1 files changed, 52 insertions, 0 deletions
diff --git a/2024/09-Disk_Fragmenter/first.hs b/2024/09-Disk_Fragmenter/first.hs
new file mode 100644
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+++ b/2024/09-Disk_Fragmenter/first.hs
@@ -0,0 +1,52 @@
+-- requires cabal install --lib megaparsec parser-combinators heap vector
+module Main (main) where
+
+import           Control.Monad        (void, when)
+import qualified Data.List            as L
+import           Data.Void            (Void)
+import           Text.Megaparsec
+import           Text.Megaparsec.Char
+
+exampleExpectedOutput = 1928
+
+type Input = [Int]
+
+type Parser = Parsec Void String
+
+parseBlockSize :: Parser Int
+parseBlockSize = read . pure <$> digitChar
+
+parseInput' :: Parser Input
+parseInput' = some parseBlockSize <* 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 input = sum $ map (\(x, y) -> x * y) $ zip [0..] $ computeFile files' free []
+  where
+    files = [s | (i,s)<-zip [0..] input, even i]
+    files' = zip [0..] files
+    free = [s | (i,s)<-zip [0..] input, odd i]
+    computeFile :: [(Int, Int)] -> [Int] -> [Int] -> [Int]
+    computeFile ((fId, fSize):fs) free acc = computeFillFree fs free (acc ++ L.replicate fSize fId)
+    computeFillFree :: [(Int, Int)] -> [Int] -> [Int] -> [Int]
+    computeFillFree [] _ acc            = acc
+    computeFillFree files (free:frees) acc | free == fSize = computeFile fs frees (acc ++ L.replicate fSize fId)
+                                           | free > fSize  = computeFillFree fs (free-fSize:frees) (acc ++ L.replicate fSize fId)
+                                           | free < fSize  = computeFile (fs++[(fId, fSize-free)]) frees (acc ++ L.replicate free fId)
+      where
+        (fId, fSize) = last files
+        fs = init files
+
+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