aoc2020/day1.hs

module Main where

main :: IO ()
main = do
  raw <- readFile "day1.txt"
  let ls = map read $ lines raw :: [Integer]
   in do
     case (solveA ls) of
       Nothing -> error "no answer"
       (Just x) -> putStrLn $ "day1a: " ++ (show x)
     case (solveB ls) of
       Nothing -> error "no answer"
       (Just x) -> putStrLn $ "day1b: " ++ (show x)

solveA :: [Integer] -> Maybe Integer
solveA = solveX (\(x,y) -> x * y)
                (\xs -> [ (x,y) | x <- xs
                                , y <- xs
                                , x + y == 2020 ])

solveB :: [Integer] -> Maybe Integer
solveB = solveX (\(x,y,z) -> x * y * z)
                (\xs -> [ (x,y,z) | x <- xs
                                  , y <- xs
                                  , z <- xs
                                  , x + y + z == 2020 ])

solveX :: Eq a => (a -> Integer) -> ([Integer] -> [a]) -> [Integer] -> Maybe Integer
solveX _ _ [] = Nothing
solveX f g l = if ls == []
               then Nothing
               else Just $ (f . head) ls
  where
    ls = g l