Finding arbitrary permutations of a list

def factorial(n)
  num = 1
  for i in 1..n do
    num *= i
  end
  num
end

def permutation(list, n)
  if n < 0
    raise "Index must be positive"
  end

  working = Array.new(list)
  n = n%factorial(working.length)

  index = 0
  for i in (working.length-1).downto(1) do
    pos = n/factorial(i)
    n = n%factorial(i)
    tmp = working[index]
    working[index] = working[index+pos]
    working[index+pos] = tmp
    index += 1
  end
  return working
end

puts "Enter a list of values."
values = gets.chomp.split(/[ ]*[ ,][ ]*/)

puts "All possible permutations:"
for index in 0...factorial(values.length) do
  print "#{permutation(values,index)}\n"
end