From 5890a28fd2161ac0c003fae8d26ae88e2fd687eb Mon Sep 17 00:00:00 2001
From: Laura Orvokki Kursula <lav@vampires.gay>
Date: Mon, 9 Dec 2024 11:12:10 +0100
Subject: 9-1

---
 9-1.hs | 51 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 51 insertions(+)
 create mode 100644 9-1.hs

diff --git a/9-1.hs b/9-1.hs
new file mode 100644
index 0000000..11b186d
--- /dev/null
+++ b/9-1.hs
@@ -0,0 +1,51 @@
+import Data.Foldable (toList)
+import Data.Sequence hiding (zip)
+import Prelude hiding (replicate)
+
+type FS = Seq (Maybe Int)
+
+showFS :: FS -> String
+showFS = Prelude.reverse . go []
+  where
+    go :: String -> FS -> String
+    go res Empty = res
+    go res (x :<| xs) = go (f x : res) xs
+
+    f :: Maybe Int -> Char
+    f Nothing = '.'
+    f (Just x) = head $ show x
+
+rearrange :: FS -> FS -> FS
+rearrange result Empty = result
+rearrange result (x@(Just _) :<| rest) =
+  rearrange (result :|> x) rest
+rearrange result (Nothing :<| (rest :|> x@(Just _))) =
+  rearrange (result :|> x) rest
+rearrange result (rest@(Nothing :<| _) :|> Nothing) =
+  rearrange result rest
+
+parse :: String -> FS
+parse = go 0 Empty
+  where
+    go :: Int -> FS -> String -> FS
+    go _ res [] = res
+    go n res (x:xs) = go' (n+1) (res <> f x (Just n)) xs
+
+    go' :: Int -> FS -> String -> FS
+    go' _ res [] = res
+    go' n res (x:xs) = go n (res <> f x Nothing) xs
+
+    f :: Char -> Maybe Int -> FS
+    f x y = let n = read . pure $ x
+            in replicate n y
+
+catMaybes :: Seq (Maybe a) -> Seq a
+catMaybes Empty = Empty
+catMaybes (Nothing :<| xs) = catMaybes xs
+catMaybes (Just x  :<| xs) = x :<| catMaybes xs
+
+checksum :: FS -> Int
+checksum = sum . fmap (uncurry (*)) . zip [0..] . toList . catMaybes
+
+main :: IO ()
+main = getLine >>= print . checksum . rearrange Empty . parse
-- 
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