29 lines
1.0 KiB
Haskell
29 lines
1.0 KiB
Haskell
-- You are given the following information, but you may prefer to do some research for yourself.
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--
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-- 1 Jan 1900 was a Monday.
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-- Thirty days has September,
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-- April, June and November.
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-- All the rest have thirty-one,
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-- Saving February alone,
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-- Which has twenty-eight, rain or shine.
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-- And on leap years, twenty-nine.
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-- A leap year occurs on any year evenly divisible by 4, but not on a century unless it is divisible by 400.
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--
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-- How many Sundays fell on the first of the month during the twentieth century (1 Jan 1901 to 31 Dec 2000)?
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import Data.Time.Calendar (Day, DayOfWeek (Sunday), dayOfWeek, fromGregorian)
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countSundaysFirst :: Day -> Day -> Int
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countSundaysFirst start end =
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let days = [start .. end]
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in length $ filter (\x -> dayOfWeek x == Sunday && [last (init (show x)), last (show x)] == "01") days
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main = do
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let startDate = fromGregorian 1901 1 1
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endDate = fromGregorian 2000 12 31
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result = countSundaysFirst startDate endDate
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putStrLn $
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"Project Euler, Problem 19\n"
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++ "Answer: "
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++ show result
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