day 13

13-a.hs

@@ -0,0 +1,52 @@
+import Prelude
+import Data.List.Split (splitOn)
+import Data.Char (isNumber)
+import Data.List (elemIndices)
+
+{- cabal:
+build-depends: base, split
+-}
+
+data Node = List [Node] | Num Int
+  deriving (Eq, Show)
+
+compareNodes :: Node -> Node -> Ordering
+-- two ints
+compareNodes (Num x) (Num y) = compare x y
+-- two lists
+compareNodes (List []) (List []) = EQ
+compareNodes (List _) (List []) = GT
+compareNodes (List []) (List _) = LT
+compareNodes (List (x:xs)) (List (y:ys))
+  | r /= EQ = r
+  | otherwise = compareNodes (List xs) (List ys)
+  where r = compareNodes x y
+-- mixed
+compareNodes (Num x) (List y) = compareNodes (List [Num x]) (List y)
+compareNodes (List x) (Num y) = compareNodes (List x) (List [Num y])
+
+-- "1,[2,3],4,[5,[6,7]]" -> ["1","[2,3]","4","[5,[6,7]]"]
+parseList :: String -> [String]
+parseList = go 0 ""
+  where
+    go :: Int -> String -> String -> [String]
+    go 0 "" [] = []
+    go 0 accum [] = [accum]
+    go _ _ [] = error "unclosed parens"
+    go depth accum (c:xs)
+      | c == '[' = go (depth + 1) (accum ++ [c]) xs
+      | c == ']' = go (depth - 1) (accum ++ [c]) xs
+      | c == ',' && depth == 0 = accum : go depth "" xs
+      | otherwise = go depth (accum ++ [c]) xs
+
+parseNode :: String -> Node
+parseNode s
+  | head s == '[' && last s == ']' = List $ map parseNode $ parseList (reverse $ drop 1 $ reverse $ drop 1 s)
+  | all isNumber s = Num $ read s
+  | otherwise = error s
+
+main = interact $
+  show
+  . sum . map (+ 1) . elemIndices LT
+  . map ((\[a, b] -> a `compareNodes` b) . map parseNode . lines)
+  . splitOn "\n\n"

13-b.hs

@@ -0,0 +1,57 @@
+import Prelude
+import Data.List.Split (splitOn)
+import Data.Char (isNumber)
+import Data.List (sortBy, findIndices, intercalate)
+
+{- cabal:
+build-depends: base, split
+-}
+
+data Node = List [Node] | Num Int
+  deriving (Eq, Show)
+
+compareNodes :: Node -> Node -> Ordering
+-- two ints
+compareNodes (Num x) (Num y) = compare x y
+-- two lists
+compareNodes (List []) (List []) = EQ
+compareNodes (List _) (List []) = GT
+compareNodes (List []) (List _) = LT
+compareNodes (List (x:xs)) (List (y:ys))
+  | r /= EQ = r
+  | otherwise = compareNodes (List xs) (List ys)
+  where r = compareNodes x y
+-- mixed
+compareNodes (Num x) (List y) = compareNodes (List [Num x]) (List y)
+compareNodes (List x) (Num y) = compareNodes (List x) (List [Num y])
+
+-- "1,[2,3],4,[5,[6,7]]" -> ["1","[2,3]","4","[5,[6,7]]"]
+parseList :: String -> [String]
+parseList = go 0 ""
+  where
+    go :: Int -> String -> String -> [String]
+    go 0 "" [] = []
+    go 0 accum [] = [accum]
+    go _ _ [] = error "unclosed parens"
+    go depth accum (c:xs)
+      | c == '[' = go (depth + 1) (accum ++ [c]) xs
+      | c == ']' = go (depth - 1) (accum ++ [c]) xs
+      | c == ',' && depth == 0 = accum : go depth "" xs
+      | otherwise = go depth (accum ++ [c]) xs
+
+parseNode :: String -> Node
+parseNode s
+  | head s == '[' && last s == ']' = List $ map parseNode $ parseList (reverse $ drop 1 $ reverse $ drop 1 s)
+  | all isNumber s = Num $ read s
+  | otherwise = error s
+
+dividerPackets = [List [List [Num 2]], List [List [Num 6]]]
+
+main = interact $
+  show
+  . product
+  . map (+ 1) . findIndices (`elem` dividerPackets)
+  . sortBy compareNodes
+  . (++ dividerPackets)
+  . map parseNode
+  . filter (not . null) . lines

13-extra.hs

@@ -0,0 +1,10 @@
+import Data.List (intercalate)
+
+data Node = List [Node] | Num Int
+  deriving (Eq)
+
+-- here's a Show instance for Node that recreates the input:
+
+instance Show Node where
+  show (List x) = "[" ++ intercalate "," (map show x) ++ "]"
+  show (Num n) = show n

README.md

@@ -10,4 +10,6 @@ you shouldn't; but for documentation purposes it's
 $ ghc 1-a.hs && cat 1_input | ./1-a
 ```
 
-you'll need to retrieve `1_input` from your aoc, but all programs will strictly take in the aoc input and ~~output what can be pasted into the anwser field~~ (thanks, day 10 part 2!)
+you'll need to retrieve `1_input` from your aoc, but all programs will strictly take in the aoc input and ~~output what can be pasted into the anwser field~~ (thanks, day 10 part 2!)
+
+occasionally, i'll also leave `-extra.hs` files for misc stuff i never ended up using but was neat anyways and i wanted to preserve