prime.rb

require 'rspec'
require 'prime'

class SocChallenge
  def self.call(*args)
    new(*args)
  end

  def initialize(number)
    @number = number
  end

  def to_s
    [smaller_prime, number, larger_prime].join(',')
  end

  private

  def smaller_prime
    number.pred.downto(1).detect(&:prime?)
  end

  def larger_prime
    (number.next..Float::INFINITY).lazy.detect(&:prime?)
  end
  attr_reader :number
end

RSpec.describe SocChallenge do
  it 'returns correct value for given inputs' do
    expect(SocChallenge.call(137).to_s).to  eq('131,137,139')
    expect(SocChallenge.call(256).to_s).to  eq('251,256,257')
  end

  it 'works for big numbers' do
    expect(SocChallenge.call(22_801_763_513).to_s).to eq('22801763489,22801763513,22801763527')
  end
end