module Day9 where
import System.IO (openFile, IOMode (ReadMode), hClose, hGetContents)
import Data.Char (digitToInt)

columns :: String -> Int
columns str = maximum $ map length $ lines str

indexOr :: Int -> [a] -> a -> a
indexOr i list j
    | i `elem` [0..length list] = list !! i
    | otherwise                 = j

compNeighbors :: [Int] -> Int -> Int -> [Int]
compNeighbors ints i width = [middle | all (middle <) [left, right, top, bottom]]
    where middle = ints !! i
          left   = getLeft (i - 1)
          right  = getRight (i + 1)
          top    = indexOr (i - width) ints 9
          bottom = indexOr (i + width) ints 9
          -- make sure that we aren't wrapping around the line edge
          getLeft  x = if (i `mod` width) == 0 then 9 else indexOr (x :: Int) ints 9
          getRight x = if (i `mod` width) == (width - 1) then 9 else indexOr (x :: Int) ints 9

main :: IO ()
main = do
    input <- openFile "inputs/9.txt" ReadMode
    contents <- hGetContents input

    let width = columns contents
    let ints = digitToInt <$> concat (lines contents)
    
    print . sum . map ((+1) . head) . filter (/= []) $ [compNeighbors ints x width | x <- [0..length ints - 1]]

    hClose input