11.6 Как подсчитывается CRC32?

Ниже приведены тексты программы для подсчета CRC32 заданного файла.

----------------------------------------------- crc.h
typedef unsigned char  uchar; /*  8 bits or more */
typedef unsigned int   uint; /* 16 - 32 bits or more */
typedef unsigned short ushort; /* 16 bits or more */
typedef unsigned long  ulong; /* 32 bits or more */
typedef ulong UCRC; /* CRC-32 */
#define CRC_MASK    0xFFFFFFFFUL
extern UCRC crc;

----------------------------------------------- crc.c
#include <stdio.h>
#include "crc.h"
#include <stdlib.h>

/* Global variables */

UCRC   crc;
FILE   *fp;

#define BUFFERSIZE  32768U
#define CRCPOLY     0xEDB88320UL
/* 1110 1101 1001 1000 1000 0011 0010 0000 */
#define UPDATE_CRC(crc, c)  \
    crc = crctable[(uchar)crc ^ (uchar)(c)] ^ (crc >> 8)
static UCRC   crctable[256];

static void make_crctable( void ) {
    uint i, j;
    UCRC r;

    for (i = 0; i <= 255; i++) {
    r = i;
    for (j = 8; j > 0; j--) {
        if (r & 1)
        r = (r >> 1) ^ CRCPOLY;
        else
        r >>= 1;
    }
    crctable[i] = r;
    /*
    printf( "%08lx  ", crctable[i] );
    if( i % 8 == 7 )
       printf( "\n" );
    */
    }
}

int main( int argc, char *argv[] ) {
  size_t len;
  char *buffer, *sav;

    if ( argc != 2 ) {
    printf("Usage: CRC32 <File_Name>\n" );
    return 1;
    }
    if ( ( fp = fopen( argv[1], "rb" ) ) == NULL ) exit(1);

    make_crctable();
    crc = CRC_MASK;
    buffer = (char *) calloc( BUFFERSIZE,1 );
  sav = buffer;
  while ( ( len = fread(buffer,1,BUFFERSIZE,fp) ) > 0 ) {
    while ( len-- ) UPDATE_CRC( crc, *buffer++ );
    buffer = sav;
  }
  free(buffer);
  len = (unsigned int)(crc>>16);
  printf( "File = %13s CRC32 = %04x%04x\n", argv[1], (int)~len, (int)~crc );

  fclose( fp );
  flushall();
    return 0;
}