Array#rotations method for Ruby

Array#rotations.rb

#! /usr/bin/env ruby

class Array

  def rotations
    enum = Enumerator.new do |yielder|
      r = dup
      length.times do
        yielder << r
        r = r.rotate
      end
    end
    if block_given?
      loop { yield enum.next }
    else
      enum
    end
  end

end

require 'minitest/autorun'

describe 'Array#rotations' do

  before do
    @ary = (1..9).to_a
  end

  it 'must return enumerator when called without block' do
    @ary.rotations.must_be_instance_of Enumerator
  end

  it 'must generate exactly Array#length number of rotations' do
    @ary.rotations.count.must_equal @ary.length
  end

  it 'must generate all possible rotations' do
    @ary = [1, 2, 3]
    rs = @ary.rotations.to_a
    [[1, 2, 3],
     [2, 3, 1],
     [3, 1, 2]].each { |r| rs.must_include r }
  end

  it 'must generate rotations in order' do
    @ary = [1, 2, 3]
    rs = @ary.rotations
    rs.next.must_equal [1, 2, 3]
    rs.next.must_equal [2, 3, 1]
    rs.next.must_equal [3, 1, 2]
  end

  it 'must raise StopIteration when #next called too many times' do
    rs = @ary.rotations
    @ary.size.times { rs.next }           # raises nothing
    lambda { (@ary.size + 1).times { rs.next } }
      .must_raise StopIteration
  end

end