/* getbits.c, 比特流程序                                           */#include <stdio.h>#include <stdlib.h>#include "config.h"#include "global.h"/* initialize buffer, call once before first getbits or showbits */void Initialize_Buffer(){  ld->Incnt = 0;  ld->Rdptr = ld->Rdbfr + 2048;  ld->Rdmax = ld->Rdptr;#ifdef VERIFY  ld->Bitcnt = 0;#endif  ld->Bfr = 0;  Flush_Buffer(0); /* fills valid data into bfr */}void Fill_Buffer(){  int Buffer_Level;  Buffer_Level = read(ld->Infile,ld->Rdbfr,2048);  ld->Rdptr = ld->Rdbfr;  if (System_Stream_Flag)    ld->Rdmax -= 2048;    /* end of the bitstream file */  if (Buffer_Level < 2048)  {    /* just to be safe */    if (Buffer_Level < 0)      Buffer_Level = 0;    /* pad until the next to the next 32-bit word boundary */    while (Buffer_Level & 3)      ld->Rdbfr[Buffer_Level++] = 0;	/* pad the buffer with sequence end codes */    while (Buffer_Level < 2048)    {      ld->Rdbfr[Buffer_Level++] = SEQUENCE_END_CODE>>24;      ld->Rdbfr[Buffer_Level++] = SEQUENCE_END_CODE>>16;      ld->Rdbfr[Buffer_Level++] = SEQUENCE_END_CODE>>8;      ld->Rdbfr[Buffer_Level++] = SEQUENCE_END_CODE&0xff;    }  }}/* MPEG-1 system layer demultiplexer */int Get_Byte(){  while(ld->Rdptr >= ld->Rdbfr+2048)  {    read(ld->Infile,ld->Rdbfr,2048);    ld->Rdptr -= 2048;    ld->Rdmax -= 2048;  }  return *ld->Rdptr++;}/* extract a 16-bit word from the bitstream buffer */int Get_Word(){  int Val;  Val = Get_Byte();  return (Val<<8) | Get_Byte();}/* return next n bits (right adjusted) without advancing */unsigned int Show_Bits(N)int N;{  return ld->Bfr >> (32-N);}/* return next bit (could be made faster than Get_Bits(1)) */unsigned int Get_Bits1(){  return Get_Bits(1);}/* advance by n bits */void Flush_Buffer(N)int N;{  int Incnt;  ld->Bfr <<= N;  Incnt = ld->Incnt -= N;  if (Incnt <= 24)  {    if (System_Stream_Flag && (ld->Rdptr >= ld->Rdmax-4))    {      do      {        if (ld->Rdptr >= ld->Rdmax)          Next_Packet();        ld->Bfr |= Get_Byte() << (24 - Incnt);        Incnt += 8;      }      while (Incnt <= 24);    }    else if (ld->Rdptr < ld->Rdbfr+2044)    {      do      {        ld->Bfr |= *ld->Rdptr++ << (24 - Incnt);        Incnt += 8;      }      while (Incnt <= 24);    }    else    {      do      {        if (ld->Rdptr >= ld->Rdbfr+2048)          Fill_Buffer();        ld->Bfr |= *ld->Rdptr++ << (24 - Incnt);        Incnt += 8;      }      while (Incnt <= 24);    }    ld->Incnt = Incnt;  }#ifdef VERIFY   ld->Bitcnt += N;#endif /* VERIFY */}/* return next n bits (right adjusted) */unsigned int Get_Bits(N)int N;{  unsigned int Val;  Val = Show_Bits(N);  Flush_Buffer(N);  return Val;}