Rock, Paper, Scissors, Lizard, Spock

rpsls.rb

# Exactly 140 characters! :)
# We rescue StandardError and re-raise a RuntimeError
# because that's nicer than getting NoMethodError or TypeError
 
def play a, b
  t = %w(scissors paper rock lizard spock)
  ['tie', b, a, b, a][(t.index(a)-t.index(b)) % 5]
rescue StandardError
  raise 'invalid'
end

rpsls_spec.rb

require 'minitest/autorun'
 
describe 'play' do
  %w(
      scissors paper
      paper    rock
      rock     lizard
      lizard   spock
      spock    scissors
      scissors lizard
      lizard   paper
      paper    spock
      spock    rock
      rock     scissors
  ).each_slice(2) do |winner, loser|
    specify "#{winner} beats #{loser}" do
      play(winner, loser).must_equal(winner)
      play(loser, winner).must_equal(winner)
    end
  end
 
  %w(scissors paper rock lizard spock).each do |tie|
    specify "#{tie} ties #{tie}" do
      play(tie, tie).must_equal('tie')
    end
  end
 
  specify "fails if first arg is invalid" do
    -> { play('foo', 'spock') }.must_raise RuntimeError
  end
 
  specify "fails if second arg is invalid" do
    -> { play('spock', 'bar') }.must_raise RuntimeError
  end
 
  specify "fails if both args are invalid" do
    -> { play('baz', 'qux') }.must_raise RuntimeError
  end
end