The partial solution as listed in the GCJ contest analysis for 2015-R1A-B-small seems to contain an infinite loop

2015_R1A_B.py

from fractions import gcd
from itertools import count


def naive_get_barber_number(N):
    customer = 1;
    for T in count(1):
        for barber in range(B):
            if T % M[barber] == 0:
                # at this point N = 0 and customer > 0..
                # and boom, infinite loop
                if customer == N:
                    return barber
                customer += 1


def slow_get_barber_number(N):
    period = M[0]
    for i in range(1, B):
        period = period // gcd(period, M[i]) * M[i]
    customers_per_phase = 0
    for i in range(B):
        customers_per_phase += period // M[i]
    N_in_my_phase = N % customers_per_phase
    return naive_get_barber_number(N_in_my_phase)

M = [7,7,7]
B = 3
N = 12

print(slow_get_barber_number(N))