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Project Euler #19

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Bill Ewanick 2024-08-03 15:58:33 -04:00
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{-
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