Advent of Code 2021
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module Day9_1 where
import System.IO (openFile, IOMode (ReadMode), hClose, hGetContents)
import Data.Char (digitToInt)
import Data.List (sort)
-- i gave up on my original solution
-- this isn't very original and is influcenced by other people's solutions
-- i'm sorry
columns :: String -> Int
columns str = maximum $ map length $ lines str
xy :: Int -> Int -> [[Int]] -> (Int, Int) -> Int
xy w h arr (x, y)
| x < 0 || x >= w = 9
| y < 0 || y >= h = 9
| otherwise = arr !! y !! x
-- left, right, up, down from current coord
neigh :: (Int, Int) -> [(Int, Int)]
neigh (x, y) = [(x - 1, y), (x + 1, y), (x, y - 1), (x, y + 1)]
basin :: Int -> Int -> [[Int]] -> [(Int, Int)] -> [(Int, Int)] -> [(Int, Int)]
basin w h ints acc [] = acc
basin w h ints acc (x:xs)
| pos x == 9 || x `elem` acc = basin w h ints acc xs
| otherwise = basin w h ints (x:acc) (xs ++ neigh x)
where pos = xy w h ints
main :: IO ()
main = do
input <- openFile "inputs/9.txt" ReadMode
contents <- hGetContents input
let ints = map (map digitToInt) $ lines contents
let width = columns contents
let height = length ints
let coordMap = [ (x, y) | x <- [0..width-1], y <- [0..height-1] ]
let pos = xy width height ints
let lowest = [ coord | coord <- coordMap, and [neighbor > pos coord | neighbor <- pos <$> neigh coord] ]
print . product . take 3 . reverse . sort $ map (length . basin width height ints [] . pure) lowest
hClose input