http://qs321.pair.com?node_id=962981


in reply to Re^2: [OT] The statistics of hashing.
in thread [OT] The statistics of hashing.

BrowserUk,

I hadn't thought about it but since the files were encrypted, more character sequences might appear.

As far as the 'SimCRC64' algorithm, it was just a simple CRC routine. I don't have the actual code, but it was something like this.


 my $crc = crc64(\$output);           ## Add crc check to output data

 sub crc64           ## Add crc check to output data
 {   my $data = shift; my $crc = ""; my $len = length($$data); my $pos
+ = 0;
     while ( $pos < $len )
     {  $crc = $crc ^ ( substr($$data,$pos,8) );
        $pos += 8;
     }
     return ( $crc );
 }

[download]

It was part of the test to see what the worst case would be. As I previously said, I didn't expect the results.

Thank you

"Well done is better than well said." - Benjamin Franklin

Replies are listed 'Best First'.
Re^4: [OT] The statistics of hashing.
by BrowserUk (Patriarch) on Apr 02, 2012 at 12:41 UTC
    I don't have the actual code, but it was something like this.

    Thanks for posting that.

    However, for my purposes -- deterministically generating a randomly distributed value -- it doesn't work well. With the MD5, every single bit change in the input generates an average of 64-bits change in the output -- which is near perfect.

    But this crc algorithm -- which was probably great for your purposes -- only generates a single bit change in the output for each single bit change in the input:

    <Reveal this spoiler or all in this thread>
    With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.

    The start of some sanity?
[reply]
[d/l]

      BrowserUK,

      I had to download your code to figure out what you were doing, but now I know your interest in uniqueness was different than mine(i.e. project).

      Each of the test encrypted files was unique and we wanted an algorithm that would produce the lowest number of duplicate hashes for the different unique files. The statistics I showed was the number of duplicate hashes produced from unique files. Each hash was generated and then: 

        ...
  $Dupes{$hash}++;
  ...

      [download]
      Then we just counted the total number of keys with values greater than one.

      Your code was very interesting and quite a mind tester.

      Thank you

      "Well done is better than well said." - Benjamin Franklin

[reply]
[d/l]

In Section Seekers of Perl Wisdom