Date : Mon, 13 Jun 2011 19:26:44 +0200
From : rick@... (Rick Murray)
Subject: Calculating CRCs
[reposted as it was sent *only* to JGH, not to the list... duh!]
On 11/06/2011 19:13, J.G.Harston wrote:
> I found it incredibly difficult tracking down actual working code to
> calculate 32-CRCs as used in ZIP files,
Anything specific to zip files? It wasn't hard to track down 32 bit CRC
code when I needed some. This one was released into the public domain by
Gary S. Brown back in '86. I chose this one over crc.c from Debian
because that version "became GPL" and I licence my own code as EUPL due
to the non-openness of GPL.
Here's the bit that goes in a header file or somesuch:
--8<--------
// Lookup table for CRC ops
static unsigned long crctable[256] = {
0x00000000L, 0x77073096L, 0xEE0E612CL, 0x990951BAL,
0x076DC419L, 0x706AF48FL, 0xE963A535L, 0x9E6495A3L,
0x0EDB8832L, 0x79DCB8A4L, 0xE0D5E91EL, 0x97D2D988L,
0x09B64C2BL, 0x7EB17CBDL, 0xE7B82D07L, 0x90BF1D91L,
0x1DB71064L, 0x6AB020F2L, 0xF3B97148L, 0x84BE41DEL,
0x1ADAD47DL, 0x6DDDE4EBL, 0xF4D4B551L, 0x83D385C7L,
0x136C9856L, 0x646BA8C0L, 0xFD62F97AL, 0x8A65C9ECL,
0x14015C4FL, 0x63066CD9L, 0xFA0F3D63L, 0x8D080DF5L,
0x3B6E20C8L, 0x4C69105EL, 0xD56041E4L, 0xA2677172L,
0x3C03E4D1L, 0x4B04D447L, 0xD20D85FDL, 0xA50AB56BL,
0x35B5A8FAL, 0x42B2986CL, 0xDBBBC9D6L, 0xACBCF940L,
0x32D86CE3L, 0x45DF5C75L, 0xDCD60DCFL, 0xABD13D59L,
0x26D930ACL, 0x51DE003AL, 0xC8D75180L, 0xBFD06116L,
0x21B4F4B5L, 0x56B3C423L, 0xCFBA9599L, 0xB8BDA50FL,
0x2802B89EL, 0x5F058808L, 0xC60CD9B2L, 0xB10BE924L,
0x2F6F7C87L, 0x58684C11L, 0xC1611DABL, 0xB6662D3DL,
0x76DC4190L, 0x01DB7106L, 0x98D220BCL, 0xEFD5102AL,
0x71B18589L, 0x06B6B51FL, 0x9FBFE4A5L, 0xE8B8D433L,
0x7807C9A2L, 0x0F00F934L, 0x9609A88EL, 0xE10E9818L,
0x7F6A0DBBL, 0x086D3D2DL, 0x91646C97L, 0xE6635C01L,
0x6B6B51F4L, 0x1C6C6162L, 0x856530D8L, 0xF262004EL,
0x6C0695EDL, 0x1B01A57BL, 0x8208F4C1L, 0xF50FC457L,
0x65B0D9C6L, 0x12B7E950L, 0x8BBEB8EAL, 0xFCB9887CL,
0x62DD1DDFL, 0x15DA2D49L, 0x8CD37CF3L, 0xFBD44C65L,
0x4DB26158L, 0x3AB551CEL, 0xA3BC0074L, 0xD4BB30E2L,
0x4ADFA541L, 0x3DD895D7L, 0xA4D1C46DL, 0xD3D6F4FBL,
0x4369E96AL, 0x346ED9FCL, 0xAD678846L, 0xDA60B8D0L,
0x44042D73L, 0x33031DE5L, 0xAA0A4C5FL, 0xDD0D7CC9L,
0x5005713CL, 0x270241AAL, 0xBE0B1010L, 0xC90C2086L,
0x5768B525L, 0x206F85B3L, 0xB966D409L, 0xCE61E49FL,
0x5EDEF90EL, 0x29D9C998L, 0xB0D09822L, 0xC7D7A8B4L,
0x59B33D17L, 0x2EB40D81L, 0xB7BD5C3BL, 0xC0BA6CADL,
0xEDB88320L, 0x9ABFB3B6L, 0x03B6E20CL, 0x74B1D29AL,
0xEAD54739L, 0x9DD277AFL, 0x04DB2615L, 0x73DC1683L,
0xE3630B12L, 0x94643B84L, 0x0D6D6A3EL, 0x7A6A5AA8L,
0xE40ECF0BL, 0x9309FF9DL, 0x0A00AE27L, 0x7D079EB1L,
0xF00F9344L, 0x8708A3D2L, 0x1E01F268L, 0x6906C2FEL,
0xF762575DL, 0x806567CBL, 0x196C3671L, 0x6E6B06E7L,
0xFED41B76L, 0x89D32BE0L, 0x10DA7A5AL, 0x67DD4ACCL,
0xF9B9DF6FL, 0x8EBEEFF9L, 0x17B7BE43L, 0x60B08ED5L,
0xD6D6A3E8L, 0xA1D1937EL, 0x38D8C2C4L, 0x4FDFF252L,
0xD1BB67F1L, 0xA6BC5767L, 0x3FB506DDL, 0x48B2364BL,
0xD80D2BDAL, 0xAF0A1B4CL, 0x36034AF6L, 0x41047A60L,
0xDF60EFC3L, 0xA867DF55L, 0x316E8EEFL, 0x4669BE79L,
0xCB61B38CL, 0xBC66831AL, 0x256FD2A0L, 0x5268E236L,
0xCC0C7795L, 0xBB0B4703L, 0x220216B9L, 0x5505262FL,
0xC5BA3BBEL, 0xB2BD0B28L, 0x2BB45A92L, 0x5CB36A04L,
0xC2D7FFA7L, 0xB5D0CF31L, 0x2CD99E8BL, 0x5BDEAE1DL,
0x9B64C2B0L, 0xEC63F226L, 0x756AA39CL, 0x026D930AL,
0x9C0906A9L, 0xEB0E363FL, 0x72076785L, 0x05005713L,
0x95BF4A82L, 0xE2B87A14L, 0x7BB12BAEL, 0x0CB61B38L,
0x92D28E9BL, 0xE5D5BE0DL, 0x7CDCEFB7L, 0x0BDBDF21L,
0x86D3D2D4L, 0xF1D4E242L, 0x68DDB3F8L, 0x1FDA836EL,
0x81BE16CDL, 0xF6B9265BL, 0x6FB077E1L, 0x18B74777L,
0x88085AE6L, 0xFF0F6A70L, 0x66063BCAL, 0x11010B5CL,
0x8F659EFFL, 0xF862AE69L, 0x616BFFD3L, 0x166CCF45L,
0xA00AE278L, 0xD70DD2EEL, 0x4E048354L, 0x3903B3C2L,
0xA7672661L, 0xD06016F7L, 0x4969474DL, 0x3E6E77DBL,
0xAED16A4AL, 0xD9D65ADCL, 0x40DF0B66L, 0x37D83BF0L,
0xA9BCAE53L, 0xDEBB9EC5L, 0x47B2CF7FL, 0x30B5FFE9L,
0xBDBDF21CL, 0xCABAC28AL, 0x53B39330L, 0x24B4A3A6L,
0xBAD03605L, 0xCDD70693L, 0x54DE5729L, 0x23D967BFL,
0xB3667A2EL, 0xC4614AB8L, 0x5D681B02L, 0x2A6F2B94L,
0xB40BBE37L, 0xC30C8EA1L, 0x5A05DF1BL, 0x2D02EF8DL
};
--8<--------
And here's the CRC code, which you'll note is very simple and pretty
damn quick:
--8<--------
static unsigned long crc32(unsigned long crc, const void *buf, size_t size)
{
const unsigned char *p;
p = buf;
crc = crc ^ ~0U;
while (size--)
crc = crctable[(crc ^ *p++) & 0xFF] ^ (crc >> 8);
return crc ^ ~0U;
}
--8<--------
It's up to you to decide whether you wish to sacrifice memory or speed.
As I was writing a utility for the OSD video recorder, a kilobyte was
neither here nor there. On a BBC Micro, it might be a different matter
(though for a Master or B+, I'd say definitely use a lookup table in
order to speed things up).
[or be like Mick's scary-looking ADFS and "just use video workspace"!]
> I've put it all together here: http://mdfs.net/Info/Comp/Comms/CRC32.htm
> I'll be adding CRC-16 code as well as used in XMODEM and the BBC RFS/CFS.
The only problem here is that there are various types of CRC-16. I
wanted to write code to create valid Psion 3 .PIC files. Without a valid
CRC it wouldn't load. I think CCITT and X-Modem are subtly different.
Here's code for the one I wrote for my S3 icon editor. It's a DLL
written in C as I didn't fancy trying bit ops in VB!
The lookup table:
--8<--------
static unsigned short crctab[256] = {
0x0000, 0x1021, 0x2042, 0x3063, 0x4084, 0x50a5, 0x60c6, 0x70e7,
0x8108, 0x9129, 0xa14a, 0xb16b, 0xc18c, 0xd1ad, 0xe1ce, 0xf1ef,
0x1231, 0x0210, 0x3273, 0x2252, 0x52b5, 0x4294, 0x72f7, 0x62d6,
0x9339, 0x8318, 0xb37b, 0xa35a, 0xd3bd, 0xc39c, 0xf3ff, 0xe3de,
0x2462, 0x3443, 0x0420, 0x1401, 0x64e6, 0x74c7, 0x44a4, 0x5485,
0xa56a, 0xb54b, 0x8528, 0x9509, 0xe5ee, 0xf5cf, 0xc5ac, 0xd58d,
0x3653, 0x2672, 0x1611, 0x0630, 0x76d7, 0x66f6, 0x5695, 0x46b4,
0xb75b, 0xa77a, 0x9719, 0x8738, 0xf7df, 0xe7fe, 0xd79d, 0xc7bc,
0x48c4, 0x58e5, 0x6886, 0x78a7, 0x0840, 0x1861, 0x2802, 0x3823,
0xc9cc, 0xd9ed, 0xe98e, 0xf9af, 0x8948, 0x9969, 0xa90a, 0xb92b,
0x5af5, 0x4ad4, 0x7ab7, 0x6a96, 0x1a71, 0x0a50, 0x3a33, 0x2a12,
0xdbfd, 0xcbdc, 0xfbbf, 0xeb9e, 0x9b79, 0x8b58, 0xbb3b, 0xab1a,
0x6ca6, 0x7c87, 0x4ce4, 0x5cc5, 0x2c22, 0x3c03, 0x0c60, 0x1c41,
0xedae, 0xfd8f, 0xcdec, 0xddcd, 0xad2a, 0xbd0b, 0x8d68, 0x9d49,
0x7e97, 0x6eb6, 0x5ed5, 0x4ef4, 0x3e13, 0x2e32, 0x1e51, 0x0e70,
0xff9f, 0xefbe, 0xdfdd, 0xcffc, 0xbf1b, 0xaf3a, 0x9f59, 0x8f78,
0x9188, 0x81a9, 0xb1ca, 0xa1eb, 0xd10c, 0xc12d, 0xf14e, 0xe16f,
0x1080, 0x00a1, 0x30c2, 0x20e3, 0x5004, 0x4025, 0x7046, 0x6067,
0x83b9, 0x9398, 0xa3fb, 0xb3da, 0xc33d, 0xd31c, 0xe37f, 0xf35e,
0x02b1, 0x1290, 0x22f3, 0x32d2, 0x4235, 0x5214, 0x6277, 0x7256,
0xb5ea, 0xa5cb, 0x95a8, 0x8589, 0xf56e, 0xe54f, 0xd52c, 0xc50d,
0x34e2, 0x24c3, 0x14a0, 0x0481, 0x7466, 0x6447, 0x5424, 0x4405,
0xa7db, 0xb7fa, 0x8799, 0x97b8, 0xe75f, 0xf77e, 0xc71d, 0xd73c,
0x26d3, 0x36f2, 0x0691, 0x16b0, 0x6657, 0x7676, 0x4615, 0x5634,
0xd94c, 0xc96d, 0xf90e, 0xe92f, 0x99c8, 0x89e9, 0xb98a, 0xa9ab,
0x5844, 0x4865, 0x7806, 0x6827, 0x18c0, 0x08e1, 0x3882, 0x28a3,
0xcb7d, 0xdb5c, 0xeb3f, 0xfb1e, 0x8bf9, 0x9bd8, 0xabbb, 0xbb9a,
0x4a75, 0x5a54, 0x6a37, 0x7a16, 0x0af1, 0x1ad0, 0x2ab3, 0x3a92,
0xfd2e, 0xed0f, 0xdd6c, 0xcd4d, 0xbdaa, 0xad8b, 0x9de8, 0x8dc9,
0x7c26, 0x6c07, 0x5c64, 0x4c45, 0x3ca2, 0x2c83, 0x1ce0, 0x0cc1,
0xef1f, 0xff3e, 0xcf5d, 0xdf7c, 0xaf9b, 0xbfba, 0x8fd9, 0x9ff8,
0x6e17, 0x7e36, 0x4e55, 0x5e74, 0x2e93, 0x3eb2, 0x0ed1, 0x1ef0
};
--8<--------
and the calc routine:
--8<--------
int WINAPI __declspec(dllexport) CRC16_CalcCRC(char *data,
register int count, long prevsum)
{
register int loop = 0;
register short sum = 0; /* 'short' to ensure a 16 bit integer */
register int tcrc = 0;
sum = (short)prevsum;
for (loop = 0; loop < count; loop++)
{
tcrc = sum;
sum = (tcrc << 8) ^ crctab[ (((tcrc >> 8) ^ data[loop]) & 255) ];
}
return (signed int) (sum & 0xFFFF);
}
--8<--------
Hope this helps.
Best wishes,
Rick.
--
Rick Murray, eeePC901 & ADSL WiFI'd into it, all ETLAs!
BBC B: DNFS, 2 x 5.25" floppies, EPROM prog, Acorn TTX
E01S FileStore, A3000/A5000/RiscPC/various PCs/blahblah...