From: Matthew Moss <matthew.moss.coder@...>
Date: 2006-03-18T02:36:24+09:00
Subject: Re: Card tricks with Ruby 2 - The Riffle

> How would you code riffle?

Start with a merge sort:

def merge(left, right, &pred)
   if right.empty?
      left.dup
   elsif left.empty?
      right.dup
   elsif pred.call(left.first, right.first)
      [left.first] + merge(left[1..-1], right, &pred)
   else
      [right.first] + merge(left, right[1..-1], &pred)
   end
end

And then riffle!

def riffle(deck)
   n = deck.size / 2
   left, right = deck[0...n], deck[n..-1]
   merge(left, right) { rand < 0.5 }
end

Or, to simulate one side's cards being more likely given the other
side's card just went down:

def riffle(deck)
   n = deck.size / 2
   left, right = deck[0...n], deck[n..-1]
   p = 0.5
   merge(left, right) do
      res = (rand < p)
      p = res ? p*p : Math.sqrt(p)
      res
   end
end