/* INTIO.C - Routines to read and write integers one byte at a time. */

/* Copyright (c) 2000 by Radford M. Neal 
 *
 * Permission is granted for anyone to copy, use, or modify this program 
 * for purposes of research or education, provided this copyright notice 
 * is retained, and note is made of any changes that have been made. 
 *
 * This program is distributed without any warranty, express or implied.
 * As this program was written for research purposes only, it has not been
 * tested to the degree that would be advisable in any important application.
 * All use of this program is entirely at the user's own risk.
 */

#include <stdlib.h>
#include <stdio.h>

#include "intio.h"


/* READ AN INTEGER ONE BYTE AT A TIME.  Four bytes are read, ordered from
   low to high order.  These are considered to represent a signed integer,
   in two's complement form.  The value returned is this integer, converted 
   to whatever a C "int" is.  The conversion should work as long as an "int" 
   is at least four bytes, even if it's not in two's complement representation 
   (except for the largest two's complement negative integer).

   If an error or eof is encountered, zero is returned.  The caller can
   check for these events using feof and ferror.  

   The file read from should have been opened as "binary".
*/

int intio_read        //從文件中讀數，返回一個32位的整數
( FILE *f   /* File to read from */
)
{ 
  unsigned char b[4];
  int top;
  int i;

  for (i = 0; i<4; i++)
  { if (fread(&b[i],1,1,f) != 1) return 0;
  }

  top = b[3]>127 ? (int)b[3] - 256 : b[3];
  
  return (top<<24) + (b[2]<<16) + (b[1]<<8) + b[0];
}


/* WRITE AN INTEGER ONE BYTE AT A TIME.  Four bytes are written, ordered from
   low to high order.  These are considered to represent a signed integer,
   in two's complement form.  This should work as long as the integer passed
   can be represented in four bytes, even if a C "int" is longer than this.

   The file written to should have been opened as "binary".
*/

void intio_write   //往文件中一次寫入一個字節
( FILE *f,  /* File to write to */
  int v     /* Value to write to file */
)
{ 
  unsigned char b;
  int i;

  for (i = 0; i<3; i++)
  { b = v&0xff;
    fwrite(&b,1,1,f);
    v >>= 8;
  }

  b = v>0 ? v : v+256;
  fwrite(&b,1,1,f);
}


/* TEST PROGRAM. */

#if TEST_INTIO

main(void)
{ 
  FILE *f;
  f = fopen("test","wb");
  intio_write(f,334);
  intio_write(f,-40000);
  intio_write(f,0x8fffffff);
  intio_write(f,-0x8fffffff);
  fclose(f);
  f = fopen("test","rb");
  if (intio_read(f)!=334
   || intio_read(f)!=-40000
   || intio_read(f)!=0x8fffffff
   || intio_read(f)!=-0x8fffffff)
  { fprintf(stderr,"got back bad data\n");
    exit(1);
  }
  if (intio_read(f)!=0 || !feof(f) || ferror(f))
  { fprintf(stderr,"eof not handled correctly\n");
    exit(1);
  }
  fclose(f);
  fprintf(stderr,"OK\n");
  exit(0);
}

#endif