gen.rb

class TrackingNumberGenerator

  include Virtus.model
  attribute :size, Integer
  attribute :letters, String, lazy: true, default: :default_letters
  attribute :numbers, String, lazy: true, default: :default_numbers
  attribute :code, String, lazy: true, default: :default_code

  DIGITS = %w(2 3 4 7 9)
  CONSONANTS = %w(C F G H J K L M N R T V W X Z)
  VOWELS = %w(A E U)

  def self.call(size)
    self.new(size: size).code
  end

  private

  def default_letters
    set = rand > 0.5
    letters_size = (0.75 * size).floor
    letters = ""

    while letters.size < letters_size
      set = !set
      char = set ? CONSONANTS.sample : VOWELS.sample
      letters << char
    end

    letters
  end

  def default_numbers
    numbers_size = size - letters.size
    numbers_size.times.map { DIGITS.sample }.join
  end

  def default_code
    [default_letters, default_numbers].join
  end

end

spec.rb

require 'rails_helper'

RSpec.describe TrackingNumberGenerator do

  it "generates a tracking number that is easy for humans to read" do
    expect(described_class.(5)).to be_present
    expect(described_class.(5).size).to eq 5
    expect(described_class.(10).size).to eq 10
  end

end