Faro Shuffle

shuffle.rb

#!/usr/bin/ruby

MAX_ROUNDS = 50

# Splits the deck of cards into two equal parts
# in the case of an odd deck size, the extra card goes in the first part
def cut(cards)
	b_size = cards.length / 2
	a_size = cards.length - b_size
	
	a = cards[0..(a_size-1)]
	b = cards[(a_size)..(cards.length - 1)]

	raise("a wrong length") unless a.length == a_size
	raise("b wrong length") unless b.length == b_size
	raise("did not cut deck evenly") unless a.length == b.length or a.length == b.length + 1

	return a, b
end

def shuffle(a, b)
	result = Array.new
	while a.any? and b.any?
		result.push(a.shift)
		result.push(b.shift)
	end
	# Handle the odd deck size case
	result.push(a.shift) if a.length == 1

	raise("a not empty") unless a.empty?
	raise("b not empty") unless b.empty?

	return result
end


puts "Deck\tFaro Shuffles" if ARGV.include?('-d')

(1..1000).each do |deck_size|
	deck = (1..deck_size).to_a
	original = deck.dup
	count = 0

	begin
		begin
			deck = shuffle(*cut(deck))
			count += 1
			raise("reached max rounds") if count >= MAX_ROUNDS
		end while (deck != original)

		if ARGV.include?('-d')
			puts "#{deck_size}\t#{count}"
		else
			puts "A #{deck_size} card deck reaches the original order after #{count} perfect faro shuffles."
		end
	rescue Exception => e
		puts "A #{deck_size} card deck caused an exception: #{e}" unless  ARGV.include?('-d')
	end
end