Day 9 of AoC 2023 in Haskell. No bullshit, just standard stuff.

AoC_2023_day_09.hs

getPairs :: [Int] -> [(Int, Int)]
getPairs [] = []
getPairs [_] = []
getPairs (x:y:xs) = (x,y) : getPairs (y:xs)

getDiff :: [(Int, Int)] -> [Int]
getDiff [] = []
getDiff ((x1, x2):xs) = (x2 - x1) : getDiff xs

generate :: [Int] -> [[Int]]
generate xs =
  let diff = getDiff . getPairs $ xs
      next = generate diff
  in xs : if all (== 0) diff
     then [diff]
     else next

solve :: ([Int] -> Int -> Int) -> [[Int]] -> Int
solve f = foldr f 0

part1 :: [[Int]] -> Int
part1 = sum . map (solve f . generate)
  where
    f xs y = last xs + y

part2 :: [[Int]] -> Int
part2 = sum . map (solve f . generate)
  where
    f xs y = head xs - y

prepare :: String -> [[Int]]
prepare = map (map read . words) . lines

main :: IO ()
main = do
  inFile <- readFile "input.txt"
  putStrLn $ "Part 1: " ++ (show . part1 . prepare) inFile
  putStrLn $ "Part 2: " ++ (show . part2 . prepare) inFile
  putStrLn "Done."