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/*
* Dr Dobb ' s Journal : http : //www.ddj.com/ftp/1992/1992.05/crcman.zip
*
* Copyright Mark R . Nelson 1992
*
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* This code used by permission of J Erickson < jerickson @ ddj . com >
* Tues 6 th October 1998. Copyright acknowledged above , as agreed .
*
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*/
# include "includes.h"
# define CRC32_POLYNOMIAL 0xEDB88320L
/*****************************************************************
Instead of performing a straightforward calculation of the 32 bit
CRC using a series of logical operations , this program uses the
faster table lookup method . This routine is called once when the
program starts up to build the table which will be used later
when calculating the CRC values .
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
static uint32 CRCTable [ 256 ] ;
void crc32_build_table ( void )
{
int i ;
int j ;
uint32 crc ;
for ( i = 0 ; i < = 255 ; i + + )
{
crc = i ;
for ( j = 8 ; j > 0 ; j - - )
{
if ( crc & 1 )
{
crc = ( crc > > 1 ) ^ CRC32_POLYNOMIAL ;
}
else
{
crc > > = 1 ;
}
}
CRCTable [ i ] = crc ;
}
}
/*****************************************************************
This routine calculates the CRC for a block of data using the
table lookup method .
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
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uint32 crc32_calc_buffer ( uint32 count , char * buffer )
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{
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char * p ;
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uint32 crc ;
p = buffer ;
crc = 0xffffffff ;
while ( count - - ! = 0 )
{
uint32 temp1 ;
uint32 temp2 ;
temp1 = ( crc > > 8 ) & 0x00FFFFFFL ;
temp2 = CRCTable [ ( ( int ) crc ^ * p + + ) & 0xff ] ;
crc = temp1 ^ temp2 ;
}
return crc ;
}
