AoC/2021/day05/day05.hs

import Control.Arrow
import Control.Monad
import Data.List.Split
import qualified Data.Map as M

main :: IO ()
main = do
  input <-
    map (map read . filter (/= "->")) <$> map (splitOneOf ", ") <$> lines <$>
    readFile "input"
  let process = length . M.filter (> 1) . M.fromListWith (+) . toMap
  print $ join (***) process $ (filterOrth input, input)

filterOrth :: [[Int]] -> [[Int]]
filterOrth [] = []
filterOrth ([x1, y1, x2, y2]:ss)
  | x1 == x2 || y1 == y2 = [x1, y1, x2, y2] : filterOrth ss
  | otherwise = filterOrth ss

toMap :: [[Int]] -> [((Int, Int), Int)]
toMap [] = []
toMap ([x1, y1, x2, y2]:xs)
  | y1 == y2 = zip ySame (repeat 1) ++ toMap xs
  | x1 == x2 = zip xSame (repeat 1) ++ toMap xs
  | otherwise = zip diag (repeat 1) ++ toMap xs
  where
    diag = zip xRange yRange
    ySame = zip xRange (repeat y1)
    xSame = zip (repeat x1) yRange
    xRange = [x1,x1 - (signum $ x1 - x2) .. x2]
    yRange = [y1,y1 - (signum $ y1 - y2) .. y2]