誕生日のパラドックス

birthday_paradox.py

def birthday(days: int, people: int):
    all_different = 1
    for i in range(1, people):
        all_different *= days - i

    return 1 - all_different / (days ** (people - 1))

birthday(365, 23)
# => 0.5072972343239854

birthday(18000000, 10000)
# => 0.9378381908765165

birthday(18000000000, 50000)
# => 0.06708680388588473