CRC table caluclation problems

[Andreas Håkansson]

Hi,

I'm trying to implement a 32bit Table based CRC caluclator based on the
guidelines in this
http://thorkildsen.no/faqsys/docs/crc.txt document. However I have run into
a small
"problem" that's made me think. When I create my CRC class I want to
precalculate the
crc table and acording to the document I should get the MSB of the byte in
question, check
if it is a 1 and if so XOR the byte, if not continue. Obviously I need to
keep shifting the byte
to the left to keep getting the "MSB" but when I shift left I get obvious
wrong precalculated
values, but when I shift right and mask the LSB bit I get what appears to be
valid
precaluclated values.

Could anyone explain WHY i should shift right and keep checkin the LSB when
the docs
claim to shift left and check the MSB ?

Here is the code I'm working on.

NOTE: this.poly is of type uint and this.table is a uint[256] array

public class CRC
{
private uint poly;
private uint[] table;

public CRC(uint poly)
{
this.poly = poly;
this.table = new uint[256];
uint indexValue = 0;

for(int index=0; index<256; index++)
{
indexValue = (uint)index;
for(int bit=7; bit>=0; bit--)
{
// THIS WORKS
if( (indexValue & 0x0000001) == 1 )
indexValue = (uint)((indexValue >> 1) ^ this.poly);
else
indexValue = (uint)(indexValue >> 1);

// THIS GIVES STRANGE VALUES
// if( (indexValue & 0x1000000) == 1 )
// indexValue = (uint)((indexValue << 1) ^
this.poly);
//else
// indexValue = (uint)(indexValue << 1);
}
this.table[index] = indexValue;
Console.WriteLine(String.Format("{0:X8}",
this.table[index]));
}
}
}

--
ANDREAS HÅKANSSON
STUDENT OF SOFTWARE ENGINEERING
andreas (at) selfinflicted.org