1 files changed, 127 insertions, 0 deletions
diff --git a/day10/main.hs b/day10/main.hs
new file mode 100644
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--- /dev/null
+++ b/day10/main.hs
@@ -0,0 +1,127 @@
+module Main where
+
+import Data.Either (fromLeft, fromRight)
+import Data.List (foldl', sort)
+import Data.Map (Map, (!))
+import qualified Data.Map as Map
+import Utils
+
+data Expr
+  = Paren [Expr]
+  | Bracket [Expr]
+  | Curly [Expr]
+  | Angle [Expr]
+  deriving (Show)
+
+data Failure
+  = Incomplete
+  | Corrupted Char
+  deriving (Show)
+
+type Computation a = Either Failure a
+
+consume expected (c : cs) =
+  if c /= expected
+    then Left (Corrupted c)
+    else Right cs
+consume expected [] = Left Incomplete
+
+pairs =
+  Map.fromList
+    [ ('(', ')'),
+      ('[', ']'),
+      ('{', '}'),
+      ('<', '>')
+    ]
+
+opening = Map.keys pairs
+
+closing = Map.elems pairs
+
+expr :: Char -> [Expr] -> Expr
+expr '(' children = Paren children
+expr '[' children = Bracket children
+expr '{' children = Curly children
+expr '<' children = Angle children
+expr c _ = error $ "invalid c: " ++ [c]
+
+parse :: String -> Computation ([Expr], String)
+parse s
+  | null s = Left Incomplete
+  | head s `elem` opening =
+    let (c : cs) = s
+        closing = pairs ! c
+     in do
+          (children, rest) <- parse cs
+          rest' <- consume closing rest
+          (nextExprs, rest'') <-
+            if not (null rest')
+              then parse rest'
+              else Right ([], rest')
+          Right (expr c children : nextExprs, rest'')
+  | head s `elem` closing = Right ([], s)
+  | otherwise = error "unreachable"
+
+isCorrupted expr = case expr of
+  Left (Corrupted c) -> True
+  _ -> False
+
+fromCorrupted expr = case expr of
+  Left (Corrupted c) -> c
+  a -> error $ "not corrupted: " ++ show a
+
+score1 ')' = 3
+score1 ']' = 57
+score1 '}' = 1197
+score1 '>' = 25137
+score1 a = error $ "not valid for scoring: " ++ [a]
+
+e1 rows =
+  let parsed = map parse rows
+      corrupted = parsed $> filter isCorrupted .> map fromCorrupted
+      scores = corrupted $> map score1
+   in sum scores
+
+repair :: [Char] -> [Char] -> Either Failure [Char]
+repair expectStack s
+  | null s = Right expectStack
+  | head s `elem` opening =
+    let (c : cs) = s
+        correspondingClosing = pairs ! c
+        expectStack' = correspondingClosing : expectStack
+     in repair expectStack' cs
+  | head s `elem` closing =
+    do
+      let (c : cs) = s
+      let (ec : ecs) = expectStack
+      if c /= ec
+        then Left (Corrupted c)
+        else repair ecs cs
+  | otherwise = error "unreachable"
+
+score2 ')' = 1
+score2 ']' = 2
+score2 '}' = 3
+score2 '>' = 4
+score2 a = error $ "not valid for scoring: " ++ [a]
+
+fixToScore = foldl' (\acc c -> 5 * acc + score2 c) 0
+
+e2 rows =
+  let parsed = map (repair []) rows
+      fixesM = parsed $> filter (not . isCorrupted)
+      fixes = fixesM $> map (\(Right fix) -> fix)
+      scores = fixes $> map fixToScore
+      scoresSorted = sort scores
+      scoresN = length scoresSorted
+   in scoresSorted !! (scoresN `div` 2)
+
+main :: IO ()
+main =
+  do
+    contents <- getContents
+
+    let input = contents $> lines
+
+    e1 input $> print
+    e2 input $> print