advent-of-code/2020/day5/day5.hs

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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
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main = runAoC (map (fromJust . binarify) . lines) part1 part2
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where part1 = foldr1 max
part2 = fromJust . findHole . sort