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POAlgorithm for determining a file’s identity (Optimisation)
 

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    <p>Further to this question: <a href="https://stackoverflow.com/questions/456175/algorithm-for-determining-a-files-identity">Algorithm for determining a file’s identity</a></p> <p><strong>Recap</strong>: I'm looking for a cheap algorithm for determining a files identity which works the vast majority of the time. </p> <p>I went ahead and implemented an algorithm that gives me a "<em>pretty unique</em>" hash per file.</p> <p>The way my algorithm works is:</p> <ul> <li><p>For files smaller than a certain threshold I use the full files content for the identity hash. </p></li> <li><p>For files larger than the threshold I take random N samples of X size.</p></li> <li><p>I include the filesize in the hashed data. (meaning all files with different sizes result in a different hash) </p></li> </ul> <p>Questions: </p> <ul> <li><p>What values should I choose for N and X (how many random samples should I take of which size?) I went with 4 samples of 8K each and am not able to stump the algorithm. I found that increasing the amount of samples quickly decreases the speed of the algorithm (cause seeks are pretty expensive) </p></li> <li><p>The maths one: how non-different do my files need to be for this algorithm to blow up. (2 different files with same length end up having the same hash) </p></li> <li><p>The optimization one: Are there any ways I can optimize my concrete implementation to improve throughput (I seem to be able to do about 100 files a second on my system).</p></li> <li><p>Does this implementation look sane? Can you think of any real world examples where this will fail. (My focus is on media files) </p></li> </ul> <p>Relevant information:</p> <p><a href="http://gist.github.com/101591" rel="nofollow noreferrer">The algorithm I implemented</a></p> <p>Thanks for your help!</p>
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    1. COnitpicking: Signiture!? you mean Signature?
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