crc.cpp

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// Created:09-25-98
// By Jeff Connelly

// Compute CRCs.  A CRC is a value based on some data to test its integerty.
// For example, when a file is transfured over the Internet, a CRC value
// is appended to it based on the file contents.  When the file is recivied
// on another computer, it performs the same CRC calculation.  If the CRC
// just calculated does not match the appended CRC, the file is downloaded
// again.

#include "stdafx.h"
#define EXPORTING
#include "comprlib.h"
#include "crc.h"

// Updates 'crc' using character 'c'.  To calculate the CRC of a file,
// read and update each character using this function:
//     while (!feof(file))
//     {
//         c = getc(file);
//         UpdateCRC (crc, c);
//     }
unsigned int EXPORT UpdateCRC (unsigned int crc, int c)
{
    int i;
    printf ("Character: %c\n", c);
    i = (crc >> 8) ^ c;
    printf ("'i' var: %d\n", i);
    crc = (crc << 8) ^ crctttab[i];
    printf ("CRC now is: %d\n", crc);
    return crc;
}

// Reverse of UpdateCRC().  See UpdateCRC() notes for explaination.
unsigned int EXPORT UpdateCRCReverse (unsigned int crc, int c)
{
    int i;
    i = crc ^ c;
    crc = (crc >> 8) ^ crc16tab[i & 0xFF];
    return crc;
}