tic_tac_toe_spec.rb

require 'rspec'
require 'set'

class TicTacToe
  attr_reader :current_player

  def initialize
    @current_player = 'O'
    @plays = {
      'O' => [],
      'X' => [],
    }
  end

  def at(x, y)
    @plays.keys.each do |player|
      return player if @plays[player].include?([x, y])
    end
    nil
  end

  def play(x, y)
    raise OutOfBoundsException if x.abs > 1 || y.abs > 1
    return if at(x, y)
    @plays[@current_player] << [x,y]
    # toggle current player
    @current_player = @current_player == 'O' ? 'X' : 'O'
  end

  def winner
    @plays.keys.each do |player|
      (-1..1).each do |index|
        return player if @plays[player].select {|tuple| tuple.first == index}.count == 3
        return player if @plays[player].select {|tuple| tuple.last == index}.count == 3
        return player if @plays[player].select {|tuple| tuple.last == tuple.first}.count == 3
        return player if @plays[player].select {|tuple| tuple.last == (-1 * tuple.first)}.count == 3
      end
    end
    nil
  end
end

class OutOfBoundsException < Exception; end


describe 'TicTacToe' do
  subject(:game) { TicTacToe.new }

  it "lets the players take turns placing markers" do
    expect(game.at(0, 0)).to be_nil
    game.play(0, 0)
    expect(game.at(0, 0)).to eql 'O'
    game.play(1, 1)
    expect(game.at(1, 1)).to eql 'X'
  end
  it "won't let a player play outside the board (3x3)?" do
    expect { game.play(-2, 0) }.to raise_error(OutOfBoundsException)
    expect { game.play(0, 2) }.to raise_error(OutOfBoundsException)
    expect { game.play(-2, -5) }.to raise_error(OutOfBoundsException)
  end
  it "does not let a player place a marker in a taken space" do
    game.play(0, 0)
    expect(game.at(0, 0)).to eql 'O'
    game.play(0, 0)
    expect(game.at(0, 0)).to eql 'O'
  end
  it "does not advance the current player after an invalid play" do
    game.play(0, 0)
    expect(game.current_player).to eql 'X'
    game.play(0, 0)
    expect(game.current_player).to eql 'X'
  end

  context "winning" do
    it "is won when one player has 3 in a row vertically" do
      expect(game.winner).to be_nil
      game.play(0, 0) # O
      game.play(-1, -1) # X
      game.play(0, -1) # O
      game.play(1, 1) # X
      game.play(0, 1) # O
      expect(game.winner).to eql 'O'
    end
    it "is won when one player has 3 in a row horizontally" do
      expect(game.winner).to be_nil
      game.play(0, 0) # O
      game.play(-1, -1) # X
      game.play(-1, 0) # O
      game.play(1, 1) # X
      game.play(1, 0) # O
      expect(game.winner).to eql 'O'
    end
    it "is won when one player has 3 in a row diagonally from right to left" do
      expect(game.winner).to be_nil
      game.play(0, 0) # O
      game.play(-1, 0) # X
      game.play(-1, -1) # O
      game.play(0, 1) # X
      game.play(1, 1) # O
      expect(game.winner).to eql 'O'
    end
    it "is won when one player has 3 in a row diagonally from left to right" do
      expect(game.winner).to be_nil
      game.play(0, 0) # O
      game.play(-1, 0) # X
      game.play(-1, 1) # O
      game.play(0, 1) # X
      game.play(1, -1) # O
      expect(game.winner).to eql 'O'
    end
  end
end