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This is my solution.  It makes use of my ArrayValue class which was 
discussed on the list previously.  It calculate the placement of pieces 
for a certain number without first calculating the placement of the 
pieces for all numbers lower than it by using the following algorithm 
(found at http://frcec.tripod.com/fischerrandomchessstartingpositions/):
1. id % 4 * 2  ight square bishop (counting left to right
2. (id / 4) % 4 * 2 + 1  ark square bishop ^ starting at 0)
3. (id / 4 / 6) % 6  ueen, number of vacant squares from the left
4. (id / 4 / 6)  eRN code of the other pieces (see the website for 
more.  The KeRN codes do have a pattern but I hard coded it).

First, an explaination of the ArrayValue class.  The code is below:
daniel@daniel-desktop:~$ cat arrayvalue.rb
class ArrayValue
         instance_methods.each do |m|
                 undef_method(m) unless m 
/^_*(method_missing|send|id)_*$/
         end

         def initialize(origArray, origIndex)
                 @origArray, @origIndex  rigArray, origIndex
         end

         def set(newObj)
                 @origArray[@origIndex]  ewObj
         end

         def get
                 @origArray[@origIndex]
         end

         def method_missing(method, *args)
                         get.send(method, *args)
                 rescue
                         super
         end

         define_method(:' ) {|other| set(other)}
end

class Array
         def to_av()
                 ret  ]
                 each_index {|x| ret << ArrayValue.new(self, x) }
                 ret
         end
end

So, an ArrayValue stores the array and index it originally came from and 
it can edit/get those values.  Otherwise, an ArrayValue inherits all of 
the methods of the value it represents because of the method_missing 
definition.  One thing missing from the ArrayValue class is what to do 
if CRD (CRUD minus the U) operations are performed on the original 
array.  Because of this, I recommend never actually storing an 
ArrayValue but creating them again every time you need one.

Also, the array class is edited to provide a to_av method, which returns 
an array of ArrayValues.  This is the only recommended way of creating 
ArrayValues.

Note that Logan Capaldo wrote something with the same concept but that 
uses some tricks with arrays and overriding [] o that calling 
ArrayValue#Set is not necessary, you just go 
ArrayOfArrayValues[0]mething and it changes the original array.

Some uses of ArrayValue:
daniel@daniel-desktop:~$ irb -r arrayvalue.rb
irb(main):001:0> ary  1, 2, 3, "a", :b]
[1, 2, 3, "a", :b]
irb(main):002:0> ary.to_av[4].set("c")
"c"
irb(main):003:0> ary
[1, 2, 3, "a", "c"]
irb(main):005:0> ary.to_av.select{|x| x.kind_of?(Numeric)}.each{|x| 
x.set(42)}
[42, 42, 42]
irb(main):006:0> ary
[42, 42, 42, "a", "c"]
irb(main):013:0> ary  "Skip me!", nil, nil, nil, 43]
["Skip me!", nil, nil, nil, 43]
irb(main):014:0> ary.to_av.select{|x| x.nil?}[1].set("The 2nd nil")
"The 2nd nil"
irb(main):015:0> ary
["Skip me!", nil, "The 2nd nil", nil, 43]

Now to the real code:
daniel@daniel-desktop:~$ cat chess960short.rb
#! /usr/bin/ruby
require 'arrayvalue.rb'

KeRN  <-END.split("\n").collect{|x| x.split(" ")}
         N N R K R
         N R N K R
         N R K N R
         N R K R N
         R N N K R
         R N K N R
         R N K R N
         R K N N R
         R K N R N
         R K R N N
         END

id  RGV[0].to_i % 960
out  rray.new(8)
1.downto(0) {|x| out[id % 4 *2 + x]  B"; id /  }
out.to_av.select{|x| x.nil?}[id % 6].set("Q"); id / 
KeRN[id].each{ |currentPiece| out.to_av.select{|x| 
x.nil?}.first.set(currentPiece) }

Another, commented and not written for brevity version will be posted 
shortly.

Ruby Quiz wrote:
> The three rules of Ruby Quiz:
> 
> 1.  Please do not post any solutions or spoiler discussion for this quiz until
> 48 hours have passed from the time on this message.
> 
> 2.  Support Ruby Quiz by submitting ideas as often as you can:
> 
> http://www.rubyquiz.com/
> 
> 3.  Enjoy!
> 
> Suggestion:  A [QUIZ] in the subject of emails about the problem helps everyone
> on Ruby Talk follow the discussion.  Please reply to the original quiz message,
> if you can.
> 
> --------------------
> 
> by Kieran Wild
> 
> Chess960, is a chess variant produced by Grandmaster Bobby Fischer by
> formalizing the rules of Shuffle Chess. Its goal was to create a chess variant
> in which chess creativity and talent would be more important than memorization
> and analysis of opening moves. His approach was to create a randomized initial
> chess position, which would thus make memorizing chess opening move sequences
> far less helpful. The initial position is set up in a special way and there are
> 960 such positions, thus the name Chess960.
> 
> The starting position for Chess960 must meet certain rules. White pawns are
> placed on the second rank as in chess. All remaining white pieces are placed
> randomly on the first rank, but with the following restrictions:
> 
> 	* The king is placed somewhere between the two rooks.
> 	* The bishops are placed on opposite-colored squares.
> 
> The black pieces are placed equal-and-opposite to the white pieces. For example,
> if the white king is placed on b1, then the black king is placed on b8. Note
> that the king never starts on file a or h, because there would be no room for a
> rook
> 
> Can I suggest a nice little ruby program to generates all 960 possible starting
> positions and outputs a random one on request.
> 
> Output could be as follows.
> 
> 	Starting Position 432:
> 	
> 	White
> 	
> 	a1 b1 c1 d1 e1 f1 g1 h1
> 	N  B  B   R  K  R  Q  N
> 	
> 	Black
> 	
> 	a8 b8 c8 d8 e8 f8 g8 h8
> 	N  B  B   R  K  R  Q  N
> 
> Or some better output.
> 
> 

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