Re: Binary comparison of files

It depends on what the expected variation is ... if it were 1 random
byte in the file then you would both be O(n) but you have twice as
large of a C

On Wed, Jan 28, 2009 at 1:26 PM, Geoff Taylor <[email protected]> wrote:
>> 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
>
> ===================================
> View archives and manage your subscription(s) at http://peach.ease.lsoft.com/archives
>



-- 
It is the mark of an educated mind to be able to entertain a thought
without accepting it.

===================================
View archives and manage your subscription(s) at http://peach.ease.lsoft.com/archives