_get_next_page(vf,og,CHUNKSIZE);
    if(ret<0)
      /* this shouldn't be possible */
      return(OV_EFAULT);
  }

  return(offset);
}

static void _add_serialno(ogg_page *og,long **serialno_list, int *n){
  long s = ogg_page_serialno(og);
  (*n)++;

  if(*serialno_list){
    *serialno_list = _ogg_realloc(*serialno_list, sizeof(**serialno_list)*(*n));
  }else{
    *serialno_list = _ogg_malloc(sizeof(**serialno_list));
  }

  (*serialno_list)[(*n)-1] = s;
}

/* returns nonzero if found */
static int _lookup_serialno(long s, long *serialno_list, int n){
  if(serialno_list){
    while(n--){
      if(*serialno_list == s) return 1;
      serialno_list++;
    }
  }
  return 0;
}

static int _lookup_page_serialno(ogg_page *og, long *serialno_list, int n){
  long s = ogg_page_serialno(og);
  return _lookup_serialno(s,serialno_list,n);
}

/* performs the same search as _get_prev_page, but prefers pages of
   the specified serial number. If a page of the specified serialno is
   spotted during the seek-back-and-read-forward, it will return the
   info of last page of the matching serial number instead of the very
   last page.  If no page of the specified serialno is seen, it will
   return the info of last page and alter *serialno.  */
static ogg_int64_t _get_prev_page_serial(OggVorbis_File *vf,
                                         long *serial_list, int serial_n,
                                         int *serialno, ogg_int64_t *granpos){
  ogg_page og;
  ogg_int64_t begin=vf->offset;
  ogg_int64_t end=begin;
  ogg_int64_t ret;

  ogg_int64_t prefoffset=-1;
  ogg_int64_t offset=-1;
  ogg_int64_t ret_serialno=-1;
  ogg_int64_t ret_gran=-1;

  while(offset==-1){
    begin-=CHUNKSIZE;
    if(begin<0)
      begin=0;

    ret=_seek_helper(vf,begin);
    if(ret)return(ret);

    while(vf->offset<end){
      ret=_get_next_page(vf,&og,end-vf->offset);
      if(ret==OV_EREAD)return(OV_EREAD);
      if(ret<0){
        break;
      }else{
        ret_serialno=ogg_page_serialno(&og);
        ret_gran=ogg_page_granulepos(&og);
        offset=ret;

        if(ret_serialno == *serialno){
          prefoffset=ret;
          *granpos=ret_gran;
        }

        if(!_lookup_serialno(ret_serialno,serial_list,serial_n)){
          /* we fell off the end of the link, which means we seeked
             back too far and shouldn't have been looking in that link
             to begin with.  If we found the preferred serial number,
             forget that we saw it. */
          prefoffset=-1;
        }
      }
    }
  }

  /* we're not interested in the page... just the serialno and granpos. */
  if(prefoffset>=0)return(prefoffset);

  *serialno = ret_serialno;
  *granpos = ret_gran;
  return(offset);

}

/* uses the local ogg_stream storage in vf; this is important for
   non-streaming input sources */
static int _fetch_headers(OggVorbis_File *vf,vorbis_info *vi,vorbis_comment *vc,
                          long **serialno_list, int *serialno_n,
                          ogg_page *og_ptr){
  ogg_page og;
  ogg_packet op;
  int i,ret;
  int allbos=0;

  if(!og_ptr){
    ogg_int64_t llret=_get_next_page(vf,&og,CHUNKSIZE);
    if(llret==OV_EREAD)return(OV_EREAD);
    if(llret<0)return(OV_ENOTVORBIS);
    og_ptr=&og;
  }

  vorbis_info_init(vi);
  vorbis_comment_init(vc);
  vf->ready_state=OPENED;

  /* extract the serialnos of all BOS pages + the first set of vorbis
     headers we see in the link */

  while(ogg_page_bos(og_ptr)){
    if(serialno_list){
      if(_lookup_page_serialno(og_ptr,*serialno_list,*serialno_n)){
        /* a dupe serialnumber in an initial header packet set == invalid stream */
        if(*serialno_list)_ogg_free(*serialno_list);
        *serialno_list=0;
        *serialno_n=0;
        ret=OV_EBADHEADER;
        goto bail_header;
      }

      _add_serialno(og_ptr,serialno_list,serialno_n);
    }

    if(vf->ready_state<STREAMSET){
      /* we don't have a vorbis stream in this link yet, so begin
         prospective stream setup. We need a stream to get packets */
      ogg_stream_reset_serialno(&vf->os,ogg_page_serialno(og_ptr));
      ogg_stream_pagein(&vf->os,og_ptr);

      if(ogg_stream_packetout(&vf->os,&op) > 0 &&
         vorbis_synthesis_idheader(&op)){
        /* vorbis header; continue setup */
        vf->ready_state=STREAMSET;
        if((ret=vorbis_synthesis_headerin(vi,vc,&op))){
          ret=OV_EBADHEADER;
          goto bail_header;
        }
      }
    }

    /* get next page */
    {
      ogg_int64_t llret=_get_next_page(vf,og_ptr,CHUNKSIZE);
      if(llret==OV_EREAD){
        ret=OV_EREAD;
        goto bail_header;
      }
      if(llret<0){
        ret=OV_ENOTVORBIS;
        goto bail_header;
      }

      /* if this page also belongs to our vorbis stream, submit it and break */
      if(vf->ready_state==STREAMSET &&
         vf->os.serialno == ogg_page_serialno(og_ptr)){
        ogg_stream_pagein(&vf->os,og_ptr);
        break;
      }
    }
  }

  if(vf->ready_state!=STREAMSET){
    ret = OV_ENOTVORBIS;
    goto bail_header;
  }

  while(1){

    i=0;
    while(i<2){ /* get a page loop */

      while(i<2){ /* get a packet loop */

        int result=ogg_stream_packetout(&vf->os,&op);
        if(result==0)break;
        if(result==-1){
          ret=OV_EBADHEADER;
          goto bail_header;
        }

        if((ret=vorbis_synthesis_headerin(vi,vc,&op)))
          goto bail_header;

        i++;
      }

      while(i<2){
        if(_get_next_page(vf,og_ptr,CHUNKSIZE)<0){
          ret=OV_EBADHEADER;
          goto bail_header;
        }

        /* if this page belongs to the correct stream, go parse it */
        if(vf->os.serialno == ogg_page_serialno(og_ptr)){
          ogg_stream_pagein(&vf->os,og_ptr);
          break;
        }

        /* if we never see the final vorbis headers before the link
           ends, abort */
        if(ogg_page_bos(og_ptr)){
          if(allbos){
            ret = OV_EBADHEADER;
            goto bail_header;
          }else
            allbos=1;
        }

        /* otherwise, keep looking */
      }
    }

    return 0;
  }

 bail_header:
  vorbis_info_clear(vi);
  vorbis_comment_clear(vc);
  vf->ready_state=OPENED;

  return ret;
}

/* Starting from current cursor position, get initial PCM offset of
   next page.  Consumes the page in the process without decoding
   audio, however this is only called during stream parsing upon
   seekable open. */
static ogg_int64_t _initial_pcmoffset(OggVorbis_File *vf, vorbis_info *vi){
  ogg_page    og;
  ogg_int64_t accumulated=0;
  long        lastblock=-1;
  int         result;
  int         serialno = vf->os.serialno;

  while(1){
    ogg_packet op;
    if(_get_next_page(vf,&og,-1)<0)
      break; /* should not be possible unless the file is truncated/mangled */

    if(ogg_page_bos(&og)) break;
    if(ogg_page_serialno(&og)!=serialno) continue;

    /* count blocksizes of all frames in the page */
    ogg_stream_pagein(&vf->os,&og);
    while((result=ogg_stream_packetout(&vf->os,&op))){
      if(result>0){ /* ignore holes */
        long thisblock=vorbis_packet_blocksize(vi,&op);
        if(lastblock!=-1)
          accumulated+=(lastblock+thisblock)>>2;
        lastblock=thisblock;
      }
    }

    if(ogg_page_granulepos(&og)!=-1){
      /* pcm offset of last packet on the first audio page */
      accumulated= ogg_page_granulepos(&og)-accumulated;
      break;
    }
  }

  /* less than zero?  This is a stream with samples trimmed off
     the beginning, a normal occurrence; set the offset to zero */
  if(accumulated<0)accumulated=0;

  return accumulated;
}

/* finds each bitstream link one at a time using a bisection search
   (has to begin by knowing the offset of the lb's initial page).
   Recurses for each link so it can alloc the link storage after
   finding them all, then unroll and fill the cache at the same time */
static int _bisect_forward_serialno(OggVorbis_File *vf,
                                    ogg_int64_t begin,
                                    ogg_int64_t searched,
                                    ogg_int64_t end,
                                    ogg_int64_t endgran,
                                    int endserial,
                                    long *currentno_list,
                                    int  currentnos,
                                    long m){
  ogg_int64_t pcmoffset;
  ogg_int64_t dataoffset=searched;
  ogg_int64_t endsearched=end;
  ogg_int64_t next=end;
  ogg_int64_t searchgran=-1;
  ogg_page og;
  ogg_int64_t ret,last;
  int serialno = vf->os.serialno;

  /* invariants:
     we have the headers and serialnos for the link beginning at 'begin'
     we have the offset and granpos of the last page in the file (potentially
       not a page we care about)
  */

  /* Is the last page in our list of current serialnumbers? */
  if(_lookup_serialno(endserial,currentno_list,currentnos)){

    /* last page is in the starting serialno list, so we've bisected
       down to (or just started with) a single link.  Now we need to
       find the last vorbis page belonging to the first vorbis stream
       for this link. */

    while(endserial != serialno){
      endserial = serialno;
      vf->offset=_get_prev_page_serial(vf,currentno_list,currentnos,&endserial,&endgran);
    }

    vf->links=m+1;
    if(vf->offsets)_ogg_free(vf->offsets);
    if(vf->serialnos)_ogg_free(vf->serialnos);
    if(vf->dataoffsets)_ogg_free(vf->dataoffsets);

    vf->offsets=_ogg_malloc((vf->links+1)*sizeof(*vf->offsets));
    vf->vi=_ogg_realloc(vf->vi,vf->links*sizeof(*vf->vi));
    vf->vc=_ogg_realloc(vf->vc,vf->links*sizeof(*vf->vc));
    vf->serialnos=_ogg_malloc(vf->links*sizeof(*vf->serialnos));
    vf->dataoffsets=_ogg_malloc(vf->links*sizeof(*vf->dataoffsets));
    vf->pcmlengths=_ogg_malloc(vf->links*2*sizeof(*vf->pcmlengths));

    vf->offsets[m+1]=end;
    vf->offsets[m]=begin;
    vf->pcmlengths[m*2+1]=endgran;

  }else{

    long *next_serialno_list=NULL;
    int next_serialnos=0;
    vorbis_info vi;
    vorbis_comment vc;

    /* the below guards against garbage seperating the last and
       first pages of two links. */
    while(searched<endsearched){
      ogg_int64_t bisect;

      if(endsearched-searched<CHUNKSIZE){
        bisect=searched;
      }else{
        bisect=(searched+endsearched)/2;
      }

      ret=_seek_helper(vf,bisect);
      if(ret)return(ret);

      last=_get_next_page(vf,&og,-1);
      if(last==OV_EREAD)return(OV_EREAD);
      if(last<0 || !_lookup_page_serialno(&og,currentno_list,currentnos)){
        endsearched=bisect;
        if(last>=0)next=last;
      }else{
        searched=last+og.header_len+og.body_len;
      }
    }

    /* Bisection point found */

    /* for the time being, fetch end PCM offset the simple way */
    {
      int testserial = serialno+1;
      vf->offset = next;
      while(testserial != serialno){
        testserial = serialno;
        vf->offset=_get_prev_page_serial(vf,currentno_list,currentnos,&testserial,&searchgran);
      }
    }

    if(vf->offset!=next){
      ret=_seek_helper(vf,next);
      if(ret)return(ret);
    }

    ret=_fetch_headers(vf,&vi,&vc,&next_serialno_list,&next_serialnos,NULL);
    if(ret)return(ret);
    serialno = vf->os.serialno;
    dataoffset = vf->offset;

    /* this will consume a page, however the next bistection always
       starts with a raw seek */
    pcmoffset = _initial_pcmoffset(vf,&vi);

    ret=_bisect_forward_serialno(vf,next,vf->offset,end,endgran,endserial,
                                 next_serialno_list,next_serialnos,m+1);
    if(ret)return(ret);

    if(next_serialno_list)_ogg_free(next_serialno_list);

    vf->offsets[m+1]=next;
    vf->serialnos[m+1]=serialno;
    vf->dataoffsets[m+1]=dataoffset;

    vf->vi[m+1]=vi;
    vf->vc[m+1]=vc;

    vf->pcmlengths[m*2+1]=searchgran;
    vf->pcmlengths[m*2+2]=pcmoffset;
    vf->pcmlengths[m*2+3]-=pcmoffset;

  }
  return(0);
}

static int _make_decode_ready(OggVorbis_File *vf){
  if(vf->ready_state>STREAMSET)return 0;
  if(vf->ready_state<STREAMSET)return OV_EFAULT;
  if(vf->seekable){
    if(vorbis_synthesis_init(&vf->vd,vf->vi+vf->current_link))
      return OV_EBADLINK;
  }else{
    if(vorbis_synthesis_init(&vf->vd,vf->vi))
      return OV_EBADLINK;
  }
  vorbis_block_init(&vf->vd,&vf->vb);
  vf->ready_state=INITSET;
  vf->bittrack=0.f;
  vf->samptrack=0.f;
  return 0;
}

static int _open_seekable2(OggVorbis_File *vf){
  ogg_int64_t dataoffset=vf->dataoffsets[0],end,endgran=-1;
  int endserial=vf->os.serialno;
  int serialno=vf->os.serialno;

  /* we're partially open and have a first link header state in
     storage in vf */

  /* fetch initial PCM offset */
  ogg_int64_t pcmoffset = _initial_pcmoffset(vf,vf->vi);

  /* we can seek, so set out learning all about this file */
  if(vf->callbacks.seek_func && vf->callbacks.tell_func){
    (vf->callbacks.seek_func)(vf->datasource,0,SEEK_END);
    vf->offset=vf->end=(vf->callbacks.tell_func)(vf->datasource);
  }else{
    vf->offset=vf->end=-1;
  }

  /* If seek_func is implemented, tell_func must also be implemented */
  if(vf->end==-1) return(OV_EINVAL);

  /* Get the offset of the last page of the physical bitstream, or, if
     we're lucky the last vorbis page of this link as most OggVorbis
     files will contain a single logical bitstream */
  end=_get_prev_page_serial(vf,vf->serialnos+2,vf->serialnos[1],&endserial,&endgran);
  if(end<0)return(end);

  /* now determine bitstream structure recursively */
  if(_bisect_forward_serialno(vf,0,dataoffset,vf->offset,endgran,endserial,
                              vf->serialnos+2,vf->serialnos[1],0)<0)return(OV_EREAD);

  vf->offsets[0]=0;
  vf->serialnos[0]=serialno;
  vf->dataoffsets[0]=dataoffset;
  vf->pcmlengths[0]=pcmoffset;
  vf->pcmlengths[1]-=pcmoffset;

  return(ov_raw_seek(vf,dataoffset));
}

/* clear out the current logical bitstream decoder */
static void _decode_clear(OggVorbis_File *vf){
  vorbis_dsp_clear(&vf->vd);
  vorbis_block_clear(&vf->vb);
  vf->ready_state=OPENED;
}

/* fetch and process a packet.  Handles the case where we're at a
   bitstream boundary and dumps the decoding machine.  If the decoding
   machine is unloaded, it loads it.  It also keeps pcm_offset up to
   date (seek and read both use this.  seek uses a special hack with
   readp).

   return: <0) error, OV_HOLE (lost packet) or OV_EOF
            0) need more data (only if readp==0)
            1) got a packet
*/

static int _fetch_and_process_packet(OggVorbis_File *vf,
                                     ogg_packet *op_in,
                                     int readp,
                                     int spanp){
  ogg_page og;

  /* handle one packet.  Try to fetch it from current stream state */
  /* extract packets from page */
  while(1){

    if(vf->ready_state==STREAMSET){
      int ret=_make_decode_ready(vf);
      if(ret<0)return ret;
    }

    /* process a packet if we can. */

    if(vf->ready_state==INITSET){
      while(1) {
              ogg_packet op;
              ogg_packet *op_ptr=(op_in?op_in:&op);
        int result=ogg_stream_packetout(&vf->os,op_ptr);
        ogg_int64_t granulepos;

        op_in=NULL;
        if(result==-1)return(OV_HOLE); /* hole in the data. */
        if(result>0){
          /* got a packet.  process it */
          granulepos=op_ptr->granulepos;
          if(!vorbis_synthesis(&vf->vb,op_ptr)){ /* lazy check for lazy
                                                    header handling.  The
                                                    header packets aren't
                                                    audio, so if/when we
                                                    submit them,
                                                    vorbis_synthesis will
                                                    reject them */

            /* suck in the synthesis data and track bitrate */
            {
              int oldsamples=vorbis_synthesis_pcmout(&vf->vd,NULL);
              /* for proper use of libvorbis within libvorbisfile,
                 oldsamples will always be zero. */
              if(oldsamples)return(OV_EFAULT);

              vorbis_synthesis_blockin(&vf->vd,&vf->vb);
              vf->samptrack+=vorbis_synthesis_pcmout(&vf->vd,NULL)-oldsamples;
              vf->bittrack+=op_ptr->bytes*8;
            }

            /* update the pcm offset. */
            if(granulepos!=-1 && !op_ptr->e_o_s){
              int link=(vf->seekable?vf->current_link:0);
              int i,samples;

              /* this packet has a pcm_offset on it (the last packet
                 completed on a page carries the offset) After processing
                 (above), we know the pcm position of the *last* sample
                 ready to be returned. Find the offset of the *first*

                 As an aside, this trick is inaccurate if we begin
                 reading anew right at the last page; the end-of-stream
                 granulepos declares the last frame in the stream, and the
                 last packet of the last page may be a partial frame.
                 So, we need a previous granulepos from an in-sequence page
                 to have a reference point.  Thus the !op_ptr->e_o_s clause
                 above */

              if(vf->seekable && link>0)
                granulepos-=vf->pcmlengths[link*2];
              if(granulepos<0)granulepos=0; /* actually, this
                                               shouldn't be possible
                                               here unless the stream
                                               is very broken */

              samples=vorbis_synthesis_pcmout(&vf->vd,NULL);

              granulepos-=samples;
              for(i=0;i<link;i++)
                granulepos+=vf->pcmlengths[i*2+1];
              vf->pcm_offset=granulepos;
            }
            return(1);
          }
        }
        else
          break;
      }
    }

    if(vf->ready_state>=OPENED){
      ogg_int64_t ret;

      while(1){
        /* the loop is not strictly necessary, but there's no sense in
           doing the extra checks of the larger loop for the common
           case in a multiplexed bistream where the page is simply
           part of a different logical bitstream; keep reading until
           we get one with the correct serialno */

        if(!readp)return(0);
        if((ret=_get_next_page(vf,&og,-1))<0){
          return(OV_EOF); /* eof. leave unitialized */
        }

        /* bitrate tracking; add the header's bytes here, the body bytes
           are done by packet above */
        vf->bittrack+=og.header_len*8;

        if(vf->ready_state==INITSET){
          if(vf->current_serialno!=ogg_page_serialno(&og)){

            /* two possibilities:
               1) our decoding just traversed a bitstream boundary
               2) another stream is multiplexed into this logical section */

            if(ogg_page_bos(&og)){
              /* boundary case */
              if(!spanp)
                return(OV_EOF);

              _decode_clear(vf);

              if(!vf->seekable){
                vorbis_info_clear(vf->vi);
                vorbis_comment_clear(vf->vc);
              }
              break;

            }else
              continue; /* possibility #2 */
          }
        }

        break;
      }
    }

    /* Do we need to load a new machine before submitting the page? */
    /* This is different in the seekable and non-seekable cases.

       In the seekable case, we already have all the header
       information loaded and cached; we just initialize the machine
       with it and continue on our merry way.

       In the non-seekable (streaming) case, we'll only be at a
       boundary if we just left the previous logical bitstream and
       we're now nominally at the header of the next bitstream
    */

    if(vf->ready_state!=INITSET){
      int link;

      if(vf->ready_state<STREAMSET){
        if(vf->seekable){
          long serialno = ogg_page_serialno(&og);

          /* match the serialno to bitstream section.  We use this rather than
             offset positions to avoid problems near logical bitstream
             boundaries */

          for(link=0;link<vf->links;link++)
            if(vf->serialnos[link]==serialno)break;

          if(link==vf->links) continue; /* not the desired Vorbis
                                           bitstream section; keep
                                           trying */

          vf->current_serialno=serialno;
          vf->current_link=link;

          ogg_stream_reset_serialno(&vf->os,vf->current_serialno);
          vf->ready_state=STREAMSET;

        }else{
          /* we're streaming */
          /* fetch the three header packets, build the info struct */

          int ret=_fetch_headers(vf,vf->vi,vf->vc,NULL,NULL,&og);
          if(ret)return(ret);
          vf->current_serialno=vf->os.serialno;
          vf->current_link++;
          link=0;
        }
      }
    }

    /* the buffered page is the data we want, and we're ready for it;
       add it to the stream state */
    ogg_stream_pagein(&vf->os,&og);

  }
}

/* if, eg, 64 bit stdio is configured by default, this will build with
   fseek64 */
static int _fseek64_wrap(FILE *f,ogg_int64_t off,int whence){
  if(f==NULL)return(-1);
  return fseek(f,off,whence);
}

static int _ov_open1(void *f,OggVorbis_File *vf,char *initial,
                     long ibytes, ov_callbacks callbacks){
  int offsettest=((f && callbacks.seek_func)?callbacks.seek_func(f,0,SEEK_CUR):-1);
  long *serialno_list=NULL;
  int serialno_list_size=0;
  int ret;

  memset(vf,0,sizeof(*vf));
  vf->datasource=f;
  vf->callbacks = callbacks;

  /* init the framing state */
  ogg_sync_init(&vf->oy);

  /* perhaps some data was previously read into a buffer for testing
     against other stream types.  Allow initialization from this
     previously read data (especially as we may be reading from a
     non-seekable stream) */
  if(initial){
    char *buffer=ogg_sync_buffer(&vf->oy,ibytes);
    memcpy(buffer,initial,ibytes);
    ogg_sync_wrote(&vf->oy,ibytes);
  }

  /* can we seek? Stevens suggests the seek test was portable */
  if(offsettest!=-1)vf->seekable=1;

  /* No seeking yet; Set up a 'single' (current) logical bitstream
     entry for partial open */
  vf->links=1;
  vf->vi=_ogg_calloc(vf->links,sizeof(*vf->vi));
  vf->vc=_ogg_calloc(vf->links,sizeof(*vf->vc));
  ogg_stream_init(&vf->os,-1); /* fill in the serialno later */

  /* Fetch all BOS pages, store the vorbis header and all seen serial
     numbers, load subsequent vorbis setup headers */
  if((ret=_fetch_headers(vf,vf->vi,vf->vc,&serialno_list,&serialno_list_size,NULL))<0){
    vf->datasource=NULL;
    ov_clear(vf);
  }else{
    /* serial number list for first link needs to be held somewhere
       for second stage of seekable stream open; this saves having to
       seek/reread first link's serialnumber data then. */
    vf->serialnos=_ogg_calloc(serialno_list_size+2,sizeof(*vf->serialnos));
    vf->serialnos[0]=vf->current_serialno;
    vf->serialnos[1]=serialno_list_size;
    memcpy(vf->serialnos+2,serialno_list,serialno_list_size*sizeof(*vf->serialnos));

    vf->offsets=_ogg_calloc(1,sizeof(*vf->offsets));
    vf->dataoffsets=_ogg_calloc(1,sizeof(*vf->dataoffsets));
    vf->offsets[0]=0;
    vf->dataoffsets[0]=vf->offset;
    vf->current_serialno=vf->os.serialno;

    vf->ready_state=PARTOPEN;
  }
  if(serialno_list)_ogg_free(serialno_list);
  return(ret);
}

static int _ov_open2(OggVorbis_File *vf){
  if(vf->ready_state != PARTOPEN) return OV_EINVAL;
  vf->ready_state=OPENED;
  if(vf->seekable){
    int ret=_open_seekable2(vf);
    if(ret){
      vf->datasource=NULL;
      ov_clear(vf);
    }
    return(ret);
  }else
    vf->ready_state=STREAMSET;

  return 0;
}


/* clear out the OggVorbis_File struct */
int ov_clear(OggVorbis_File *vf){
  if(vf){
    vorbis_block_clear(&vf->vb);
    vorbis_dsp_clear(&vf->vd);
    ogg_stream_clear(&vf->os);

    if(vf->vi && vf->links){
      int i;
      for(i=0;i<vf->links;i++){
        vorbis_info_clear(vf->vi+i);
        vorbis_comment_clear(vf->vc+i);
      }
      _ogg_free(vf->vi);
      _ogg_free(vf->vc);
    }
    if(vf->dataoffsets)_ogg_free(vf->dataoffsets);
    if(vf->pcmlengths)_ogg_free(vf->pcmlengths);
    if(vf->serialnos)_ogg_free(vf->serialnos);
    if(vf->offsets)_ogg_free(vf->offsets);
    ogg_sync_clear(&vf->oy);
    if(vf->datasource && vf->callbacks.close_func)
      (vf->callbacks.close_func)(vf->datasource);
    memset(vf,0,sizeof(*vf));
  }
#ifdef DEBUG_LEAKS
  _VDBG_dump();
#endif
  return(0);
}

/* inspects the OggVorbis file and finds/documents all the logical
   bitstreams contained in it.  Tries to be tolerant of logical
   bitstream sections that are truncated/woogie.

   return: -1) error
            0) OK
*/

int ov_open_callbacks(void *f,OggVorbis_File *vf,char *initial,long ibytes,
    ov_callbacks callbacks){
  int ret=_ov_open1(f,vf,initial,ibytes,callbacks);
  if(ret)return ret;
  return _ov_open2(vf);
}

int ov_open(FILE *f,OggVorbis_File *vf,char *initial,long ibytes){
  ov_callbacks callbacks = {
    (size_t (*)(void *, size_t, size_t, void *))  fread,
    (int (*)(void *, ogg_int64_t, int))              _fseek64_wrap,
    (int (*)(void *))                             fclose,
    (long (*)(void *))                            ftell
  };

  return ov_open_callbacks((void *)f, vf, initial, ibytes, callbacks);
}

int ov_fopen(char *path,OggVorbis_File *vf){
  int ret;
  FILE *f = fopen(path,"rb");
  if(!f) return -1;

  ret = ov_open(f,vf,NULL,0);
  if(ret) fclose(f);
  return ret;
}


/* cheap hack for game usage where downsampling is desirable; there's
   no need for SRC as we can just do it cheaply in libvorbis. */

int ov_halfrate(OggVorbis_File *vf,int flag){
  int i;
  if(vf->vi==NULL)return OV_EINVAL;
  if(!vf->seekable)return OV_EINVAL;
  if(vf->ready_state>=STREAMSET)
    _decode_clear(vf); /* clear out stream state; later on libvorbis
                          will be able to swap this on the fly, but
                          for now dumping the decode machine is needed
                          to reinit the MDCT lookups.  1.1 libvorbis
                          is planned to be able to switch on the fly */

  for(i=0;i<vf->links;i++){
    if(vorbis_synthesis_halfrate(vf->vi+i,flag)){
      ov_halfrate(vf,0);
      return OV_EINVAL;
    }
  }
  return 0;
}

int ov_halfrate_p(OggVorbis_File *vf){
  if(vf->vi==NULL)return OV_EINVAL;
  return vorbis_synthesis_halfrate_p(vf->vi);
}

/* Only partially open the vorbis file; test for Vorbisness, and load
   the headers for the first chain.  Do not seek (although test for
   seekability).  Use ov_test_open to finish opening the file, else
   ov_clear to close/free it. Same return codes as open. */

int ov_test_callbacks(void *f,OggVorbis_File *vf,char *initial,long ibytes,
    ov_callbacks callbacks)
{
  return _ov_open1(f,vf,initial,ibytes,callbacks);
}

int ov_test(FILE *f,OggVorbis_File *vf,char *initial,long ibytes){
  ov_callbacks callbacks = {
    (size_t (*)(void *, size_t, size_t, void *))  fread,
    (int (*)(void *, ogg_int64_t, int))              _fseek64_wrap,
    (int (*)(void *))                             fclose,
    (long (*)(void *))                            ftell
  };

  return ov_test_callbacks((void *)f, vf, initial, ibytes, callbacks);
}

int ov_test_open(OggVorbis_File *vf){
  if(vf->ready_state!=PARTOPEN)return(OV_EINVAL);
  return _ov_open2(vf);
}

/* How many logical bitstreams in this physical bitstream? */
long ov_streams(OggVorbis_File *vf){
  return vf->links;
}

/* Is the FILE * associated with vf seekable? */
long ov_seekable(OggVorbis_File *vf){
  return vf->seekable;
}

/* returns the bitrate for a given logical bitstream or the entire
   physical bitstream.  If the file is open for random access, it will
   find the *actual* average bitrate.  If the file is streaming, it
   returns the nominal bitrate (if set) else the average of the
   upper/lower bounds (if set) else -1 (unset).

   If you want the actual bitrate field settings, get them from the
   vorbis_info structs */

long