# From: http://www.rubyonrailsblog.com/articles/2006/08/31/permutations-in-ruby-can-be-fun (in the comments)
# Author: Brian Mitchell

class Array
  # The accumulation is a bit messy but it works ;-)
  def sequence(i = 0, *a)
    return [a] if i == size
    self[i].map {|x|
      sequence(i+1, *(a + [x]))
    }.inject([]) {|m, x| m + x}     # this has to be used instead of flatten so I can sequence something
                                    # like [[[4]]] -> [[[4]]] rather than -> [[4]]; ruby 1.9 has an option for flatten
  end
end


p [(0..3), [4,6]].sequence             #=> [[0, 4], [0, 6], [1, 4], [1, 6], [2, 4], [2, 6], [3, 4], [3, 6]]      
p [(0..3).collect, [4, 6]].sequence



# http://wiki.rubygarden.org/Ruby/page/show/ArrayPermute
# Permute an array, and call a block for each permutation
# Author: Paul Battley

    class Array
        def permute(prefixed=[])
            if (length < 2)
                # there are no elements left to permute
                yield(prefixed + self)
            else
                # recursively permute the remaining elements
                each_with_index do |e, i|
                    (self[0,i]+self[(i+1)..-1]).permute(prefixed+[e]) { |a| yield a }
                end
            end
        end
    end


%w[a b c].permute { |x| puts(x.join('')) } 

[0, 1, 2, 3 ].permute { |x| puts(x.join('-')) } 



# http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/32844
# Author: Steven Grady

class Array
  def permutations
    return [self] if size < 2
    perm = []
    each { |e| (self - [e]).permutations.each { |p| perm << ([e] + p) } }
    perm
  end
end

p (1..4).to_a.permutations



# http://blade.nagaokaut.ac.jp/~sinara/ruby/math/combinatorics/array-perm.rb
# Author: Shin-ichiro Hara
# For many more permutation snippets see: 
# http://blade.nagaokaut.ac.jp/~sinara/ruby/math/combinatorics/

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



# Based on:
# http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/139290
# Author: Endy Tjahjono

class String
   def perm
       return [self] if self.length < 2
       ret = []
    
       0.upto(self.length - 1) do |n|
          #rest = self.split('')                
          rest = self.split(//u)            # for UTF-8 encoded strings
          picked = rest.delete_at(n)
          rest.join.perm.each { |x| ret << picked + x }
       end

       ret
   end
end

p "abc".perm      #=>  ["abc", "acb", "bac", "bca", "cab", "cba"]



require 'permutation'

# http://permutation.rubyforge.org 
# http://permutation.rubyforge.org/doc/classes/Permutation.html
# sudo gem install permutation --remote
# For more examples see permutation-0.1.4/examples/tsp.rb and permutation_0.1.4/lib/permutation.rb:
# curl http://files.rubyforge.vm.bytemark.co.uk/permutation/permutation-0.1.4.tgz | tar xfz -

perm = Permutation.new(3)   
p perm.map { |p| p.value }        #=>  [[0, 1, 2], [0, 2, 1], [1, 0, 2], [1, 2, 0], [2, 0, 1], [2, 1, 0]]