crc.zip_The Program_crc

/********************************************************************** * * Filename: crc.c * * Description: Slow and fast implementations of the CRC standards. * * Notes: The parameters for each supported CRC standard are * defined in the header file crc.h. The implementations * here should stand up to further additions to that list. * * * Copyright (c) 2000 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 "crc.h" /* * Derive parameters from the standard-specific parameters in crc.h. */ #define WIDTH (8 * sizeof(crc)) #define TOPBIT (1 << (WIDTH - 1)) #if (REFLECT_DATA == TRUE) #undef REFLECT_DATA #define REFLECT_DATA(X) ((unsigned char) reflect((X), 8)) #else #undef REFLECT_DATA #define REFLECT_DATA(X) (X) #endif #if (REFLECT_REMAINDER == TRUE) #undef REFLECT_REMAINDER #define REFLECT_REMAINDER(X) ((crc) reflect((X), WIDTH)) #else #undef REFLECT_REMAINDER #define REFLECT_REMAINDER(X) (X) #endif /********************************************************************* * * Function: reflect() * * Description: Reorder the bits of a binary sequence, by reflecting * them about the middle position. * * Notes: No checking is done that nBits <= 32. * * Returns: The reflection of the original data. * *********************************************************************/ static unsigned long reflect(unsigned long data, unsigned char nBits) { unsigned long reflection = 0x00000000; unsigned char bit; /* * Reflect the data about the center bit. */ for (bit = 0; bit < nBits; ++bit) { /* * If the LSB bit is set, set the reflection of it. */ if (data & 0x01) { reflection |= (1 << ((nBits - 1) - bit)); } data = (data >> 1); } return (reflection); } /* reflect() */ /********************************************************************* * * Function: crcSlow() * * Description: Compute the CRC of a given message. * * Notes: * * Returns: The CRC of the message. * *********************************************************************/ crc crcSlow(unsigned char const message[], int nBytes) { crc remainder = INITIAL_REMAINDER; int byte; unsigned char bit; /* * Perform modulo-2 division, a byte at a time. */ for (byte = 0; byte < nBytes; ++byte) { /* * Bring the next byte into the remainder. */ remainder ^= (REFLECT_DATA(message[byte]) << (WIDTH - 8)); /* * Perform modulo-2 division, a bit at a time. */ for (bit = 8; bit > 0; --bit) { /* * Try to divide the current data bit. */ if (remainder & TOPBIT) { remainder = (remainder << 1) ^ POLYNOMIAL; } else { remainder = (remainder << 1); } } } /* * The final remainder is the CRC result. */ return (REFLECT_REMAINDER(remainder) ^ FINAL_XOR_VALUE); } /* crcSlow() */ crc crcTable[256]; /********************************************************************* * * Function: crcInit() * * Description: Populate the partial CRC lookup table. * * Notes: This function must be rerun any time the CRC standard * is changed. If desired, it can be run "offline" and * the table results stored in an embedded system's ROM. * * Returns: None defined. * *********************************************************************/ void crcInit(void) { crc remainder; int dividend; unsigned char bit; /* * Compute the remainder of each possible dividend. */ for (dividend = 0; dividend < 256; ++dividend) { /* * Start with the dividend followed by zeros. */ remainder = dividend << (WIDTH - 8); /* * Perform modulo-2 division, a bit at a time. */ for (bit = 8; bit > 0; --bit) { /* * Try to divide the current data bit. */ if (remainder & TOPBIT) { remainder = (remainder << 1) ^ POLYNOMIAL; } else { remainder = (remainder << 1); } } /* * Store the result into the table. */ crcTable[dividend] = remainder; } } /* crcInit() */ /********************************************************************* * * Function: crcFast() * * Description: Compute the CRC of a given message. * * Notes: crcInit() must be called first. * * Returns: The CRC of the message. * *********************************************************************/ crc crcFast(unsigned char const message[], int nBytes) { crc remainder = INITIAL_REMAINDER; unsigned char data; int byte; /* * Divide the message by the polynomial, a byte at a time. */ for (byte = 0; byte < nBytes; ++byte) { data = REFLECT_DATA(message[byte]) ^ (remainder >> (WIDTH - 8)); remainder = crcTable[data] ^ (remainder << 8); } /* * The final remainder is the CRC. */ return (REFLECT_REMAINDER(remainder) ^ FINAL_XOR_VALUE); } /* crcFast() */