permutation implementation for array

perm.rb

class Array
  def generate_permutations
    return [self] if self.size < 2
    permutation = []
    self.each do |element|
      (self - [element]).generate_permutations.each do |perm_accumulator|
        permutation.push ([element] + perm_accumulator)
      end
    end
    permutation
  end
end

Whenever you see code with the following structure:

results = initial_thing
enumerable.each do |thing|
  results = some_method(results, thing)
end
return results

you can write

return enumerable.inject(initial_thing) do |results, thing|
  some_method(results, thing)
end

That's pretty much the definition of inject.

Removing or adding elements from anywhere but the end of an array is very expensive. Why?

You should avoid it if you can.