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108 lines
4.7 KiB
Plaintext
108 lines
4.7 KiB
Plaintext
/*
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* Compute 16-bit CRCs.
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*
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* Adapted from code in NuLib, http://www.nulib.com/.
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*/
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#include <assert.h>
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/* define this if you want a table-based lookup */
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#define CRC_TAB
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#ifdef CRC_TAB
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/*
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* updcrc macro derived from article Copyright (C) 1986 Stephen Satchell.
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* NOTE: First srgument must be in range 0 to 255.
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* Second argument is referenced twice.
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*
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* Programmers may incorporate any or all code into their programs,
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* giving proper credit within the source. Publication of the
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* source routines is permitted so long as proper credit is given
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* to Stephen Satchell, Satchell Evaluations and Chuck Forsberg,
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* Omen Technology.
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*/
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/*#define updcrc(cp, crc) ( crctab[((crc >> 8) & 255)] ^ (crc << 8) ^ cp)*/
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#define updcrc(cp, crc) ( (crctab[((crc >> 8) & 0xFF) ^ cp] ^ (crc << 8)) & 0xFFFF)
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/* crctab calculated by Mark G. Mendel, Network Systems Corporation */
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const unsigned short crctab[256] = {
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0x0000, 0x1021, 0x2042, 0x3063, 0x4084, 0x50a5, 0x60c6, 0x70e7,
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0x8108, 0x9129, 0xa14a, 0xb16b, 0xc18c, 0xd1ad, 0xe1ce, 0xf1ef,
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0x1231, 0x0210, 0x3273, 0x2252, 0x52b5, 0x4294, 0x72f7, 0x62d6,
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0x9339, 0x8318, 0xb37b, 0xa35a, 0xd3bd, 0xc39c, 0xf3ff, 0xe3de,
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0x2462, 0x3443, 0x0420, 0x1401, 0x64e6, 0x74c7, 0x44a4, 0x5485,
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0xa56a, 0xb54b, 0x8528, 0x9509, 0xe5ee, 0xf5cf, 0xc5ac, 0xd58d,
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0x3653, 0x2672, 0x1611, 0x0630, 0x76d7, 0x66f6, 0x5695, 0x46b4,
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0xb75b, 0xa77a, 0x9719, 0x8738, 0xf7df, 0xe7fe, 0xd79d, 0xc7bc,
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0x48c4, 0x58e5, 0x6886, 0x78a7, 0x0840, 0x1861, 0x2802, 0x3823,
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0xc9cc, 0xd9ed, 0xe98e, 0xf9af, 0x8948, 0x9969, 0xa90a, 0xb92b,
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0x5af5, 0x4ad4, 0x7ab7, 0x6a96, 0x1a71, 0x0a50, 0x3a33, 0x2a12,
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0xdbfd, 0xcbdc, 0xfbbf, 0xeb9e, 0x9b79, 0x8b58, 0xbb3b, 0xab1a,
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0x6ca6, 0x7c87, 0x4ce4, 0x5cc5, 0x2c22, 0x3c03, 0x0c60, 0x1c41,
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0xedae, 0xfd8f, 0xcdec, 0xddcd, 0xad2a, 0xbd0b, 0x8d68, 0x9d49,
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0x7e97, 0x6eb6, 0x5ed5, 0x4ef4, 0x3e13, 0x2e32, 0x1e51, 0x0e70,
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0xff9f, 0xefbe, 0xdfdd, 0xcffc, 0xbf1b, 0xaf3a, 0x9f59, 0x8f78,
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0x9188, 0x81a9, 0xb1ca, 0xa1eb, 0xd10c, 0xc12d, 0xf14e, 0xe16f,
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0x1080, 0x00a1, 0x30c2, 0x20e3, 0x5004, 0x4025, 0x7046, 0x6067,
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0x83b9, 0x9398, 0xa3fb, 0xb3da, 0xc33d, 0xd31c, 0xe37f, 0xf35e,
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0x02b1, 0x1290, 0x22f3, 0x32d2, 0x4235, 0x5214, 0x6277, 0x7256,
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0xb5ea, 0xa5cb, 0x95a8, 0x8589, 0xf56e, 0xe54f, 0xd52c, 0xc50d,
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0x34e2, 0x24c3, 0x14a0, 0x0481, 0x7466, 0x6447, 0x5424, 0x4405,
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0xa7db, 0xb7fa, 0x8799, 0x97b8, 0xe75f, 0xf77e, 0xc71d, 0xd73c,
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0x26d3, 0x36f2, 0x0691, 0x16b0, 0x6657, 0x7676, 0x4615, 0x5634,
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0xd94c, 0xc96d, 0xf90e, 0xe92f, 0x99c8, 0x89e9, 0xb98a, 0xa9ab,
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0x5844, 0x4865, 0x7806, 0x6827, 0x18c0, 0x08e1, 0x3882, 0x28a3,
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0xcb7d, 0xdb5c, 0xeb3f, 0xfb1e, 0x8bf9, 0x9bd8, 0xabbb, 0xbb9a,
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0x4a75, 0x5a54, 0x6a37, 0x7a16, 0x0af1, 0x1ad0, 0x2ab3, 0x3a92,
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0xfd2e, 0xed0f, 0xdd6c, 0xcd4d, 0xbdaa, 0xad8b, 0x9de8, 0x8dc9,
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0x7c26, 0x6c07, 0x5c64, 0x4c45, 0x3ca2, 0x2c83, 0x1ce0, 0x0cc1,
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0xef1f, 0xff3e, 0xcf5d, 0xdf7c, 0xaf9b, 0xbfba, 0x8fd9, 0x9ff8,
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0x6e17, 0x7e36, 0x4e55, 0x5e74, 0x2e93, 0x3eb2, 0x0ed1, 0x1ef0
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};
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#endif
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/*
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* Calculate CRC on a region. Pass in seed (which can be an initial
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* value -- typically zero -- or the result of an earlier computation),
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* and the pointer and length of the buffer to compute on.
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*
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* A CRC is the result of a mathematical operation based on the
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* coefficients of a polynomial when multiplied by X^16 then divided by
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* the generator polynomial (X^16 + X^12 + X^5 + 1) using modulo two
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* arithmetic.
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*
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* This routine is a slightly modified verison of one found in:
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* _Advanced Programming Techniques for the Apple //gs Toolbox_
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* By Morgan Davis and Dan Gookin (Compute! Publications, Inc.)
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*
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* This can either calculate the CRC bit-by-bit or use a table.
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* Depending on CPU architecture, one may be dramatically faster than
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* the other.
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*/
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unsigned short
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CalcCRC16(unsigned short seed, const unsigned char* ptr, int count)
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{
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unsigned short CRC = seed;
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#ifndef CRC_TAB
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int x;
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assert(sizeof(unsigned short) == 2); /* I think this is assumed */
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#endif
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do {
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#ifndef CRC_TAB
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CRC ^= *ptr++ << 8; /* XOR hi-byte of CRC w/dat */
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for (x = 8; x; --x) /* Then, for 8 bit shifts... */
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if (CRC & 0x8000) /* Test hi order bit of CRC */
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CRC = CRC << 1 ^ 0x1021; /* if set, shift & XOR w/$1021 */
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else
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CRC <<= 1; /* Else, just shift left once. */
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#else
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CRC = updcrc(*ptr++, CRC); /* look up new value in table */
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#endif
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} while (--count);
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return (CRC);
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}
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