# File: calendar.py

# Some global values
# January 1, 1900 was a Monday

firstYear = 1900
firstYearWeekDay = 1
monthNames = [ "Not a Month", "January", "February", "March",
   "April", "May", "June", "July", "August", "September",
   "October", "November", "Decenber" ]
   

# Ask the user for a year for which to print
# a calendar.
#
def getYear():
   done = False
   while not done:
      aString = input("Enter year: ")
      year = int(aString) 
      if year >= firstYear:
         done = True
      else:
         print("Enter a year after ", firstYear - 1) 
   # end of while()

   return year

# end of getYear()


# Returns True or False depending on
# whether the parameter year is a leap year.
#
def isLeapYear(year):

   if year % 4 != 0:
      return False

   if year % 100 != 0:
      return True

   if year % 400 == 0:
      return True
   else:
      return False

# end of isLeapYear()


# If the current day is a day of the week
# given by the parameter weekday (e.g. Monday),
# what day will it be in the number of days
# given by the parameter days?
#
# For example, nextWeekDay(12, 1) return 6
# because 12 days from Monday is a Saturday.
#
def nextWeekDay(days, weekday):
   return (weekday + days) % 7


# Returns the day of the week for Jan 1
# in the year specified by the parameter year.
#
def weekDayJan1(year):

   # Doesn't work for years before firstYear

   if year < firstYear:
      print("Error: can't print calendars before", firstYear)
      exit()

   # start at firstYear
   weekday = firstYearWeekDay

   # figure out the week day of Jan 1 for
   # each year between firstYear and year.

   for y in range(firstYear, year):

      # figure out the start of the next year
      #
      if isLeapYear(y):
         weekday = nextWeekDay(366, weekday)
      else:
         weekday = nextWeekDay(365, weekday)

   return weekday

# end of weekDayJan1()


# Returns the number of days in month m of year y
#
def numberDays(m, y):

   # month numbered: Jan=1, Feb=2, ..., Dec=12

   if m == 1 or m == 3 or m == 5 or m == 7 or m == 8 or m == 10 or m == 12:
      return 31

   if m == 4 or m == 6 or m == 9 or m == 11:
      return 30
 
   # must be February
   if isLeapYear(y):
      return 29
   else:
      return 28

# end of numberDays()


#
# Prints the header for each month in the calendar
#
def printLabel(m, y):
   print("   ", monthNames[m], y)
# end of printLabel()


#
# Prints the "body" of the monthly calendar for
# a month with number of days specified by days
# that starts on the day of the week specified
# by weekday.
#
def printMonth(days, weekday):
   print(" Su Mo Tu We Th Fr Sa")

   # print blank spaces up to first day
   for d in range(weekday):
      print("   ", end="")

   for d in range(1, days+1):
      if (d + weekday) % 7 == 0:
         print("%3d" % d)
      else:
         print("%3d" % d, end="")

   print()
   if (d + weekday) % 7 != 0:
      print()

   # print("last d is", d) ;

# end printMonth()


def main():

   # prompt user for a year
   year = getYear()

   # January 1 is Monday? Tuesday?
   weekday = weekDayJan1(year)

   for month in range(1,13):

      # make nice label for each month
      printLabel(month, year)
      
      # find out how long this month is
      days = numberDays(month, year)

      printMonth(days, weekday)

      weekday = nextWeekDay(days, weekday)

# end of main()

main()