Re: Binary comparison of files

Heh - so it does! That's what I get for changing code half-way through  
an email. Apologies.

     Geoff

On 28 Jan 2009, at 20:03, Kim Major <[email protected]> wrote:

> The code below also reads the whole file " byte[] referenceFile =
> File.ReadAllBytes (referenceFilename);"
>
> You would probably be better off by comparing 1) the size and 2)  
> then two
> file streams byte by byte.
> Like this:
> http://jopinblog.wordpress.com/2008/05/06/compare-files-method-in-c-unit-tes
> ting/
>
>
> Kim Major
> Renaissance Computer Systems Ltd.
> Blog: http://blogs.microsoft.co.il/blogs/kim
> http://www.renaissance.co.il
>
>
> -----Original Message-----
> From: Discussion of advanced .NET topics.
> [mailto:[email protected]] On Behalf Of Eddie Lascu
> Sent: Wednesday, January 28, 2009 9:53 PM
> To: [email protected]
> Subject: Re: [ADVANCED-DOTNET] Binary comparison of files
>
> Geoff,
>
> Are you working on the same project as me? It's like you are doing  
> exactly
> what I need. I agree 100% with your comments. Hashing is not the best
> approach because it will require the processing of the whole file.
>
> Thanks bunches,
> Eddie
>
>
>
> -----Original Message-----
> From: Discussion of advanced .NET topics.
> [mailto:[email protected]] On Behalf Of Geoff  
> Taylor
> Sent: Wednesday, January 28, 2009 1:27 PM
> To: [email protected]
> Subject: Re: [ADVANCED-DOTNET] Binary comparison of files
>
>> If all you need to know is that the files are different; I would  
>> use a
>> hash.  CRC32 is fairly fast.  For more reliability you can use a
>> cryptographic hash.
>>
>> If you want to know what the differences are; the performance would
>> depend on how you plan on processing the differences.
>
> Surely a hash will require processing all the file before returning  
> its
> result?  So if the first byte is different, it's still going to read  
> in all
> of both files and calculate both hashes.
>
> Why not just go for a simpler approach using buffered IO.  You read  
> two
> chunks (one chunk from each file) into byte buffers then do a byte- 
> by-byte
> comparison, breaking the loop at the first byte that doesn't match.
>
> Since they're small files, you could probably just do a ReadAllBytes  
> instead
> of reading into buffers.  That would give you the length of both, so  
> you
> could check that first, giving you another shortcut to a fast  
> negative.
>
> Here's a similar method that uses assertions instead of failure  
> conditions
> that should show you what I mean:
>
>        public static void Compare (string referenceFilename, string
> testFilename)
>        {
>            byte[] referenceFile = File.ReadAllBytes  
> (referenceFilename);
>            byte[] testFile = File.ReadAllBytes (testFilename);
>            Assert.AreEqual (testFile.Length, referenceFile.Length,  
> "Files
> are of different lengths.  Reference file is {0} bytes, test file is  
> {1}
> bytes.", referenceFile.Length, testFile.Length);
>            for (int counter = 0; counter < referenceFile.Length;  
> counter++)
>            {
>                if (referenceFile [counter] != testFile [counter])
>                {
>                    Assert.Fail ("Files do not match (at position " +
> counter + " - [before '" + indicatorString + "']).");
>                }
>            }
>
>            return;
>        }
>
> I'm pretty sure this'll be faster than using a hash (although I'd  
> love to
> see a comparison of timings).
>
> You might be able to make the method faster by looking at how the  
> data is
> read in.
>
> Good luck,
>
>            Geoff
>
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