Project Euler #19

src/projectEuler/question19.hs

@@ -0,0 +1,64 @@
+{-
+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