-- 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)?

import Data.Time.Calendar (Day, DayOfWeek(Sunday), dayOfWeek, fromGregorian)

countSundaysFirst :: Day -> Day -> Int
countSundaysFirst start end = let days = [start .. end]
                              in length $ filter (\x -> dayOfWeek x == Sunday && [last (init (show x)), last (show x)] == "01") days

main = do
    let startDate = fromGregorian 1901 1 1
        endDate = fromGregorian 2000 12 31
        result = countSundaysFirst startDate endDate
    putStrLn $ "Project Euler, Problem 19\n"
        ++ "Answer: " ++ show result