Euler: Counting Sundays

Problem 19. You are given the following information, but you may prefer to do some research for yourself.

• 1 Jan 1900 was a Monday.
• Thirty days has September, April, June and November.
  All the rest have thirty-one,
  Saving February alone, Which has twenty-eight, rain or shine. And on leap years, twenty-nine.
• A leap year occurs on any year evenly divisible by 4, but not on a century unless it is divisible by 400.

How many Sundays fell on the first of the month during the twentieth century (1 Jan 1901 to 31 Dec 2000)?

Method:
1. Couting date day by day, with attention to the months and leap years.
2. Couting weekday day by day.
3. Count the Sundays falling on 1st.

def isLeapYear(year):
    if year%4 != 0:
        return False
    if year%400 == 0:
        return True
    if year%100 == 0:
        return False
    return True

def add_date(year, month, day):
    if month in [4,6,9,11]:
        if day < 30:
            day += 1
        else:
            day = 1
            month += 1
    elif month == 2:
        if isLeapYear(year):
            feb = 29
        else:
            feb = 28
        if day < feb:
            day += 1
        else:
            day = 1
            month += 1
    elif month == 12:
        if day < 31:
            day += 1
        else:
            day = 1
            month = 1
            year += 1
    else:
        if day < 31:
            day += 1
        else:
            day = 1
            month += 1
    return year, month, day

def count():
    count_sun = 0
    year = 1900
    month = 1
    day = 1
    weekday = 1
    while  (year,month,day) != (2000,12,31):
        year,month,day = add_date(year,month,day)
        weekday += 1
        #print year,month,day
        if day == 1 and weekday == 7:
            print year, month, day
            count_sun += 1
        if weekday == 7:
            weekday = 0
    print count_sun
    return


count()