day 13

Signed-off-by: Amneesh Singh <natto@weirdnatto.in>

day13.hs

@@ -0,0 +1,59 @@
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE ViewPatterns #-}
+
+import Data.Char (isDigit)
+import Data.List (elemIndex, sort)
+import Data.Maybe (fromJust)
+import Data.Text (Text)
+import qualified Data.Text as T (filter, init, lines, null, span, tail, uncons)
+import qualified Data.Text.Read as T (decimal)
+import Lib (readFile')
+
+main :: IO ()
+main = do
+  input <-
+    map (fst . parse (List []) . T.init . T.tail) . filter (not . T.null)
+      . T.lines
+      <$> readFile' "day13.in"
+  putStr "Q1: "
+  print $ q1 input
+  putStr "Q2: "
+  print $ q2 input (List [List [Elem 2]]) (List [List [Elem 6]])
+
+q1 :: [List] -> Int
+q1 input =
+  foldl
+    (\xs x -> xs + if input !! (2 * x - 2) < input !! (2 * x - 1) then x else 0)
+    0
+    [1 .. length input `div` 2]
+
+q2 :: [List] -> List -> List -> Int
+q2 input a b =
+  let sorted = sort (a : b : input)
+      a' = fromJust $ elemIndex a sorted
+      b' = fromJust $ elemIndex b sorted
+   in (a' + 1) * (b' + 1)
+
+data List = Elem Int | List [List] deriving (Show, Eq)
+
+-- is this cheating :P
+instance Ord List where
+  compare (List xs) (List ys) = compare (reverse xs) (reverse ys)
+  compare (List xs) (Elem y) = compare (List xs) (List [Elem y])
+  compare (Elem x) (List ys) = compare (List [Elem x]) (List ys)
+  compare (Elem x) (Elem y) = compare x y
+
+-- Weak parser but it works
+-- I do not really know how to use Parsec and ReadP with Data.Text yet
+
+parse :: List -> Text -> (List, Text)
+parse (List xs) txt
+  | T.null txt || cur == ']' = (List xs, rest)
+  | cur == '[' =
+    let (new, next) = parse (List []) rest
+     in parse (List (new : xs)) next
+  | isDigit cur = parse (List (Elem num : xs)) rest'
+  | otherwise = parse (List xs) rest
+  where
+    (cur, rest) = fromJust $ T.uncons txt
+    (T.decimal -> Right (num, ""), rest') = T.span isDigit txt