1 files changed, 53 insertions, 0 deletions
diff --git a/2023/09-Mirage_Maintenance/second.hs b/2023/09-Mirage_Maintenance/second.hs
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+++ b/2023/09-Mirage_Maintenance/second.hs
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+-- requires cabal install --lib megaparsec parser-combinators
+module Main (main) where
+
+import Control.Applicative.Permutations
+import Control.Monad (void, when)
+import Data.Char qualified as C
+import Data.Either
+import Data.Functor
+import Data.List qualified as L
+import Data.Map qualified as M
+import Data.Maybe
+import Data.Set qualified as S
+import Data.Void (Void)
+import Text.Megaparsec
+import Text.Megaparsec.Char
+
+import Debug.Trace
+
+exampleExpectedOutput = 2
+
+type History = [Int]
+type Input = [History]
+
+type Parser = Parsec Void String
+
+parseNumber :: Parser Int
+parseNumber = read <$> some (char '-' <|> digitChar) <* hspace
+
+parseInput' :: Parser Input
+parseInput' = some (some parseNumber <* eol) <* eof
+
+parseInput :: String -> IO Input
+parseInput filename = do
+  input <- readFile filename
+  case runParser parseInput' filename input of
+    Left bundle -> error $ errorBundlePretty bundle
+    Right input' -> return input'
+
+compute :: Input -> Int
+compute = sum . map (next . reverse)
+  where
+    next :: History -> Int
+    next = sum . map last . takeWhile (any (/= 0)) . iterate differences
+    differences :: History -> History
+    differences xs = zipWith (flip (-)) xs (tail xs)
+
+main :: IO ()
+main = do
+  example <- parseInput "example"
+  let exampleOutput = compute example
+  when  (exampleOutput /= exampleExpectedOutput)  (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+  input <- parseInput "input"
+  print $ compute input