A class which generates the prime factors for a given number. It tries to do it in a relatively optimised way by using a lazy prime generator inside an enumerator.

prime_factorizer.rb

class Prime
  def self.primes
    isComposite = -> (n) {(3..Math.sqrt(n)).step(2).any?{ |i| n % i == 0 }}
    Enumerator.new do |e|
      e.yield 2                           # start at 2
      (3..Float::INFINITY)                # start generator from 3
        .step(2)                          # sieve all even numbers
        .lazy                             # calculate on demand
        .reject(&isComposite)             # sieve all composites numbers
        .map(&:to_i)                      # reduce to ints
        .each(&e.method(:yield).to_proc)  # yield only when asked
    end.lazy
  end
  
  def self.prime_division(n, primes = Prime.primes)
    raise ZeroDivisionError if n == 0
    pv = []
    return pv unless n > 1

    primes.each do |p|
      count = 0
      while (quot, mod = n.divmod(p)
            mod) == 0
        n = quot
        count += 1
      end
      pv.push [p, count] if count > 0
      break if quot <= p
    end
    pv.push [n, 1] if n > 1

    return pv
  end
end

class PrimeFactorizer
  def factors
    @factors
  end

  private

  def initialize(n)
    @factors = Hash[Prime.prime_division(n)]
  end
end

Some related links:

http://www.sievesofchaos.com
https://www.jasondavies.com/primos/