2020 day3/haskell: add solution

2020/day3/day3.hs

@@ -0,0 +1,23 @@
+import Control.Applicative (empty)
+import Control.Monad.Zip (mzip)
+import Data.List (intercalate)
+import Data.Maybe (catMaybes)
+
+compose2 :: (a' -> b' -> c) -> (a -> a') -> (b -> b') -> a -> b -> c
+compose2 f g h x y = f (g x) (h y)
+
+(.:) :: (c -> d) -> (a -> b -> c) -> a -> b -> d
+f .: g = (f .) . g
+infixl 8 .:
+
+countHits :: (Int, Int) -> [[Bool]] -> Int
+countHits = length . filter id . map (uncurry $ flip (!!)) . catMaybes .: compose2 (zipWith mzip) (uncurry . flip $ compose2 intercalate (flip replicate empty . pred) (fmap pure . map pure . enumFromThen 0)) (map pure)
+
+treeCharToBool :: Char -> Bool
+treeCharToBool = (== '#')
+
+main = do
+    charLines <- lines <$> readFile "input.txt"
+    let boolLines = cycle <$> map treeCharToBool <$> charLines
+    print $ countHits (3, 1) boolLines
+    print $ product $ flip countHits boolLines <$> [(1,1), (3,1), (5,1), (7,1), (1,2)]