day 9 part one

aoc2024.cabal

@@ -37,6 +37,7 @@ library
       Day6.Guard
       Day7
       Day8
+      Day9
   other-modules:
       Paths_aoc2024
   autogen-modules:
@@ -65,6 +66,7 @@ test-suite aoc2024-test
       Day6Spec
       Day7Spec
       Day8Spec
+      Day9Spec
       Paths_aoc2024
   autogen-modules:
       Paths_aoc2024

src/Day9.hs

@@ -0,0 +1,45 @@
+module Day9 (partOne, partTwo) where
+
+import Data.Char (digitToInt, isDigit)
+
+type FileId = Int
+
+data Block = File FileId | Free
+  deriving (Show, Eq)
+
+partOne :: String -> Int
+partOne input = sum $ zipWith (*) [0 ..] $ compactFiles $ parseDisk input
+
+partTwo :: String -> Int
+partTwo _ = 0
+
+-- Compacting
+compactFiles :: [Block] -> [FileId]
+compactFiles blocks = go enumeratedBlocks (reverse enumeratedBlocks) []
+  where
+    enumeratedBlocks :: [(Int, Block)]
+    enumeratedBlocks = zip [0 ..] blocks
+
+    go [] _ acc = reverse acc
+    go _ [] acc = reverse acc
+    go ((lIndex, lBlock) : xs) ((rIndex, rBlock) : ys) acc
+      | lIndex > rIndex = reverse acc
+      | otherwise = case (lBlock, rBlock) of
+          (File fileId, _) -> go xs ((rIndex, rBlock) : ys) (fileId : acc)
+          (Free, File fileId) -> go xs ys (fileId : acc)
+          (Free, Free) -> go ((lIndex, lBlock) : xs) ys acc
+
+-- Parsing
+
+data ParseStep = ParseFile | ParseFree
+
+parseDisk :: String -> [Block]
+parseDisk input = go (parseInts input) ParseFile 0 []
+  where
+    go [] _ _id acc = reverse acc
+    go (space : xs) _ fileId [] = go xs ParseFree (fileId + 1) (replicate space (File fileId))
+    go (space : xs) ParseFree fileId acc = go xs ParseFile fileId (replicate space Free ++ acc)
+    go (space : xs) ParseFile fileId acc = go xs ParseFree (fileId + 1) (replicate space (File fileId) ++ acc)
+
+    parseInts :: String -> [Int]
+    parseInts = map digitToInt . filter isDigit

test/Day9Spec.hs

@@ -0,0 +1,15 @@
+module Day9Spec (spec) where
+
+import Day9 (partOne, partTwo)
+import Test.Hspec
+
+spec :: Spec
+spec = do
+  describe "PartOne" $ do
+    it "works" $ do
+      partOne input `shouldBe` 1928
+  describe "PartTwo" $ do
+    it "works" $ do
+      partTwo input `shouldBe` 0
+  where
+    input = "2333133121414131402"