AoC/2019/day3/day3.hs

import Data.List.Split
import Data.List
import qualified Data.Set as Set
import Data.Maybe
import Linear.V2

main = do
    content <- readFile "input"
    let [trace1, trace2] = map gensteps $ parse content
    print (day3a trace1 trace2)
    print (day3b trace1 trace2)

parse :: String -> [[(Char,Int)]]
parse = map (map (\x->(head x,read $ tail x))) . map (splitOn ",") . lines

gensteps :: [(Char,Int)] -> [V2 Int]
gensteps = scanl1 (+) . concatMap (\(x,y)->replicate y $ step x)

step :: Char -> V2 Int
step x
 | x=='U'=V2 0 1
 | x=='R'=V2 1 0
 | x=='D'=V2 0 (-1)
 | x=='L'=V2 (-1) 0

day3a :: [V2 Int] -> [V2 Int] -> Int
day3a xs ys = Set.findMin $ Set.map manhattan (intersections xs ys)
    where manhattan (V2 x y) = abs x + abs y

intersections :: [V2 Int] -> [V2 Int] -> Set.Set (V2 Int)
intersections xs ys = (Set.fromList xs) `Set.intersection` (Set.fromList ys)

day3b xs ys = minimum [let f = succ . fromJust . elemIndex x in
                       f xs + f ys | x<-Set.toList $ intersections xs ys]