Add CRC16 with reverse bit order
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Nils Faerber
parent
5c3d6461a1
commit
0d6cba4530
5 changed files with 450 additions and 4 deletions
367
crc16ccitt.c
Normal file
367
crc16ccitt.c
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/*
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* derived from crctester.c
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* original header:
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* ----------------------------------------------------------------------------
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* CRC tester v1.3 written on 4th of February 2003 by Sven Reifegerste (zorc/reflex)
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* This is the complete compilable C program, consisting only of this .c file.
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* No guarantee for any mistakes.
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*
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* changes to CRC tester v1.2:
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*
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* - remove unneccessary (!(polynom&1)) test for invalid polynoms
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* (now also XMODEM parameters 0x8408 work in c-code as they should)
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*
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* changes to CRC tester v1.1:
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*
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* - include an crc&0crcmask after converting non-direct to direct initial
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* value to avoid overflow
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*
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* changes to CRC tester v1.0:
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*
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* - most int's were replaced by unsigned long's to allow longer input strings
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* and avoid overflows and unnecessary type-casting's
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* ----------------------------------------------------------------------------
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*/
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static const int order = 16;
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static const unsigned long polynom = 0x1021;
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static const int direct = 1;
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static const unsigned long crcinit = 0xffff;
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static const unsigned long crcxor = 0x0000;
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static const int refin = 1;
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static const int refout = 0;
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// 'order' [1..32] is the CRC polynom order, counted without the leading '1' bit
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// 'polynom' is the CRC polynom without leading '1' bit
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// 'direct' [0,1] specifies the kind of algorithm: 1=direct, no augmented zero bits
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// 'crcinit' is the initial CRC value belonging to that algorithm
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// 'crcxor' is the final XOR value
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// 'refin' [0,1] specifies if a data byte is reflected before processing (UART) or not
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// 'refout' [0,1] specifies if the CRC will be reflected before XOR
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// internal global values:
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static unsigned long crcmask;
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static unsigned long crchighbit;
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static unsigned long crcinit_direct;
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static unsigned long crcinit_nondirect;
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static unsigned long crctab[256];
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// subroutines
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static unsigned long reflect (unsigned long crc, int bitnum) {
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// reflects the lower 'bitnum' bits of 'crc'
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unsigned long i, j=1, crcout=0;
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for (i=(unsigned long)1<<(bitnum-1); i; i>>=1) {
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if (crc & i) crcout|=j;
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j<<= 1;
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}
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return (crcout);
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}
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static void generate_crc_table(void)
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{
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// make CRC lookup table used by table algorithms
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int i, j;
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unsigned long bit, crc;
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for (i=0; i<256; i++) {
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crc=(unsigned long)i;
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if (refin) crc=reflect(crc, 8);
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crc<<= order-8;
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for (j=0; j<8; j++) {
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bit = crc & crchighbit;
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crc<<= 1;
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if (bit) crc^= polynom;
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}
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if (refin) crc = reflect(crc, order);
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crc&= crcmask;
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crctab[i]= crc;
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}
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}
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static unsigned long crctablefast (unsigned char* p, unsigned long len)
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{
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// fast lookup table algorithm without augmented zero bytes, e.g. used in pkzip.
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// only usable with polynom orders of 8, 16, 24 or 32.
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unsigned long crc = crcinit_direct;
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if (refin) crc = reflect(crc, order);
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if (!refin) while (len--) crc = (crc << 8) ^ crctab[ ((crc >> (order-8)) & 0xff) ^ *p++];
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else while (len--) crc = (crc >> 8) ^ crctab[ (crc & 0xff) ^ *p++];
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if (refout^refin) crc = reflect(crc, order);
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crc^= crcxor;
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crc&= crcmask;
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return(crc);
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}
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static unsigned long crctable (unsigned char* p, unsigned long len)
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{
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// normal lookup table algorithm with augmented zero bytes.
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// only usable with polynom orders of 8, 16, 24 or 32.
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unsigned long crc = crcinit_nondirect;
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if (refin)
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crc = reflect(crc, order);
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if (!refin)
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while (len--)
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crc = ((crc << 8) | *p++) ^ crctab[ (crc >> (order-8)) & 0xff];
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else
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while (len--)
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crc = ((crc >> 8) | (*p++ << (order-8))) ^ crctab[ crc & 0xff];
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if (!refin)
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while (++len < order/8)
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crc = (crc << 8) ^ crctab[ (crc >> (order-8)) & 0xff];
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else
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while (++len < order/8)
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crc = (crc >> 8) ^ crctab[crc & 0xff];
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if (refout^refin)
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crc = reflect(crc, order);
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crc^= crcxor;
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crc&= crcmask;
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return(crc);
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}
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unsigned long crcbitbybit(unsigned char* p, unsigned long len) {
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// bit by bit algorithm with augmented zero bytes.
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// does not use lookup table, suited for polynom orders between 1...32.
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unsigned long i, j, c, bit;
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unsigned long crc = crcinit_nondirect;
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for (i=0; i<len; i++) {
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c = (unsigned long)*p++;
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if (refin)
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c = reflect(c, 8);
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for (j=0x80; j; j>>=1) {
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bit = crc & crchighbit;
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crc<<= 1;
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if (c & j)
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crc|= 1;
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if (bit)
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crc^= polynom;
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}
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}
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for (i=0; i<order; i++) {
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bit = crc & crchighbit;
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crc <<= 1;
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if (bit)
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crc^= polynom;
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}
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if (refout)
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crc=reflect(crc, order);
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crc ^= crcxor;
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crc &= crcmask;
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return(crc);
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}
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unsigned long crcbitbybitfast(unsigned char* p, unsigned long len) {
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// fast bit by bit algorithm without augmented zero bytes.
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// does not use lookup table, suited for polynom orders between 1...32.
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unsigned long i, j, c, bit;
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unsigned long crc = crcinit_direct;
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for (i=0; i<len; i++) {
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c = (unsigned long)*p++;
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if (refin)
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c = reflect(c, 8);
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for (j=0x80; j; j>>=1) {
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bit = crc & crchighbit;
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crc <<= 1;
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if (c & j)
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bit^= crchighbit;
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if (bit)
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crc^= polynom;
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}
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}
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if (refout)
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crc=reflect(crc, order);
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crc^= crcxor;
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crc&= crcmask;
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return(crc);
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}
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void crc16ccitt_init(void)
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{
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int i;
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unsigned long bit, crc;
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crcmask = ((((unsigned long)1<<(order-1))-1)<<1)|1;
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crchighbit = (unsigned long)1<<(order-1);
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generate_crc_table();
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if (!direct) {
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crcinit_nondirect = crcinit;
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crc = crcinit;
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for (i=0; i<order; i++) {
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bit = crc & crchighbit;
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crc <<= 1;
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if (bit)
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crc^= polynom;
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}
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crc &= crcmask;
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crcinit_direct = crc;
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} else {
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crcinit_direct = crcinit;
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crc = crcinit;
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for (i=0; i<order; i++) {
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bit = crc & 1;
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if (bit)
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crc^= polynom;
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crc >>= 1;
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if (bit)
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crc|= crchighbit;
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}
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crcinit_nondirect = crc;
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}
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}
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unsigned short crc16ccitt (unsigned char *data, int len)
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{
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// call CRC algorithms using the CRC parameters above and print result to the console
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return crcbitbybitfast((unsigned char *)data, len);
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}
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#if 0
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int main() {
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// test program for checking four different CRC computing types that are:
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// crcbit(), crcbitfast(), crctable() and crctablefast(), see above.
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// parameters are at the top of this program.
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// Result will be printed on the console.
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int i;
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unsigned long bit, crc;
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// at first, compute constant bit masks for whole CRC and CRC high bit
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crcmask = ((((unsigned long)1<<(order-1))-1)<<1)|1;
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crchighbit = (unsigned long)1<<(order-1);
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// check parameters
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if (order < 1 || order > 32) {
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printf("ERROR, invalid order, it must be between 1..32.\n");
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return(0);
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}
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if (polynom != (polynom & crcmask)) {
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printf("ERROR, invalid polynom.\n");
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return(0);
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}
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if (crcinit != (crcinit & crcmask)) {
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printf("ERROR, invalid crcinit.\n");
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return(0);
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}
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if (crcxor != (crcxor & crcmask)) {
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printf("ERROR, invalid crcxor.\n");
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return(0);
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}
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// generate lookup table
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generate_crc_table();
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// compute missing initial CRC value
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if (!direct) {
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crcinit_nondirect = crcinit;
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crc = crcinit;
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for (i=0; i<order; i++) {
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bit = crc & crchighbit;
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crc<<= 1;
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if (bit) crc^= polynom;
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}
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crc&= crcmask;
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crcinit_direct = crc;
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}
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else {
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crcinit_direct = crcinit;
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crc = crcinit;
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for (i=0; i<order; i++) {
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bit = crc & 1;
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if (bit) crc^= polynom;
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crc >>= 1;
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if (bit) crc|= crchighbit;
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}
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crcinit_nondirect = crc;
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}
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// call CRC algorithms using the CRC parameters above and print result to the console
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printf("\n");
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printf("CRC tester v1.1 written on 13/01/2003 by Sven Reifegerste (zorc/reflex)\n");
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printf("-----------------------------------------------------------------------\n");
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printf("\n");
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printf("Parameters:\n");
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printf("\n");
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printf(" polynom : 0x%x\n", polynom);
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printf(" order : %d\n", order);
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printf(" crcinit : 0x%x direct, 0x%x nondirect\n", crcinit_direct, crcinit_nondirect);
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printf(" crcxor : 0x%x\n", crcxor);
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printf(" refin : %d\n", refin);
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printf(" refout : %d\n", refout);
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printf("\n");
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printf(" data string : '%s' (%d bytes)\n", string, strlen(string));
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printf("\n");
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printf("Results:\n");
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printf("\n");
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printf(" crc bit by bit : 0x%x\n", crcbitbybit((unsigned char *)string, strlen(string)));
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printf(" crc bit by bit fast : 0x%x\n", crcbitbybitfast((unsigned char *)string, strlen(string)));
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if (!(order&7)) printf(" crc table : 0x%x\n", crctable((unsigned char *)string, strlen(string)));
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if (!(order&7)) printf(" crc table fast : 0x%x\n", crctablefast((unsigned char *)string, strlen(string)));
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return(0);
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}
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#endif
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