heads_or_tails.py

"""
https://m.whitehouse.gov/blog/2015/05/17/hello-world

Alice and Bob are playing a game.  They are teammates, so they will win or lose
together.  Before the game starts, they can talk to each other and agree on a
strategy.

When the game starts, Alice and Bob go into separate soundproof rooms – they
cannot communicate with each other in any way.  They each flip a coin and note
whether it came up Heads or Tails.  (No funny business allowed – it has to be
an honest coin flip and they have to tell the truth later about how it came
out.)  Now Alice writes down a guess as to the result of Bob’s coin flip; and
Bob likewise writes down a guess as to Alice’s flip.

If either or both of the written-down guesses turns out to be correct, then
Alice and Bob both win as a team.  But if both written-down guesses are wrong,
then they both lose.

The puzzle is this: Can you think of a strategy Alice and Bob can use that is
guaranteed to win every time?
"""


"""
| H | T | Bob || H | T | Alice
--------------------------------
| x |   |  T  || x |   |   H   |
--------------------------------
| x |   |  T  ||   | x |   T   |
--------------------------------
|   | x |  H  || x |   |   H   |
--------------------------------
|   | x |  H  ||   | x |   T   |
--------------------------------
"""


def bob(res):
    if res == 'heads':
        return 'tails'
    return 'heads'


def alice(res):
    if res == 'heads':
        return 'heads'
    return 'tails'


x = (
    ('heads', 'heads'),
    ('heads', 'tails'),
    ('tails', 'heads'),
    ('tails', 'tails'),
)

def test_it():
    for a_res, b_res in x:
        a_guess = alice(a_res)
        b_guess = bob(b_res)
        if a_guess != b_res and b_guess != a_res:
            raise ValueError("A: {0}/{1} - B: {2}/{3}".format(
                a_guess, b_res, b_guess, a_res,
            ))
    print "Win!"