Question

So I need to get all possible permutations of a string.

What I have now is this:

def uniq_permutations string
  string.split(//).permutation.map(&:join).uniq
end

Ok, now what is my problem: This method works fine for small strings but I want to be able to use it with strings with something like size of 15 or maybe even 20. And with this method it uses a lot of memory (>1gb) and my question is what could I change not to use that much memory?

Is there a better way to generate permutation? Should I persist them at the filesystem and retrieve when I need them (I hope not because this might make my method slow)?

What can I do?

Update:

I actually don’t need to save the result anywhere I just need to lookup for each in a table to see if it exists.

Answer 1

Just to reiterate what Sawa said. You do understand the scope? The number of permutations for any n elements is n!. It’s about the most aggressive mathematical progression operation you can get. The results for n between 1-20 are:

[1, 2, 6, 24, 120, 720, 5040, 40320, 362880, 3628800, 39916800, 479001600, 
 6227020800, 87178291200, 1307674368000, 20922789888000, 355687428096000,
 6402373705728000, 121645100408832000, 2432902008176640000]

Where the last number is approximately 2 quintillion, which is 2 billion billion.

That is 2265820000 gigabytes.

You can save the results to disk all day long – unless you own all the Google datacenters in the world you’re going to be pretty much out of luck here 🙂