deck.rb

require 'pp'

class Card < Struct.new(:suit, :rank, :color)
end

class Deck

  attr_accessor :cards
  def initialize
  end

  def suits
    %w(clubs hearts spades diamonds)
  end

  def ranks
    %w(ace 2 3 4 5 6 7 8 9 10 jack queen king)
  end

  def cards
    @cards ||= build_cards
  end

  def shuffle
    @cards.shuffle!
  end

  def deal
    # shuffle.pop
    shuffle.shift
  end

  private

  def build_cards
    cards = []
    suits.product(ranks) do |suit, rank|
        cards.push Card.new(suit, rank)
    end
    cards
  end
end

deck_spec.rb

require_relative 'deck'
require 'rspec'

RSpec.describe Deck do
  it 'has 52 cards' do
    expect(Deck.new.cards.size).to eq 52
  end

  describe '#deal' do
    it 'reduces the amount of cards' do
      deck = Deck.new
      expect { deck.deal }.to change { deck.cards.size }.from(52).to(51)
    end
  end
end

# $ rspec deck_spec.rb
# F
#
# Failures:
#
#   1) Deck has 52 cards
#      Failure/Error: expect(Deck.new.cards.size).to eq 52
#
#      NoMethodError:
#        undefined method `size' for nil:NilClass
#      # ./deck_spec.rb:6:in `block (2 levels) in <top (required)>'
#
# Finished in 0.00386 seconds (files took 0.35181 seconds to load)
# 1 example, 1 failure
#
# Failed examples:
#
# rspec ./deck_spec.rb:5 # Deck has 52 cards


# $ rspec deck_spec.rb
# .
#
# Finished in 0.00318 seconds (files took 0.36684 seconds to load)
# 1 example, 0 failures

# $ rspec deck_spec.rb
# F
#
# Failures:
#
#   1) Deck has 52 cards
#      Failure/Error: expect(Deck.new.cards.size).to eq 52
#
#        expected: 52
#             got: 65
#
#        (compared using ==)
#      # ./deck_spec.rb:6:in `block (2 levels) in <top (required)>'
#
# Finished in 0.02266 seconds (files took 0.42261 seconds to load)
# 1 example, 1 failure
#
# Failed examples:
#
# rspec ./deck_spec.rb:5 # Deck has 52 cards


# $ rspec deck_spec.rb
# ..
#
# Finished in 0.00473 seconds (files took 0.29317 seconds to load)
# 2 examples, 0 failures
#
# $ rspec deck_spec.rb
# .F
#
# Failures:
#
#   1) Deck#deal reduces the amount of cards
#      Failure/Error: expect { deck.deal }.to change { deck.cards.size }.from(52).to(51)
#        expected result to have changed from 52 to 51, but did not change
#      # ./deck_spec.rb:12:in `block (3 levels) in <top (required)>'

# Finished in 0.02081 seconds (files took 0.26372 seconds to load)
# 2 examples, 1 failure
#
# Failed examples:
#
# rspec ./deck_spec.rb:10 # Deck#deal reduces the amount of cards
#
# $ rspec deck_spec.rb
# ..
#
# Finished in 0.00442 seconds (files took 0.27671 seconds to load)
# 2 examples, 0 failures

run.rb

require_relative 'deck'

deck = Deck.new

pp deck.build_deck