Prime calculator on Reddit: https://www.reddit.com/r/ruby/comments/ct3yop/just_started_coding_a_couple_months_ago_and_have

new_new_primes_retrieval_service.rb

class NewNewPrimesRetrievalService 
  def self.process(max)
    new(max).process
  end

  def initialize(max)
    @max = max
  end

  def process
    (2..max).each do |number|
      primes_so_far << number if prime?(number)
    end
    primes_so_far
  end

  private

  attr_reader :max

  def prime?(number)
    primes_so_far.none? do |prime|
      number % prime == 0
    end
  end

  def primes_so_far
    @_primes_so_far ||= []
  end
end

new_primes_retrieval_service.rb

class NewPrimesRetrievalService 
  def self.process(max)
    (2..max).each_with_object([]) do |number, primes|
      if primes.all? { |divisor| number % divisor != 0 }
        primes << number
      end
    end
  end
end

primes_retrieval_service.rb

class PrimesRetrievalService 
  def self.process(options)
    n = options['number']
    # 1 is NOT a prime number!
    #all_numbers = [*1..n]
    all_numbers = [*2..n]
    primes = []
    all_numbers.each do |number|
      solutions = []
      divisors = [*2..(number-1)]
      divisors.each do |divisor|
        solution = (number.to_f/divisor.to_f)
        solution = 1 if solution - solution.to_i == 0
        solutions << solution
      end
      if solutions.all? {|i| i.is_a?(Float)} == true
        primes << number
      end
    end
    primes
  end
end

primes_test.rb

require "minitest/autorun"
require "benchmark"
require "prime"

require_relative "./primes_retrieval_service.rb"
require_relative "./new_primes_retrieval_service.rb"
require_relative "./new_new_primes_retrieval_service.rb"


module TestHelper
  PRIMES_SAMPLE = Prime.first(10)

  def primes_sample_before(n)
    PRIMES_SAMPLE.select do |prime|
      prime <= n
    end
  end

  def test_primes_before_0
    test_primes_before(0)
  end

  def test_primes_before_1
    test_primes_before(1)
  end

  def test_primes_before_10
    test_primes_before(10)
  end

  def test_primes_before_30
    test_primes_before(30)
  end

  private

  def test_primes_before(n)
    assert_equal primes_sample_before(n), get_primes_before(n)
  end
end

class TestPrimesRetrieval < Minitest::Test
  include TestHelper

  private

  def get_primes_before(n)
    PrimesRetrievalService.process("number" => n)
  end
end

class TestNewPrimesRetrieval < Minitest::Test
  include TestHelper

  private

  def get_primes_before(n)
    NewPrimesRetrievalService.process(n)
  end
end

class TestNewNewPrimesRetrieval < Minitest::Test
  include TestHelper

  private

  def get_primes_before(n)
    NewNewPrimesRetrievalService.process(n)
  end
end

class TestBenchmark < Minitest::Test
  def test_benchmark
    Benchmark.bm(20) do |x|
      max = 10000
      x.report("Original retriever: ") do
        PrimesRetrievalService.process("number" => max)
      end
      x.report("New retriever: ") do
        NewPrimesRetrievalService.process(max)
      end
      x.report("New New retriever: ") do
        NewNewPrimesRetrievalService.process(max)
      end
      x.report("Trial division: ") do
        Prime::TrialDivisionGenerator.new.each do |prime|
          break if prime >= max
        end
      end
      x.report("Eratosthenes' sieve: ") do
        Prime::EratosthenesGenerator.new.each do |prime|
          break if prime >= max
        end
      end
    end
  end
end

results.txt

$ ruby -I:lib/test primes_test.rb 
Run options: --seed 20111

# Running:

                           user     system      total        real
Original retriever:    8.261264   0.458415   8.719679 (  8.806964)
New retriever:         0.046797   0.001929   0.048726 (  0.049026)
New New retriever:     0.045379   0.000119   0.045498 (  0.045645)
Trial division:        0.004188   0.000009   0.004197 (  0.004240)
Eratosthenes' sieve:   0.000579   0.000008   0.000587 (  0.000587)
.............

Finished in 8.909285s, 1.4592 runs/s, 1.3469 assertions/s.
13 runs, 12 assertions, 0 failures, 0 errors, 0 skips