tennis_spec.rb

class Tennis
  def initialize
    @player1_score = 'Zero'
    @player2_score = 'Zero'
  end

  def won_point(player)
    if player == :player1
      @player1_score = next_score_for(@player1_score)
    else
      @player2_score = next_score_for(@player2_score)
    end
  end

  def score
    "#{@player1_score}-#{@player2_score}"
  end

  private

  NEXT_SCORES = {
    'Zero' => 'Fifteen',
    'Fifteen' => 'Thirty',
    'Thirty' => 'Forty'
  }.freeze

  def next_score_for(player_score)
    NEXT_SCORES[player_score]
  end
end

describe Tennis do
  def expect_score_to_be(expected)
    expect(tennis.score).to eq(expected)
  end

  let(:tennis) { Tennis.new }

  it 'can score a game where no player has scored' do
    expect_score_to_be('Zero-Zero')
  end

  it 'can score a game where player one has won a single point' do
    tennis.won_point(:player1)

    expect_score_to_be('Fifteen-Zero')
  end

  it 'can score a game where player one has won two points' do
    2.times { tennis.won_point(:player1) }

    expect_score_to_be('Thirty-Zero')
  end

  it 'can score a game where player one has won three points' do
    3.times { tennis.won_point(:player1) }

    expect_score_to_be('Forty-Zero')
  end

  it 'can score a game where player two has won one point' do
    1.times { tennis.won_point(:player2) }

    expect_score_to_be('Zero-Fifteen')
  end

  it 'can score a game where player two has won three points' do
    3.times { tennis.won_point(:player2) }

    expect_score_to_be('Zero-Forty')
  end
end