class Hamster::Vector

def permutation(n = @size)
  return enum_for(:permutation, n) if not block_given?
  if n < 0 || @size < n
    # yield nothing
  elsif n == 0
    yield []
  elsif n == 1
    each { |item| yield [item] }
  else
    used, result = [], []
    perms = lambda do |index|
      0.upto(@size-1) do |i|
        if !used[i]
          result[index] = get(i)
          if index < n-1
            used[i] = true
            perms[index+1]
            used[i] = false
          else
            yield result.dup
          end
        end
      end
    end
    perms[0]
  end
  self
end