Day 10

src/AOC-10a.hs

 module Main where
 
+score :: String -> Int
+score = go [] where
+  match [] _ = False
+  match (a:_) c = (a,c) `elem` [('(',')'), ('[',']'),('{','}'),('<','>')]
+  isOpen c = c `elem` ['(','[','{','<']
+  go _ [] = 0   -- out of input
+  go as (c:cs)
+    | isOpen c = go (c:as) cs
+    | match as c = go (tail as) cs
+    | otherwise = charScore c
+  charScore ')' = 3
+  charScore ']' = 57
+  charScore '}' = 1197
+  charScore '>' = 25137
+  charScore c = error $ "score.charScore -- unexpected character: " ++ [c]
+  
 main :: IO ()
-main = pure ()
+main = do
+  input <- lines <$> readFile "data/syntax.txt"
+  print . sum . map score $ input
+

src/AOC-10b.hs

 module Main where
 
+import Data.List (sort)
+import Data.Maybe (mapMaybe)
+
+score :: String -> Maybe Int
+score = go [] where
+  match [] _ = False
+  match (a:_) c = (a,c) `elem` [('(',')'), ('[',']'),('{','}'),('<','>')]
+  isOpen c = c `elem` ['(','[','{','<']
+  go as [] = Just $ stackScore as
+  go as (c:cs)
+    | isOpen c = go (c:as) cs
+    | match as c = go (tail as) cs
+    | otherwise = Nothing -- bad syntax
+  stackScore  = foldl (\s c -> charScore c + 5 * s) 0
+  charScore '(' = 1
+  charScore '[' = 2
+  charScore '{' = 3
+  charScore '<' = 4
+  charScore c = error $ "score.charScore -- unexpected character: " ++ [c]
+
+middle :: [a] -> a
+middle as = head . drop (length as `div` 2) $ as
+
 main :: IO ()
-main = pure ()
+main = do
+  input <- lines <$> readFile "data/syntax.txt"
+  putStr . show . middle . sort . mapMaybe score $ input
+

test/syntax.txt

+[({(<(())[]>[[{[]{<()<>>
+[(()[<>])]({[<{<<[]>>(
+{([(<{}[<>[]}>{[]{[(<()>
+(((({<>}<{<{<>}{[]{[]{}
+[[<[([]))<([[{}[[()]]]
+[{[{({}]{}}([{[{{{}}([]
+{<[[]]>}<{[{[{[]{()[[[]
+[<(<(<(<{}))><([]([]()
+<{([([[(<>()){}]>(<<{{
+<{([{{}}[<[[[<>{}]]]>[]]
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