re "enlighten" - I think under the circumstances the most appropriate enlightment I can offer is to improve your lateral thinking In particular I recommend Six Thinking Hats. You would benefit from the insight that you tend to be stuck wearing a black hat (negative logic) and need to learn how to take it off at will and replace it with other forms of thinking when needed. If you need evidence - In your post you say on the one hand you find it difficult to see how I expect it to work and yet you go on to imply an ability to assess its performance when corrected. That's like saying the performance of anything in a category is X without realising the scope you have already accepted for that category, making it impossible to make any sensible such judgement. Conversely, all I am offering is a means to use hashes to solve the problem and I sincerely expect this to perform well when corrected given the pure simplicity of the algorithm and the opportunities it uses to cut out unnecessary iteration. I think you will find that many beginners out there will find it easy enough to correct this code and get a fast-performing result. So why not you, eh? You're clearly not nearly a beginner are you. So what else can it be ... ?