#!/usr/bin/ruby -Ke

# Permutation Library
# http://blade.nagaokaut.ac.jp/~sinara/ruby/math/combinatorics/index.html

class Array
  def perm(n = size)
    if size < n or n < 0
    elsif n == 0
      yield([])
    else
      self[1..-1].perm(n - 1) do |x|
        (0...n).each do |i|
          yield(x[0...i] + [first] + x[i..-1])
        end
      end
      self[1..-1].perm(n) do |x|
        yield(x)
      end
    end
  end
end

if $0 == __FILE__
  ["a", "b", "c", "d"].perm(3) do |x|
    p x
  end
end