Expiration Date

expire.py

def Permute(list):
    if len(list) == 0:
        return []
    elif len(list) == 1:
        return [list]
    else:
        perms = []
        for n in list:
            rest = list[:]
            rest.remove(n)
            rest_perms = Permute(rest)
            for perm in rest_perms:
                perms.append([n] + perm)
        return perms


def IsLeapYear(year):
    return (year % 400 == 0) or (year % 4 == 0 and year % 100 != 0)


def IsValidDate(year, month, day):
    if year >= 2000 and year <= 2999:
        if month >= 1 and month <= 12:
            if month in [4, 6, 9, 11]:
                return (day >= 1 and day <= 30)
            elif month in [1, 3, 5, 7, 8, 10, 12]:
                return (day >= 1 and day <= 31)
            elif IsLeapYear(year):
                return (day >= 1 and day <= 29)
            else:
                return (day >= 1 and day <= 28)
    return False


def FormatDate(year, month, day):
    if year < 2000:
        year += 2000
    return (year, month, day)


def ParseEarliestDate(input_date):
    date_array = input_date.split("/")
    if len(date_array) != 3:
        return input_date + " is illegal"
    try:
        date_array[0] = int(date_array[0])
        date_array[1] = int(date_array[1])
        date_array[2] = int(date_array[2])
    except:
        return input_date + " is illegal"

    date_perms = [FormatDate(*date) for date in Permute(date_array)]
    valid_dates = [date for date in date_perms if IsValidDate(*date)]
    if len(valid_dates) == 0:
        return input_date + " is illegal"
    
    year, month, day = sorted(valid_dates)[0]
    return "%.4d-%.2d-%.2d" % (year, month, day)
    

if __name__ == '__main__':
    import sys
    if len(sys.argv) != 2:
        print "Usage: %s date_string" % (sys.argv[0])
    else:
        print ParseEarliestDate(sys.argv[1])