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71 lines
1.4 KiB
C++
71 lines
1.4 KiB
C++
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#define STRICT
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#include <windows.h>
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/*
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* Instead of performing a straightforward calculation of the 32 bit
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* CRC using a series of logical operations, this program uses the
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* faster table lookup method. This routine is called once when the
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* program starts up to build the table which will be used later
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* when calculating the CRC values.
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*/
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#define CRC32_POLYNOMIAL 0xEDB88320L
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unsigned long CRCTable[256];
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static void BuildCRCTable(void)
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{
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static BOOL TableBuilt = FALSE;
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int i;
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int j;
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unsigned long crc;
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if(!TableBuilt)
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{
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for(i = 0; i <= 255; i++)
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{
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crc = i;
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for(j = 8; j > 0; j--)
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if(crc & 1) crc = (crc >> 1) ^ CRC32_POLYNOMIAL; else crc >>= 1;
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CRCTable[i] = crc;
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}
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}
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}
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/*
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* This routine calculates the CRC for a block of data using the
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* table lookup method. It accepts an original value for the crc,
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* and returns the updated value.
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*/
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static unsigned long CalculateBufferCRC(unsigned long count, BYTE *p)
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{
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unsigned long temp1;
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unsigned long temp2;
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unsigned long crc = 0xFFFFFFFFL;
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while (count-- != 0)
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{
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temp1 = ( crc >> 8 ) & 0x00FFFFFFL;
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temp2 = CRCTable[((int) crc ^ *p++) & 0xff];
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crc = temp1 ^ temp2;
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}
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return crc;
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}
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BOOL VerifyChecksum(LPBYTE buf, LPDWORD CheckSum, DWORD sz)
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{
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DWORD tempcs;
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tempcs = *CheckSum;
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*CheckSum = 0;
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BuildCRCTable();
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return tempcs == CalculateBufferCRC(sz, buf);
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}
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