From: dblack@...
Date: 2006-03-18T02:40:00+09:00
Subject: Re: Card tricks with Ruby 2 - The Riffle

Hi --

On Sat, 18 Mar 2006, grrr wrote:

> First of all I wish to humbly send my thanks everyone taking their time to
> share their pearls of wisdom! You have taught me alot! Hehe my honorable
> mention goes to James for the cards.sort_by{ rand } which is nice and
> snappy. Also, I thought the 'deck = deck[x..-1] + deck[0...x]' was clear
> to me, even if I am a ruby newb. Now lets move on..
>
> On Fri, 17 Mar 2006 22:14:47 +0900, dblack wrote:
>> I have to say, it only dawned on me just now that it's kind of funny to
>> cut the deck when the dealer is a computer.  The idea of cutting the deck
>> is to thwart attempts by the dealer to stack the deck.  I actually kind of
>> love the idea that one has to do this in the case of a computer :-)
>
> Yes, I want the whole deal to be as realistic as possible! Down to
> last detail! In real life cards are not really shuffled into random
> order!
>
> And thus theres need for realistic Riffle! 8)
>
> Around here, before any card games, after the deck has been
> cut into two by the dealer it is of course 'recombined' by taking the two
> decks, and mixing them back into one deck, in a manouver called 'the
> riffle'.

That's different from the person next to the dealer cutting the deck,
though, which happens after all the shuffling is over.

> "A riffle, in which half of the deck is held in each hand with the thumbs
> inward, then cards are released by the thumbs so that they fall to the
> table intertwined." Thus *almost* every card other card is from the other
> half of the deck, and vice versa - with the odd double, triple or
> quadruple cards sometimes falling on the table from either pack!
>
> You can find a photo and detailed description of the riffle here:
> http://en.wikipedia.org/wiki/Shuffle
>
> This is done three times, and finally one of the players
> gets to cut the deck and the dealer then recombines it and deals out
> cards. Ie. dealer does 'cut + riffle' three times, then player 'cuts',
> then dealer riffles, and game can then begin.
>
> How would you code riffle?

   class Deck
     def riffle
       sort_by { rand }   # just don't tell anyone :-)
     end
   end

But seriously... here's a first attempt, maybe useful at least as a
starting point for better:

class Deck < Array
   def initialize
     concat([*1..52])
   end

   def split            # an even cut; you'd want a real cut too
     [self[0...26],self[26..52]]
   end

   def pure_riffle
     s = split
     s[0].zip(s[1]).flatten
   end

   def riffle(max_consec = 3)
     s,t = split
     done = []
     until s.empty? || t.empty?
       done.push(s.shift,t.shift)
       x = if rand(2).zero? then s else t end
       if time_for_glitch?
         rand(max_consec + 1).times { done.push(x.shift) }
       end
     end
     done + s + t
   end

   def time_for_glitch?
     rand(5).zero?
   end
end


-- 
David A. Black (dblack@wobblini.net)
Ruby Power and Light, LLC (http://www.rubypowerandlight.com)

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