Day 9 - placed 917 and 905 on the leaderboardgit add .

9.hs

@@ -0,0 +1,49 @@
+
+removeCancelled :: String -> String
+removeCancelled str =
+    let (_, removed) = foldl (\(prev, rs) curr -> 
+            case prev of 
+                '!'  -> ('\0',        rs)
+                '\0' -> (curr,        rs)
+                _    -> (curr, prev : rs))
+            (head str, "") (tail str)
+    in  reverse $ filter (/= '\0') removed
+
+removeGarbage :: String -> String
+removeGarbage str =
+    let (_, removed) = foldl (\(isGarbage, rs) curr ->
+            if   isGarbage
+            then (curr /= '>', rs)
+            else (curr == '<',
+                if   curr == '<' 
+                then rs 
+                else curr : rs))
+            (False, "") str
+    in  reverse $ filter (/= ',') removed
+
+countNonGarbage :: String -> Int
+countNonGarbage str =
+    let (_, removed) = foldl (\(isGarbage, rs) curr ->
+            if   isGarbage
+            then (curr /= '>', 
+                if   curr == '>'
+                then rs
+                else curr : rs)
+            else (curr == '<', rs))
+            (False, "") str
+    in  length removed
+
+countGroups :: String -> Int
+countGroups str =
+    let (_, total) = foldl (\(score, acc) curr -> 
+            case curr of
+                '{' -> (score + 1, acc + score)
+                '}' -> (score - 1, acc)) 
+            (1, 0) str
+    in total
+
+main :: IO ()
+main = do
+    input <- readFile "9.txt"
+    print $ countGroups . removeGarbage . removeCancelled $ input
+    print $ countNonGarbage . removeCancelled $ input