Day 9, Part 1

Day9/puzzle1.hs

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+import Data.List (intersperse)
+import Data.Char (digitToInt)
+import Data.Maybe (fromJust, fromMaybe)
+import qualified Data.Sequence as S
+import qualified Data.Foldable as F
+import Control.Applicative
+
+parseDiskMap :: [Int] -> S.Seq Int
+parseDiskMap xs = let values = intersperse (-1) [0..]
+                  in  S.fromList . concat . getZipList $ replicate <$> ZipList xs <*> ZipList values
+
+compact :: S.Seq Int -> S.Seq Int
+compact xs
+    | fileIndex == -1 = xs
+    | otherwise       = S.filter (/= -1) $ startDisk S.>< (compact . S.insertAt 0 fileVal . S.deleteAt 0 $ S.deleteAt fileIndex endDisk)
+        where spaceIndex           = fromJust $ S.elemIndexL (-1) xs
+              (startDisk, endDisk) = S.splitAt spaceIndex xs
+              fileIndex            = fromMaybe (-1) (S.findIndexR (/= -1) endDisk)
+              fileVal              = S.index endDisk fileIndex
+
+checksum :: [Int] -> Int
+checksum xs = sum $ zipWith (*) xs [0..]
+
+main = do
+    contents <- init <$> readFile "day9.txt"
+    let diskMap = map digitToInt contents
+    print . checksum . F.toList . compact $ parseDiskMap diskMap