Prime numbers

sieve.rb

class PrimeNumbers
  include Enumerable

  def self.prime_numbers_below(edge)
    new(max_number: edge)
  end

  def initialize(max_number:)
    @max_number = max_number
  end

  def each(&block)
    prime_numbers.each(&block)
  end

  private

    attr_accessor :max_number

    def prime_numbers
      @_prime_number = sieve.map.with_index do |value, index|
        index if value
      end.select { |number| number }
    end

    def sieve
      @_sieve ||= ::Sieve.new(size: max_number)
    end
end

class Sieve < Array
  def initialize(size:)
    super(size) do |index|
      if index == 0 || index == 1 || (index > 2 && index.even?)
        false
      else
        true
      end
    end
    sift
    freeze
  end

  private

    def sift
      current_index = 3
      while (current_index < size)
        unless self[current_index]
          current_index += 1
          next
        end
        non_prime_index = current_index * current_index
        while non_prime_index < size
          self[non_prime_index] = false
          non_prime_index += current_index * 2
        end
        current_index += 1
      end
    end
end

test_sieve.rb

require 'minitest/autorun'

class TestPrimeNumbers < MiniTest::Unit::TestCase
  def test_prime_numbers_below_10
    assert_equal(PrimeNumbers.prime_numbers_below(10).to_a, [2, 3, 5, 7])
  end

  def test_prime_numbers_below_100
    assert_equal(PrimeNumbers.prime_numbers_below(100).to_a,
      [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97])
  end
end