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+-- requires cabal install --lib megaparsec parser-combinators
+module Main (main) where
+import Control.Monad (void, when)
+import Data.Char qualified as C
+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
+
+exampleExpectedOutput = 281
+
+type Input = [(Int, Int)]
+
+type Parser = Parsec Void String
+
+parseDigit :: Parser Int
+parseDigit = (C.digitToInt <$> digitChar)
+ <|> (string "one" $> 1)
+ <|> (string "two" $> 2)
+ <|> (string "three" $> 3)
+ <|> (string "four" $> 4)
+ <|> (string "five" $> 5)
+ <|> (string "six" $> 6)
+ <|> (string "seven" $> 7)
+ <|> (string "eight" $> 8)
+ <|> (string "nine" $> 9)
+ <|> (letterChar *> parseDigit)
+
+parseDigitReversed :: Parser Int
+parseDigitReversed = (C.digitToInt <$> digitChar)
+ <|> (string "eno" $> 1)
+ <|> (string "owt" $> 2)
+ <|> (string "eerht" $> 3)
+ <|> (string "ruof" $> 4)
+ <|> (string "evif" $> 5)
+ <|> (string "xis" $> 6)
+ <|> (string "neves" $> 7)
+ <|> (string "thgie" $> 8)
+ <|> (string "enin" $> 9)
+ <|> (letterChar *> parseDigitReversed)
+
+parseFirstAndLast :: String -> (Int, Int)
+parseFirstAndLast str = (f, l)
+ where
+ f = case runParser parseDigit "parseDigit" str of
+ Left bundle -> error $ errorBundlePretty bundle
+ Right input' -> input'
+ l = case runParser parseDigitReversed "parseDigit" (reverse str) of
+ Left bundle -> error $ errorBundlePretty bundle
+ Right input' -> input'
+
+parseInput' :: Parser Input
+parseInput' = fmap parseFirstAndLast <$> some (some alphaNumChar <* 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'
+
+process :: Int -> (Int, Int) -> Int
+process acc l = acc + fst l * 10 + snd l
+
+compute :: Input -> Int
+compute = L.foldl' process 0
+
+main :: IO ()
+main = do
+ example <- parseInput "example2"
+ let exampleOutput = compute example
+ when (exampleOutput /= exampleExpectedOutput) (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+ input <- parseInput "input"
+ print $ compute input