putbits.c

基于H.263的图像压缩编解码的C源码

/************************************************************************
 *
 *  putbits.c, bitstream handling for tmn (TMN encoder)
 *  
 ************************************************************************/
 

#include "sim.h"

extern FILE *streamfile; /* the only global var we need here */

/* private data */
static unsigned char outbfr;
static int outcnt;
static int bytecnt;

/* initialize buffer, call once before first putbits or alignbits */
void initbits()
{
  outcnt = 8;
  bytecnt = 0;
}

/* write rightmost n (0<=n<=32) bits of val to outfile */

void putbits (int n, int val)
{
  int i;
  unsigned int mask;
  char bitstring[32];

  if (trace) {
    if (n > 0) {
      BitPrint(n,val,bitstring);
      fprintf(tf,bitstring);
    }
  }

  mask = 1 << (n-1); /* selects first (leftmost) bit */

  for (i=0; i<n; i++) {
    outbfr <<= 1;

    if (val & mask)
      outbfr|= 1;

    mask >>= 1; /* select next bit */
    outcnt--;

    if (outcnt==0) /* 8 bit buffer full */ {
      putc(outbfr,streamfile);
      outcnt = 8;
      bytecnt++;
    }
  }
}


/* zero bit stuffing to next byte boundary (5.2.3, 6.2.1) */

int alignbits ()
{
  int ret_value;
  
  if (outcnt!=8) {
    ret_value = outcnt;	/* outcnt is reset in call to putbits () */
    putbits (outcnt, 0);
    return ret_value;
  }
  else
    return 0;
}

/* return total number of generated bits */
int bitcount()
{
  return 8*bytecnt + (8-outcnt);
}

/* convert to binary number */
void BitPrint(int length, int val, char *bit)
{
  int m;
  
  m = length;
  bit[0] = '"';
  while (m--) {
    bit[length-m] = (val & (1<<m)) ? '1' : '0';
  }
  bit[length+1] = '"';
  bit[length+2] = '\n';
  bit[length+3] = '\0';
  return;
}