{-
https://projecteuler.net/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)?
-}

daysInMonth :: [Int]
daysInMonth =
  [ 31 -- Jan
  , 28 -- Feb
  , 31 -- Mar
  , 30 -- Apr
  , 31 -- May
  , 30 -- Jun
  , 31 -- Jul
  , 31 -- Aug
  , 30 -- Sep
  , 31 -- Oct
  , 30 -- Nov
  , 31 -- Dec
  ]

isLeapYear :: Int -> Bool
isLeapYear yyyy
  | yyyy `isDivBy` 400 = True
  | yyyy `isDivBy` 100 = False
  | otherwise          = yyyy `isDivBy` 4
  where
    isDivBy yyyy n = yyyy `rem` n == 0

daysOfWeek :: [Int]
daysOfWeek = cycle
  [ 1 -- Sun
  , 0 -- Mon
  , 0 -- Tue
  , 0 -- Wed
  , 0 -- Thu
  , 0 -- Fri
  , 0 -- Sat
  ]

daysInYear :: Int -> Int
daysInYear yyyy =
  if isLeapYear yyyy
    then 366
    else 365

years :: [Int]
years = [1901..2000]

ans :: Int
ans = sum $ drop 2 $ take (sum $ map daysInYear years) daysOfWeek
-- 5217