advent09/MainComplicated.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
   6 newtype Sequence = Sequence [[Int]] deriving (Show, Eq)
   7
   8 main :: IO ()
   9 main =
  10   do  dataFileName <- getDataFileName
  11       text <- readFile dataFileName
  12       let histories = readInput text
  13       -- print histories
  14       let seqs = fmap (expand . Sequence . pure) histories
  15       -- print seqs
  16       -- let seqs' = fmap extend seqs
  17       -- print seqs'
  18       print $ part1 seqs
  19       let rseqs = fmap (expand . Sequence . pure . reverse) histories
  20       print $ part1 rseqs
  21
  22 part1 :: [Sequence] -> Int
  23 part1 = sum . fmap (evaluate . extend)
  24
  25
  26 readInput :: String -> [[Int]]
  27 readInput = fmap (fmap read . words) . lines
  28
  29 expand :: Sequence -> Sequence
  30 expand (Sequence xss)
  31   | all (== 0) $ last xss = Sequence xss
  32   | otherwise = expand $ Sequence $ xss ++ [differences $ last xss]
  33
  34 differences :: [Int] -> [Int]
  35 differences xs = zipWith (-) (tail xs) xs
  36
  37 extend :: Sequence -> Sequence
  38 extend (Sequence xss) = Sequence $ fst $ foldr extendRow ([], 0) xss
  39
  40 extendRow :: [Int] -> ([[Int]], Int) -> ([[Int]], Int)
  41 extendRow row (seq, n) = ((row ++ [n']) : seq, n')
  42   where n' = last row + n
  43
  44 evaluate :: Sequence -> Int
  45 evaluate (Sequence xss) = last $ head xss