Ολοκληρωμένο παράδειγμα αλγορίθμου CRC.
/**********************************************************************
*
* Filename: crc.c
*
* Description: A table-driven implementation of CRC-CCITT checksums.
*
* Notes: Some of the constants in this file are specific to
* Arcom's Target188EB hardware.
*
* This code can be easily modified to implement any
* "non-reflective" CRC algorithm. Simply change the
* constants POLYNOMIAL, INITIAL_REMAINDER, FINAL_XOR,
* and--if an 8 or 32-bit CRC is required--the definition
* of type 'width'.
*
*
* Copyright (c) 1998 by Michael Barr. This software is placed into
* the public domain and may be used for any purpose. However, this
* notice must not be changed or removed and no warranty is either
* expressed or implied by its publication or distribution.
**********************************************************************/
#include <string.h>
#include "led.h"
/*
* The CRC parameters. Currently configured for CCITT.
* Simply modify these to switch to another CRC standard.
*/
#define POLYNOMIAL 0x1021
#define INITIAL_REMAINDER 0xFFFF
#define FINAL_XOR_VALUE 0x0000
/*
* The width of the CRC calculation and result.
* Modify the typedef for an 8 or 32-bit CRC standard.
*/
typedef unsigned short width;
#define WIDTH (8 * sizeof(width))
#define TOPBIT (1 << (WIDTH - 1))
/*
* An array containing the pre-computed intermediate result for each
* possible byte of input. This is used to speed up the computation.
*/
width crcTable[256];
/**********************************************************************
*
* Function: crcInit()
*
* Description: Initialize the CRC lookup table. This table is used
* by crcCompute() to make CRC computation faster.
*
* Notes: The mod-2 binary long division is implemented here.
*
* Returns: None defined.
*
**********************************************************************/
void
crcInit(void)
{
width remainder;
width dividend;
int bit;
/*
* Perform binary long division, a bit at a time.
*/
for (dividend = 0; dividend < 256; dividend++)
{
/*
* Initialize the remainder.
*/
remainder = dividend << (WIDTH - 8);
/*
* Shift and XOR with the polynomial.
*/
for (bit = 0; bit < 8; bit++)
{
/*
* Try to divide the current data bit.
*/
if (remainder & TOPBIT)
{
remainder = (remainder << 1) ^ POLYNOMIAL;
}
else
{
remainder = remainder << 1;
}
}
/*
* Save the result in the table.
*/
crcTable[dividend] = remainder;
}
} /* crcInit() */
/**********************************************************************
*
* Function: crcCompute()
*
* Description: Compute the CRC checksum of a binary message block.
*
* Notes: This function expects that crcInit() has been called
* first to initialize the CRC lookup table.
*
* Returns: The CRC of the data.
*
**********************************************************************/
width
crcCompute(unsigned char * message, unsigned int nBytes)
{
unsigned int offset;
unsigned char byte;
width remainder = INITIAL_REMAINDER;
/*
* Divide the message by the polynomial, a byte at time.
*/
for (offset = 0; offset < nBytes; offset++)
{
byte = (remainder >> (WIDTH - 8)) ^ message[offset];
remainder = crcTable[byte] ^ (remainder << 8);
}
/*
* The final remainder is the CRC result.
*/
return (remainder ^ FINAL_XOR_VALUE);
} /* crcCompute() */
/**********************************************************************
*
* Function: main()
*
* Description: Test the CRC functions by computing the CRC-CCITT of
* the check string "123456789". The expected result
* was provided by an independent third-party.
*
* Notes:
*
* Returns: 0 on success.
* Otherwise -1 indicates failure.
*
**********************************************************************/
main()
{
#define CCITT_CHECK 0x29B1
char * s = "123456789";
/*
* Initialize the CRC lookup table.
*/
crcInit();
/*
* Compute the CRC of the check string.
*/
if (crcCompute(s, strlen(s)) != CCITT_CHECK)
{
toggleLed(LED_RED);
return (-1);
}
else
{
toggleLed(LED_GREEN);
return (0);
}
} /* main() */
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