advent09/Main.hs

   1 -- Writeup at https://work.njae.me.uk/2023/12/09/advent-of-code-2023-day-09/
   2
   3 import AoC
   4 import Data.List
   5 import Prelude hiding (seq)
   6
   7 type Sequence = [[Int]]
   8
   9 main :: IO ()
  10 main =
  11   do  dataFileName <- getDataFileName
  12       text <- readFile dataFileName
  13       let histories = readInput text
  14       -- print histories
  15       let seqs = fmap expand histories
  16       -- print seqs
  17       -- let seqs' = fmap extend seqs
  18       -- print seqs'
  19       print $ part1 seqs
  20       let rseqs = fmap (expand . reverse) histories
  21       print $ part1 rseqs
  22
  23 part1 :: [Sequence] -> Int
  24 part1 = sum . fmap (evaluate . extend)
  25
  26
  27 readInput :: String -> [[Int]]
  28 readInput = fmap (fmap read . words) . lines
  29
  30 expand :: [Int] -> Sequence
  31 expand seq = unfoldr go seq
  32   where go xs
  33           | all (== 0) xs = Nothing
  34           | otherwise = Just (xs, differences xs)
  35
  36 differences :: [Int] -> [Int]
  37 differences xs = zipWith (-) (tail xs) xs
  38
  39 extend :: Sequence -> Sequence
  40 extend = fst . foldr extendRow ([], 0)
  41
  42 extendRow :: [Int] -> ([[Int]], Int) -> ([[Int]], Int)
  43 extendRow row (seq, n) = ((row ++ [n']) : seq, n')
  44   where n' = last row + n
  45
  46 evaluate :: Sequence -> Int
  47 evaluate = last . head