Re: [MacPerl] CRC.pm for Macintosh

[bart.lateur@skynet.be (Bart Lateur)]

On Tue, 5 Jan 1999 22:18:12 +0000, Lindsay Davies wrote:

>You probably don't need to do this, as you can just use perl's unpack 
>function to generate a CRC. Camel p.237 gives the following example 
>for a 32 bit CRC...
>
>	undef $/;
>	$checksum = unpack("%32C*", <FILEHANDLE>) % 32767;

I think you're confusing a CRC with a plain checksum.

In checksums, the bytes are simply added, and the result is given modulo
some upper value (only the lower bits preserved). In a CRC, the result
depends on the order of bytes as well. That's one of the things that
makes it so powerful!

	$\ = "\n";
	print unpack("%32C*", 'ab') % 32767;
	print unpack("%32C*", 'ba') % 32767;
	print ord('a') + ord('b');

Result:

	195
	195
	195

Yup. A plain checksum, not a CRC.

	Bart.

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