Fibonacci sequence - using infinite enumerator in Ruby.

fibonacci_enumerator.rb

# Enumerable Fibonacci sequence
module Fibonacci  
  class << self
    include Enumerable  
    
    def [](index)
      raise "index must be >= 0" unless index >= 0
      raise "index must be an Integer" unless index.is_a? Integer

      find.with_index {|n, i| i == index }
    end
  
    def each(&block)
      if block_given?
        # First two positions are already defined.
        yield 0     
        yield 1

        @penultimate = 0
        @ultimate = 1 
          
        # indices for 2...length
        loop do
          # Append the sum of the last two numbers and
          # forget the first of the stored numbers.
          @penultimate, @ultimate = @ultimate, @penultimate + @ultimate
          
          yield @ultimate
        end
      else
        enum_for :each
      end
    end
  end
end

p Fibonacci.take(0) #=> []
p Fibonacci.take(1) #=> [0]
p Fibonacci.take(2) #=> [0, 1]
p Fibonacci.take(3) #=> [0, 1, 1]

p Fibonacci.take(15) #=> [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377]
puts

puts "15th value is #{Fibonacci[14]}" #=> 377
puts

# Used as an infinite sequence, where we don't know how many we want.
puts "All values <= 1000"
p Fibonacci.take_while {|i| i < 1000 }
#=> [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987]