Re: [crc] Simple example please

From: Daryle Walker (darylew_at_[hidden])
Date: 2003-06-10 14:10:46

On Monday, June 9, 2003, at 5:48 PM, Alex Henderson wrote:

> I'd like to calculate a value for a cyclic redundancy check (crc) for 
> a number of files.
>
> This isn't going to be transmitted anywhere, I simply want to compare 
> the values to see whether the files are exactly the same. I know I can 
> simply check byte for byte, but I've got a few thousand files and it's 
> easier to calculate the crc for each and see whether any are the same 
> first.
>
> I've looked at the crc library, but can't get my head round the 
> example. It seems to test the library rather than show how its used.

Yes, the sample program is meant as a test of the library.  You can 
still get an idea of usage from it.  There are also short examples in 
the HTML documentation.

> I'm after something like
>
>   std::vector<char> MyStream1; // the whole file (about 500kb)
>
>   boost::SomeCRCTypeThatIsOptimisedForSafetyAndLargeishBlocks<???> x;
>
>   long = x.checkme(MyStream1);
>
> Could someone please point me to some code where a simple example is 
> detailed?

What's in the documentation is it.

Do you have a specific CRC algorithm in mind?  If so, you just pick the 
appropriate template parameters.  If not, you can choose one of the 
samples I provided, like boost::crc_32_type.

The optimized CRC function objects can work with a single byte at a 
time or an array-segment of bytes.  By putting your file's data in a 
std::vector as you have in your example, you limit yourself to just the 
single-byte computing call, as the elements of a vector may not be 
contiguous.  Just allocate a simple char array and use one of the CRC 
object's array computing calls or use one of the CRC functions.  Based 
on what you said, I don't think you need the object-wrapping that 
std::vector gives.

Daryle