Day 9 - Part 2

Hope you have a fast machine. This is long to compute :)

day9/main.hs

@@ -7,10 +7,13 @@
 {-# OPTIONS_GHC -Wno-unused-top-binds -Wno-unused-imports #-}
 {-# LANGUAGE OverloadedStrings #-}
 
+import Data.IntMap.Strict (IntMap)
 import Data.IntSet (IntSet)
-import Data.List (tails)
+import Data.List (inits, tails, sortOn)
+import Data.Monoid
 import Debug.Trace (trace)
 import Text.Pretty.Simple
+import qualified Data.IntMap.Strict as M
 import qualified Data.IntSet as S
 
 
@@ -38,6 +41,23 @@ solvePart1 size message =
   where
     go xs = (head $ drop size xs,isSumOfPreamble size xs)
 
+
+--------------------------------------------------------------------------------
+
+sortedContiguousSubLists :: [Int] -> [[Int]]
+sortedContiguousSubLists =
+  dropWhile (== []) . sortOn length . concat . fmap inits . tails
+
+listSums :: [[Int]] -> IntMap [Int]
+listSums xss = M.fromList $ do
+  xs <- xss
+  pure $ (getSum $ foldMap Sum xs, xs)
+
+solvePart2 :: Int -> [Int] -> Int
+solvePart2 target message = (\x -> minimum x + maximum x) $  bigMap M.! target
+  where
+    bigMap = listSums $ sortedContiguousSubLists message
+
 main :: IO ()
 main = do
   putStrLn "Test"
@@ -48,3 +68,8 @@ main = do
   print $ solvePart1 5 exampleData
   input <- lines <$> readFile "day9/input"
   print $ solvePart1 25 $ map read input
+  putStrLn "Test"
+  print $ take 10 $ drop 20 $ sortedContiguousSubLists exampleData
+  putStrLn "Day 9 - Part 2"
+  print $ solvePart2 127 exampleData
+  print $ solvePart2 3199139634 $ map read input