Here is a version of Olfen’s CRC32 function, which is somewhat shorter, inverts the bits of the result and can be continued, when only sections of the data are available at a time (e.g. when computing the CRC of a large file, read piece-by-piece). The corresponding C code is
Code:
typedef unsigned long uint;
void CRC32_Init(uint* table) { // uint table[256]
uint i, j, poly = 0xEDB88320, CRC;
for(i = 0; i < 256; i++) {
CRC = i;
for(j = 0; j < 8; j++)
if(CRC & 1)
CRC = (CRC >> 1) ^ poly;
else
CRC >>= 1;
table[i] = CRC;
}
}
uint CRC32(unsigned char* buffer, uint len, uint crc32val, uint* table) { // init: crc32val = 0xFFFFFFFF
uint i;
for (i = 0; i < len; i++)
crc32val = table[(crc32val ^ buffer[i]) & 255] ^ (crc32val >> 8);
return ~crc32val;
}
Their machine code is converted to AHK functions by
Code:
MCode(CRC32_Init,"33c06a088bc85af6c101740ad1e981f12083b8edeb02d1e94a75ec8b542404890c82403d0001000072d8c3")
MCode(CRC32,"558bec33c039450c7627568b4d080fb60c08334d108b55108b751481e1ff000000c1ea0833148e403b450c89551072db5e8b4510f7d05dc3")
When the CRC of a string or a binary buffer is computed, crc32val=-1:
Code:
a = ABCDEFGHIJKLMNOPQRSTUVWXYZ ; CRC = 0xabf77822
MsgBox % DllCall(&CRC32, "uint",&a, "uint",StrLen(a), "int",-1, "uint",&CRC32LookupTable, "cdecl uint")
The call is the same, when the first section of long data is processed. Subsequent sections need modified calls: the third parameter be the bitwise-not of the previous CRC value:
Code:
a = clip
b = board ; CRC("clipboard") = 0x8fdbc496
c := DllCall(&CRC32, "uint",&a, "uint",StrLen(a), "int",-1, "uint",&CRC32LookupTable, "cdecl uint")
MsgBox % DllCall(&CRC32, "uint",&b, "uint",StrLen(b), "int",~c, "uint",&CRC32LookupTable, "cdecl uint")
This version is very fast, so it can be used to compare large files. First compare the size of the files. If they are equal, compare their CRC values. Because the CRC’s are only 32 bits long, there is a 50% chance that among 65,536 random files of the same size, two will be falsely labeled as equal. We don’t normally have that many files of the same size. If you do, you can compute more CRC values for the files, which possibly equal to another one. These CRC’s should use different starting values (like -1, 0, 0x55555555).
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