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1624 giacomo 1
/* getbits.c, bit level routines                                            */
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/*
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 * All modifications (mpeg2decode -> mpeg2play) are
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 * Copyright (C) 1996, Stefan Eckart. All Rights Reserved.
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 */
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/* Copyright (C) 1996, MPEG Software Simulation Group. All Rights Reserved. */
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/*
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 * Disclaimer of Warranty
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 *
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 * These software programs are available to the user without any license fee or
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 * royalty on an "as is" basis.  The MPEG Software Simulation Group disclaims
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 * any and all warranties, whether express, implied, or statuary, including any
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 * implied warranties or merchantability or of fitness for a particular
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 * purpose.  In no event shall the copyright-holder be liable for any
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 * incidental, punitive, or consequential damages of any kind whatsoever
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 * arising from the use of these programs.
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 *
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 * This disclaimer of warranty extends to the user of these programs and user's
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 * customers, employees, agents, transferees, successors, and assigns.
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 *
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 * The MPEG Software Simulation Group does not represent or warrant that the
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 * programs furnished hereunder are free of infringement of any third-party
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 * patents.
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 *
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 * Commercial implementations of MPEG-1 and MPEG-2 video, including shareware,
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 * are subject to royalty fees to patent holders.  Many of these patents are
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 * general enough such that they are unavoidable regardless of implementation
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 * design.
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 *
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 */
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#include <stdio.h>
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#include <stdlib.h>
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#include "config.h"
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#include "global.h"
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/* initialize buffer, call once before first getbits or showbits */
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void Initialize_Buffer()
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{
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  ld->Incnt = 0;
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  ld->Rdptr = ld->Rdbfr + 2048;
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  ld->Rdmax = ld->Rdptr;
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#ifdef VERIFY
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  /*  only the verifier uses this particular bit counter
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   *  Bitcnt keeps track of the current parser position with respect
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   *  to the video elementary stream being decoded, regardless
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   *  of whether or not it is wrapped within a systems layer stream
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   */
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  ld->Bitcnt = 0;
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#endif
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  ld->Bfr = 0;
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  Flush_Buffer(0); /* fills valid data into bfr */
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}
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void Fill_Buffer()
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{
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  int Buffer_Level;
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  Buffer_Level = read(ld->Infile,ld->Rdbfr,2048);
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  ld->Rdptr = ld->Rdbfr;
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  if (System_Stream_Flag)
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    ld->Rdmax -= 2048;
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  /* end of the bitstream file */
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  if (Buffer_Level < 2048)
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  {
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    /* just to be safe */
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    if (Buffer_Level < 0)
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      Buffer_Level = 0;
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    /* pad until the next to the next 32-bit word boundary */
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    while (Buffer_Level & 3)
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      ld->Rdbfr[Buffer_Level++] = 0;
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        /* pad the buffer with sequence end codes */
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    while (Buffer_Level < 2048)
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    {
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      ld->Rdbfr[Buffer_Level++] = SEQUENCE_END_CODE>>24;
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      ld->Rdbfr[Buffer_Level++] = SEQUENCE_END_CODE>>16;
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      ld->Rdbfr[Buffer_Level++] = SEQUENCE_END_CODE>>8;
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      ld->Rdbfr[Buffer_Level++] = SEQUENCE_END_CODE&0xff;
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    }
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  }
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}
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/* MPEG-1 system layer demultiplexer */
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int Get_Byte()
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{
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  while(ld->Rdptr >= ld->Rdbfr+2048)
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  {
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    read(ld->Infile,ld->Rdbfr,2048);
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    ld->Rdptr -= 2048;
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    ld->Rdmax -= 2048;
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  }
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  return *ld->Rdptr++;
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}
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/* extract a 16-bit word from the bitstream buffer */
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int Get_Word()
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{
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  int Val;
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  Val = Get_Byte();
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  return (Val<<8) | Get_Byte();
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}
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/* return next n bits (right adjusted) without advancing */
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unsigned int Show_Bits(N)
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int N;
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{
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  return ld->Bfr >> (32-N);
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}
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/* return next bit (could be made faster than Get_Bits(1)) */
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unsigned int Get_Bits1()
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{
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  return Get_Bits(1);
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}
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/* advance by n bits */
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void Flush_Buffer(N)
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int N;
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{
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  int Incnt;
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  ld->Bfr <<= N;
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  Incnt = ld->Incnt -= N;
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  if (Incnt <= 24)
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  {
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    if (System_Stream_Flag && (ld->Rdptr >= ld->Rdmax-4))
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    {
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      do
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      {
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        if (ld->Rdptr >= ld->Rdmax)
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          Next_Packet();
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        ld->Bfr |= Get_Byte() << (24 - Incnt);
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        Incnt += 8;
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      }
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      while (Incnt <= 24);
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    }
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    else if (ld->Rdptr < ld->Rdbfr+2044)
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    {
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      do
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      {
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        ld->Bfr |= *ld->Rdptr++ << (24 - Incnt);
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        Incnt += 8;
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      }
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      while (Incnt <= 24);
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    }
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    else
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    {
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      do
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      {
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        if (ld->Rdptr >= ld->Rdbfr+2048)
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          Fill_Buffer();
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        ld->Bfr |= *ld->Rdptr++ << (24 - Incnt);
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        Incnt += 8;
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      }
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      while (Incnt <= 24);
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    }
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    ld->Incnt = Incnt;
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  }
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#ifdef VERIFY 
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  ld->Bitcnt += N;
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#endif /* VERIFY */
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}
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/* return next n bits (right adjusted) */
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unsigned int Get_Bits(N)
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int N;
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{
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  unsigned int Val;
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  Val = Show_Bits(N);
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  Flush_Buffer(N);
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  return Val;
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}
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