From: Morton Goldberg <m_goldberg@...>
Date: 2006-12-22T00:24:37+09:00
Subject: Re: [SUMMARY] Chess960 (#106)

On Dec 21, 2006, at 8:37 AM, Ruby Quiz wrote:

> Here's the main algorithm from Morton's code (with a minor fix from  
> me):
>
> 	   # ...
> 	
> 	   def initialize(number)
> 	      q, @bishop_index = number.divmod 16
> 	      @knight_index, @queen_index = q.divmod 6
> 	      @white_pieces = BISHOP_TABLE[@bishop_index].split('')
> 	      @white_pieces[nth_dash(@queen_index)] = 'Q'
> 	      knights = N5N_TABLE[@knight_index]
> 	      m = knights.index('N')
> 	      n = knights.index('N', m + 1)
> 	      m, n = nth_dash(m), nth_dash(n)
> 	      @white_pieces[m] = 'N'
> 	      @white_pieces[n] = 'N'
> 	      @white_pieces[@white_pieces.index('-')] = 'R'
> 	      @white_pieces[@white_pieces.index('-')] = 'K'
> 	      @white_pieces[@white_pieces.index('-')] = 'R'
> 	   end
> 	
> 	   def nth_dash(n)
> 	      dashes = []
> 	      @white_pieces.each_with_index { |ch, i| dashes << i if ch ==  
> '-' }
> 	      dashes[n]
> 	   end
>

If you're going to make the change

-   q, @bishop_index = (number - 1).divmod 16
+   q, @bishop_index = number.divmod 16

then, to maintain consistency, you've got to make the following  
changes as well:

> 	if __FILE__ == $0
> 	   begin

-              if ARGV.empty? then n = 1 + rand(960)
+              if ARGV.empty? then n = rand(960)


> 	      else
> 	         n = ARGV.first.to_i

-	          raise StandardError unless (1..960).include?(n)
+	          raise StandardError unless (0..959).include?(n)

> 	      end
> 	      puts "Initial position #{n}"
> 	      print Chess960.new(n).to_s
> 	   rescue StandardError
> 	      puts "Usage:  #{$PROGRAM_NAME} [<integer>]"

-	       puts "where <integer> is in 1..960"
+	       puts "where <integer> is in 0..959"

> 	      puts "Omitting <integer> produces a random initial position"
> 	   end
> 	end

Regards, Morton