On Mon, 18 Dec 2006 01:44:16 +0900, James Edward Gray II wrote:

> Begin forwarded message:
> 
>> From: darrenks / ml1.net
>> Date: December 16, 2006 9:51:18 AM CST
>> To: submission / rubyquiz.com
>> Subject: Please Forward: Ruby Quiz Submission
>>
>> Hi, here is my solution to the chess 960 quiz. Since white and  
>> black are the same I decided to just print it once and omit the  
>> whole white/black thing. It is not totally efficient, for position  
>> #960 it will have to iterate about 20,000 times, however it still  
>> runs in about a one second in this case.
>>
>> # Chess960 minimalist version
>> # Author: Darren Smith
>> # Usage: ruby $0 [1..960] (omit for random position)
>>
>> i=$*[0]||rand(960)+1
>> srand 1558
>> p=%w'K R N B B Q N R'
>> h={}
>> while h.size<i.to_i
>> 	p=p.sort_by{rand}
>> 	$_=p*'  '
>> 	h[$_]=1if~/R.*K.*R/&&~/B(..)*B/
>> end
>> puts"Position ##{i}:",$_
>>

My solution borrows heavily from Darren's, but I don't like the idea of
creating permutations by bogosorting, since there's no guarantee you'll
ever see every necessary permutation. (Here, he does see every
permutation, quickly, but that's not guaranteed for different random seeds.)

#!/usr/bin/env ruby
require 'rubygems'
require 'facets/core/enumerable/permutation'
require 'set'
i=ARGV[0] || rand(960)+1
i=i.to_i
fail "A number between 1 and 960 is required" if not (1..960).include? i
pieces=%w'K R N B B Q N R'
generated=Set.new
pieces.each_permutation do |p|
  arrangement=p.join
  if arrangement=~/R.*K.*R/ and arrangement=~/B(..)*B/ and
      not generated.include?(arrangement)
   
    #the "generated" set is needed because identical objects will
    #nevertheless appear in both possible permutations, like so:
    # [1,1].each_permutation {|pe| p pe}
    # gives the output
    # [1, 1]
    # [1, 1]
    generated.add arrangement

    if generated.size==i
      print "Position ##{generated.size}:", arrangement, "\n"
      break
    end
  end
end

-- 
Ken Bloom. PhD candidate. Linguistic Cognition Laboratory.
Department of Computer Science. Illinois Institute of Technology.
http://www.iit.edu/~kbloom1/