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235 lines
5.7 KiB
C++
235 lines
5.7 KiB
C++
/**********************************************************************
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*
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* Filename: crc.c
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*
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* Description: Slow and fast implementations of the CRC standards.
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*
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* Notes: The parameters for each supported CRC standard are
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* defined in the header file crc.h. The implementations
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* here should stand up to further additions to that list.
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*
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*
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* Copyright (c) 2000 by Michael Barr. This software is placed into
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* the public domain and may be used for any purpose. However, this
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* notice must not be changed or removed and no warranty is either
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* expressed or implied by its publication or distribution.
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**********************************************************************/
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#include "crc.h"
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/*
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* Derive parameters from the standard-specific parameters in crc.h.
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*/
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#define WIDTH (8 * sizeof(crc))
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#define TOPBIT (1 << (WIDTH - 1))
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#if (REFLECT_DATA == TRUE)
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#undef REFLECT_DATA
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#define REFLECT_DATA(X) ((unsigned char) reflect((X), 8))
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#else
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#undef REFLECT_DATA
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#define REFLECT_DATA(X) (X)
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#endif
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#if (REFLECT_REMAINDER == TRUE)
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#undef REFLECT_REMAINDER
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#define REFLECT_REMAINDER(X) ((crc) reflect((X), WIDTH))
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#else
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#undef REFLECT_REMAINDER
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#define REFLECT_REMAINDER(X) (X)
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#endif
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/*********************************************************************
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*
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* Function: reflect()
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*
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* Description: Reorder the bits of a binary sequence, by reflecting
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* them about the middle position.
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*
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* Notes: No checking is done that nBits <= 32.
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*
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* Returns: The reflection of the original data.
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*
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*********************************************************************/
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static unsigned long
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reflect(unsigned long data, unsigned char nBits) {
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unsigned long reflection = 0x00000000;
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unsigned char bit;
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/*
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* Reflect the data about the center bit.
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*/
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for (bit = 0; bit < nBits; ++bit) {
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/*
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* If the LSB bit is set, set the reflection of it.
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*/
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if (data & 0x01) {
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reflection |= (1 << ((nBits - 1) - bit));
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}
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data = (data >> 1);
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}
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return (reflection);
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} /* reflect() */
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/*********************************************************************
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*
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* Function: crcSlow()
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*
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* Description: Compute the CRC of a given message.
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*
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* Notes:
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*
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* Returns: The CRC of the message.
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*
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*********************************************************************/
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crc
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crcSlow(unsigned char const message[], int nBytes) {
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crc remainder = INITIAL_REMAINDER;
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int byte;
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unsigned char bit;
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/*
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* Perform modulo-2 division, a byte at a time.
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*/
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for (byte = 0; byte < nBytes; ++byte) {
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/*
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* Bring the next byte into the remainder.
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*/
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remainder ^= (REFLECT_DATA(message[byte]) << (WIDTH - 8));
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/*
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* Perform modulo-2 division, a bit at a time.
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*/
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for (bit = 8; bit > 0; --bit) {
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/*
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* Try to divide the current data bit.
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*/
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if (remainder & TOPBIT) {
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remainder = (remainder << 1) ^ POLYNOMIAL;
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} else {
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remainder = (remainder << 1);
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}
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}
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}
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/*
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* The final remainder is the CRC result.
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*/
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return (REFLECT_REMAINDER(remainder) ^ FINAL_XOR_VALUE);
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} /* crcSlow() */
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crc crcTable[256];
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/*********************************************************************
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*
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* Function: crcInit()
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*
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* Description: Populate the partial CRC lookup table.
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*
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* Notes: This function must be rerun any time the CRC standard
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* is changed. If desired, it can be run "offline" and
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* the table results stored in an embedded system's ROM.
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*
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* Returns: None defined.
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*
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*********************************************************************/
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void
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crcInit(void) {
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crc remainder;
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int dividend;
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unsigned char bit;
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/*
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* Compute the remainder of each possible dividend.
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*/
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for (dividend = 0; dividend < 256; ++dividend) {
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/*
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* Start with the dividend followed by zeros.
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*/
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remainder = dividend << (WIDTH - 8);
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/*
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* Perform modulo-2 division, a bit at a time.
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*/
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for (bit = 8; bit > 0; --bit) {
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/*
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* Try to divide the current data bit.
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*/
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if (remainder & TOPBIT) {
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remainder = (remainder << 1) ^ POLYNOMIAL;
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} else {
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remainder = (remainder << 1);
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}
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}
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/*
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* Store the result into the table.
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*/
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crcTable[dividend] = remainder;
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}
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} /* crcInit() */
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/*********************************************************************
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*
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* Function: crcFast()
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*
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* Description: Compute the CRC of a given message.
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*
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* Notes: crcInit() must be called first.
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*
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* Returns: The CRC of the message.
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*
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*********************************************************************/
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crc
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crcFast(unsigned char const message[], int nBytes) {
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crc remainder = INITIAL_REMAINDER;
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unsigned char data;
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int byte;
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/*
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* Divide the message by the polynomial, a byte at a time.
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*/
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for (byte = 0; byte < nBytes; ++byte) {
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data = (unsigned char)(REFLECT_DATA(message[byte]) ^ (remainder >> (WIDTH - 8)));
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remainder = crcTable[data] ^ (remainder << 8);
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}
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/*
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* The final remainder is the CRC.
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*/
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return (REFLECT_REMAINDER(remainder) ^ FINAL_XOR_VALUE);
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} /* crcFast() */
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crc crc_initialize(void) {
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crcInit();
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return INITIAL_REMAINDER;
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}
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crc crc_process_byte(unsigned char byteVal, crc remainder) {
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unsigned char data;
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data = (unsigned char)(REFLECT_DATA(byteVal) ^ (remainder >> (WIDTH - 8)));
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remainder = crcTable[data] ^ (remainder << 8);
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return remainder;
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}
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crc crc_finalize(crc remainder) {
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return (REFLECT_REMAINDER(remainder) ^ FINAL_XOR_VALUE);
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}
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