76 lines
2.0 KiB
C++
76 lines
2.0 KiB
C++
#include "stdafx.h"
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#include "crc32.h"
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Crc32Gen::Crc32Gen()
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{
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init_CRC32_Table();
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}
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// Call this function only once to initialize the CRC table.
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void Crc32Gen::init_CRC32_Table()
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{
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// This is the official polynomial used by CRC-32
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// in PKZip, WinZip and Ethernet.
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unsigned int ulPolynomial = 0x04c11db7;
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// 256 values representing ASCII character codes.
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for(int i = 0; i <= 0xFF; i++)
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{
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crc32_table[i] = reflect(i, 8) << 24;
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for (int j = 0; j < 8; j++)
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crc32_table[i] = (crc32_table[i] << 1) ^ (crc32_table[i] & (1 << 31) ? ulPolynomial : 0);
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crc32_table[i] = reflect(crc32_table[i], 32);
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}
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}
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unsigned int Crc32Gen::reflect(unsigned int ref, char ch)
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{// Used only by Init_CRC32_Table().
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unsigned int value = 0;
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// Swap bit 0 for bit 7
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// bit 1 for bit 6, etc.
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for(int i = 1; i < (ch + 1); i++)
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{
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if(ref & 1)
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value |= 1 << (ch - i);
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ref >>= 1;
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}
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return value;
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}
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unsigned int Crc32Gen::get_CRC32( const char *data, int size, unsigned int ulCRC )
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{// Pass a text string to this function and it will return the CRC.
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// Once the lookup table has been filled in by the two functions above,
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// this function creates all CRCs using only the lookup table.
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// Be sure to use unsigned variables, because negative values introduce high bits where zero bits are required.
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// Start out with all bits set high.
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int len;
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unsigned char* buffer;
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// Get the length.
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len = size;
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// Save the text in the buffer.
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buffer = (unsigned char*)data;
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// Perform the algorithm on each character in the string, using the lookup table values.
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while(len--)
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ulCRC = (ulCRC >> 8) ^ crc32_table[(ulCRC & 0xFF) ^ *buffer++];
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// Exclusive OR the result with the beginning value.
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return ulCRC ^ 0xffffffff;
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}
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unsigned int Crc32Gen::GetCRC32( const char *text )
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{
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int len = strlen(text);
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return GetCRC32( text,len,0xffffffff );
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}
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unsigned int Crc32Gen::GetCRC32( const char *data, int size, unsigned int ulCRC )
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{
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static Crc32Gen stdCrc32Generator;
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return stdCrc32Generator.get_CRC32( data,size,ulCRC );
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} |