audrey_random.rb

#!/usr/bin/ruby
# AudreyRandom
# Randomly returns elements out of an enumerable, without picking previous elements.
#
# Author: Sunny Ripert - http://sunfox.org
# Licence: WTFPL
#
# You see, my ex-girlfriend used to blame her music player's random algorythm
# because it sometimes gave her a song which played just a few minutes ago.
# This is for you, Audrey ;).
#
# This algorithm makes sure of two things:
#   - it will give you a previously returned element only after _all_ the
#     other elements were returned.
#   - it will never return the same element twice
#
# Example:
#
#   letters = AudreyRandom.new([:a, :b, :c])
#   letters.next # => :b   # could have been a, b or c
#   letters.next # => :a   # could have been a or c
#   letters.next # => :c   # could only have been c
#   letters.next # => :a   # could have been anything but c

class AudreyRandom
  def initialize(enum)
    @enum = enum.to_a
    @prev ||= nil
    @i = 0
    randomize
  end
  def randomize
    size, orig, @enum = @enum.size, @enum, []
    @enum << orig.slice!(rand(orig.size)) until @enum.size == size
  end
  def next
    initialize(@enum) if @i == @enum.size
    elem = @enum[@i]
    @i += 1
    return self.next if @prev == elem
    @prev = elem
  end
end