From: Robert Dober <robert.dober@...>
Date: 2006-12-18T04:28:23+09:00
Subject: Re: [QUIZ] Chess960 (#106)

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Here goes my second solution, looks a little bit better, using Jim Weirich's
Amb class
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#!/usr/bin/ruby
# This solution uses a cut down version of Jim Weirich's Amb class
# submitted to Ruby Quiz # 70. Hope that's ok?
#
#  The purpose of this solution is to show how #generate becomes more
readable
#  and there is a fix of the "place 2 Knights instead of 1 Queen" error.
class Amb
  class ExhaustedError < RuntimeError; end

  def initialize
    @fail = proc { fail ExhaustedError, "amb tree exhausted" }
  end

  def choose(*choices)
    prev_fail = @fail
    callcc { |sk|
      choices.each { |choice|
        callcc { |fk|
            @fail = proc {
                @fail = prev_fail
                    fk.call(:fail)
                }
                sk.call(choice)
            }
        }
        @fail.call
    }
  end

  def assert(cond)
    choose unless cond
  end
end


class Chess960
    N = 8
    Queen = ?Q
    King = ?K
    Rook = ?R
    Bishop = ?B
    def initialize
        @amb = Amb.new
        @solutions = []
        init
        generate
        raise RuntimeError, "Illegal Number of solutions #{@solutions.length}"
unless
            @solutions.length == 960
    end
    def [] sol_nb
        @solutions[ sol_nb ]
    end

    private
    def init
        @sol="N " * N
    end
    def generate
        @b1 = @amb.choose( *1..N-1 )
        @b2 = @amb.choose( *@b1.succ..N )
        @amb.assert @b1 & 1 != @b2 & 1
        @r1 = @amb.choose( *1..N-2 )
        @k = @amb.choose( *@r1.succ..N-1 )
        @r2 = @amb.choose( *@k.succ..N )
        @q = @amb.choose( *1..N )
        # This late check makes the whole thing more readable
        # we can easily afford the additional computations
        @amb.assert [@b1,@b2,@r1,@k,@r2,@q].uniq.length == 6
        save_solution
            rescue Amb::ExhaustedError
    end
    def save_solution
        @sol[2*(@b1-1)]= Bishop
        @sol[2*(@b2-1)]= Bishop
        @sol[2*(@r1-1)]= Rook
        @sol[2*(@r2-1)]= Rook
        @sol[2*(@q-1)]= Queen
        @sol[2*(@k-1)] = King
        @solutions << @sol
        init
        @amb.choose
    end

end

c = Chess960.new
puts %<enter a number to show a specific solution (0 based) or
enter r for a random solution or
enter q to go back to work>
until (n = gets.strip) =~ /^q/i
    i = n.to_i
    i = rand(960) if n =~ /^r/i
    puts "Solution #{i}"
    puts c[i]
end


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Cheers
Robert
-- 
"The real romance is out ahead and yet to come. The computer revolution
hasn't started yet. Don't be misled by the enormous flow of money into bad
defacto standards for unsophisticated buyers using poor adaptations of
incomplete ideas."

- Alan Kay

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