[Boost-users] Compile-time constant CRCs

From: Michael Marcin (mmarcin_at_[hidden])
Date: 2006-06-30 22:36:44

I didn't tag this because I'm not sure if it should belong to 
[preprocessor], [mpl] or [crc].

Given the black-box (macro, function, class, whatever) I'll call ConstCRC32 
that is assignable to a unsigned int is there a way to write:

unsigned int hash = ConstCRC32( "HELLO" );

and have an optimized build yield the equivalent of:

unsigned int hash = 0x58a10fc0;

I had a solution (although obviously non-portable) in msvc7.1 but after 
moving to msvc8 and stepping through the code I noticed that msvc8's 
"improved" optimizing compiler cannot optimize away the calculation like 
msvc7.1's could.  I also doubt that the code would work if I ever had to 
port it to another compiler.

I tried using simple TMP techniques I've used in the past but couldn't find 
a way to access the characters of the string.

So looking around I came upon boost.crc which is geared towards computing 
crcs of blocks of data which isn't really what I care about right now.

I also found boost.mpl which is very cool but I have a hard time grasping 
still.
The problems seems solvable with mpl if I could transform the data into 
boost::mpl::list_c<char,'H','E','L','L','O'> but I still coulnd't find a way 
to access the characters of the string and I think others at my company 
would rather revert to the old
#define CRC( str, crc ) crc
unsigned int hash = CRC( "HELLO", 0x58a10fc0 );
than move to that syntax directly.

So looking further I found boost.preprocessor which I haven't explored 
enough to see if there is away to have PP calculate the CRC directly or have 
it transform the string into the boost::mpl::list_c form.

Can anyone offer some advice?

Thanks

Mike

P.S.

The old version was actually fairly simple it just used a precomputed static 
const unsigned int lookup table and did a strlen and a for loop with a 
lookup into the table for each character (in a __forceinline function that 
took a const char * const string).