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diff --git a/2023/05-If_You_Give_A_Seed_A_Fertilizer/first.hs b/2023/05-If_You_Give_A_Seed_A_Fertilizer/first.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.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 = 35
+
+data Map = Map Int Int Int deriving Show -- destination source range
+data Mapping = Mapping String String [Map] deriving Show -- sourceStr destStr
+data Input = Input [Int] [Mapping] deriving Show -- seeds mappings
+
+type Parser = Parsec Void String
+
+parseNumber :: Parser Int
+parseNumber = read <$> some digitChar <* many (char ' ')
+
+parseMap :: Parser Map
+parseMap = Map <$> parseNumber
+               <*> parseNumber
+               <*> parseNumber <* eol
+
+parseMapping :: Parser Mapping
+parseMapping = Mapping <$> (some letterChar <* string "-to-")
+                       <*> (some letterChar <* string " map:" <* eol)
+                       <*> some parseMap <* optional eol
+
+parseInput' :: Parser Input
+parseInput' = Input <$> (string "seeds: " *> some parseNumber <* some eol)
+                    <*> some parseMapping <* 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'
+
+step :: [Map] -> Int -> Int
+step [] n = n
+step (Map dest src range:ms) n | n>=src && n<src+range = n - src + dest
+                               | otherwise = step ms n
+
+stepMapping :: [Int] -> Mapping -> [Int]
+stepMapping values (Mapping _ _ ms) = map (step ms) values
+
+compute :: Input -> Int
+compute (Input values mappings) = minimum $ L.foldl' stepMapping values mappings
+
+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