Improved performance and readability for day 7

7/solution.hs

@@ -1,22 +1,18 @@
+import Data.List
 import Data.List.Split
+import Debug.Trace
 
 main :: IO ()
 main = do
     input <- readFile "7/input.txt"
-    let x = parse input
+    let x = sort $ parse input
     -- Problem 1
     print $ f x
     -- Problem 2
     print $ g x
     where
-        f x = solve distance [ minimum x .. maximum x ] x
+        f = solve distance
-        g x | length x >= 100 = solve triangle [ sum x `div` length x ] x     
+        g = solve triangle
-            | otherwise       = solve triangle [ minimum x .. maximum x ] x
-
--- NOTE for part 2 (`g`):
--- Just testing against the mean value of `x` only gives accurate results if the 
--- sample is large enough. Here, lists of size 100 or larger are deemed "large enough",
--- but that's pretty arbitrary.
 
 -- | Split on comma and interpret each element as an integer
 parse :: String -> [Int]
@@ -26,13 +22,19 @@ parse = map read . wordsBy (== ',')
 -- | each element is the total amount of fuel needed for each crab to reach
 -- | that position.
 solve :: (Int -> Int -> Int) -- ^ Fuel calculation to get between two points
-  -> [Int]                   -- ^ List of possible target positions
   -> [Int]                   -- ^ List of initial positions
   -> Int
-solve f a x = minimum $ map g a
+solve f x = minimum $ map g (narrow x)
     where
         g t = sum $ map (f t) x
 
+-- | Get a slice of numbers around the mean in order to narrow the
+-- | range of possible best positions to search.
+narrow :: [Int] -> [Int]
+narrow x = [ m - n .. m + n ]
+    where m = sum x `div` length x 
+          n = 5
+
 -- | Calculate the fuel usage based on the triangle number of the distance
 -- | between the two numbers.
 triangle :: Int -> Int -> Int