day5: add haskell solution

2020/day5/day5.hs

@@ -0,0 +1,23 @@
+import AoC
+
+import Data.List (sort)
+import Data.Maybe (fromJust)
+import Numeric (readInt)
+import Text.ParserCombinators.ReadP
+
+binarify :: String -> Maybe Int
+binarify = oneCompleteResult . readS_to_P $ readInt 2 (`elem` "BFLR") digitValue
+    where digitValue 'F' = 0
+          digitValue 'B' = 1
+          digitValue 'L' = 0
+          digitValue 'R' = 1
+
+findHole :: (Enum a, Eq a) => [a] -> Maybe a
+findHole (x:y:ys) | y == next = findHole $ y:ys
+                  | otherwise = Just next
+                  where next = succ x
+findHole _ = Nothing
+
+main = runAoC (fmap (fromJust . binarify) <$> lines) part1 part2
+    where part1 = foldr1 max
+          part2 = fromJust . findHole . sort