This is a function I wrote that takes an array, and returns all possible permutations of arrays of that same size.

permutation.rb

def permute(arr)
  permutation(arr.sort)
end

def permutation(arr, result=[])
  k = nil
  result += [arr.dup]
  (arr.length-1).times do |i|
    if arr[i] < arr[i+1]
      k = i
    end
  end
  if k.nil?
    return result
  else
    l = nil
    arr.length.times do |i|
      if arr[k] < arr[i]
        l = i
      end
    end
    arr[k], arr[l] = arr[l], arr[k]
    arr = arr[0..k] + arr[k+1..-1].reverse
    return permutation(arr, result)
  end
end