Re: [Boost-Users] Re: [crc] Simple example please

From: Duane Murphy (duanemurphy_at_[hidden])
Date: 2003-06-10 14:20:21

--- At Tue, 10 Jun 2003 15:10:46 -0400, Daryle Walker wrote:

>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.

[ I expect a correction flood on this one. :-) ]

In fact, vector<> is guaranteed to be contiguous. It is the only standard
container that satisfies this requirement.

 ...Duane