http://www.problemotd.com/problem/balancing-act/ -- find the smallest of 9 items only accessing the 'scales' twice

balancing_act.py

"""
http://www.problemotd.com/problem/balancing-act/
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Problem: You are given 9 balls and a balance scale. One of the balls weighs
slightly less than the other 8. Using the scale only twice how can you figure
out which ball weighs less than the others? Do not attempt to mark the ball
once you find it.
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find the smallest of 9 items only accessing the 'scales' twice
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Cameron Phillips - 19/03/2014 - Python 3.3.3
"""

# equivalent to using weighing scales
def lightest_of_pair(a, b):
    print("USED SCALES")
    if isinstance(a, list):
        va, vb = sum(i[1] for i in a), sum(i[1] for i in b)
    else:
        va, vb = a[1], b[1]

    if va == vb:
        return None
    if va < vb:
        return a
    return b

def lightest_of_group(group):
    lightest = lightest_of_pair(group[0],group[1])
    if lightest == None:
        return group[2]
    return lightest

def balancing_act(balls):
    # split into 3 groups of 3
    (a, b, c) = (balls[i:i+3] for i in range(0,8,3))

    lightest = lightest_of_pair(a, b)
    if lightest == None:
        return lightest_of_group(c)

    return lightest_of_group(lightest)

# balls 'A' to 'H' have weight 5, ball 'X', the target, has weight 4
balls = [(chr(65+i), 5) for i in range(8)] + [('X', 4)]
print(balancing_act(balls))