Inspired by http://people.ischool.berkeley.edu/~nick/aaronson-oracle/index.html

freewill.py

import getpass

def compute_freqs(sequence):
    freqs = {
        'd|d' : 0,
        'f|d' : 0,
        'd|f' : 0,
        'f|f' : 0,
    }

    prev = sequence[0]
    for char in sequence:
        cur = char
        freqs["%s|%s" % (cur, prev)] += 1
        prev = cur

    return freqs

def compute_probs(freq):
    d_count = float(freq['d|d'] + freq['f|d'])
    f_count = float(freq['d|f'] + freq['f|f'])

    probs = {
        'd|d' : freq['d|d']/d_count if d_count > 0 else 0,
        'f|d' : freq['f|d']/d_count if d_count > 0 else 0,
        'd|f' : freq['d|f']/f_count if f_count > 0 else 0,
        'f|f' : freq['f|f']/f_count if f_count > 0 else 0,
    }

    return probs

print "FREE WILL GAME v 1.0"
print "Randomly type 'd' or 'f' for a little while. You must press <enter> after typing each character."

brain_size = 20
sequence = []
i = 0
while True:
    letter = getpass.getpass('')
    
    if letter not in ('d' , 'f'):
        print "Please input only 'd' or 'f'."
        continue
            
    i += 1
    sequence.append(letter)

    if i == brain_size:
        break

accuracy = []
probs = compute_probs(compute_freqs(sequence))
while True:
    prev = sequence[-1]
    prob_d = probs["d|%s" % prev]
    prob_f = probs["f|%s" % prev]

    guess = 'd' if prob_d > prob_f else 'f'
    cur = getpass.getpass('>')

    if cur not in ('d' , 'f'):
        print "Please input only 'd' or 'f'."
        continue

    if guess == cur:
        accuracy.append(1)
    else:
        accuracy.append(0)

    if len(accuracy) > brain_size:
        accuracy.pop(0)

    correct = 1.0 * sum(accuracy)
    total = len(accuracy)
    result = correct/total
    print "%.4f" % result

    sequence.append(cur)
    sequence.pop(0)
    probs = compute_probs(compute_freqs(sequence))