Lazily generate primes in ruby

prime.rb

# Functional implementation of the Sieve of Eratosthenes
#
# Once initialized, an instance of this class may be used to generate an
# infinite number of primes.
#
# @see https://www.cs.hmc.edu/~oneill/papers/Sieve-JFP.pdf
# @see http://raganwald.com/2013/02/23/sieve.html
#
# Example:
#   @sieve = EratosthenesIncrementalSieve.new
#   @sieve.take(1000) # First 1000 primes
class EratosthenesIncrementalSieve
  include Enumerable

  attr_reader :numbers
  attr_reader :composites

  def initialize
    @candidate_primes = {}
    @next_candidate = 2
    @candidate_primes[@next_candidate] = @next_candidate
  end

  # Find and return the next prime number
  def succ
    loop do
      @candidate_primes[@next_candidate] = @next_candidate
      new_prime = @next_candidate if sieve!
      @next_candidate += 1
      return new_prime if new_prime
    end
  end

  # Enumerate over all prime numbers returned by #succ
  def each
    return dup unless block_given?
    loop { yield succ }
  end

  private

  # Mark all candidate primes that are multiples of `number` as non-prime
  #
  # @return [Boolean] indicates if a prime number has been found given the
  #   current list of candidate primes
  def sieve!
    @candidate_primes
      .reject { |_, v| v == false } # Ignore preveiously sieved candidates
      .each do |n, _|
      # Don't factor past the square root of the current
      # candidate prime; we've already checked those factors
      break if n * n > @next_candidate

      @candidate_primes[@next_candidate] = false if @next_candidate % n == 0
    end

    @candidate_primes[@next_candidate] ? true : false
  end
end