{- 
 - Zeller's congruence (Haskell)
 - by Project Nayuki, 2022. Public domain.
 - https://www.nayuki.io/page/zellers-congruence
 -}


{- 
 - Returns the day-of-week dow for the given date
 - (y, m, d) on the proleptic Gregorian calendar.
 - Values of dow are 0 = Sunday, 1 = Monday, ..., 6 = Saturday.
 - Strict values of m are 1 = January, ..., 12 = December.
 - Strict values of d start from 1.
 - The handling of months and days-of-month is lenient.
 -}
dayOfWeek :: Int -> Int -> Int -> Int
dayOfWeek y m d =
	let
		m' = mod ((mod m 4800) - 3) 4800
		y' = (mod y 400) + (div m' 12)
		m'' = mod m' 12
		d' = mod d 7
		temp = y' + (div y' 4) - (div y' 100) + (div y' 400)
	in
		mod (temp + (div (m'' * 13 + 12) 5) + d') 7