Performance optimizations to day 7

7/solution.hs

@@ -9,18 +9,37 @@ main = do
     -- Problem 2
     print $ g x
     where
-        f = solve distance
-        g = solve (\a -> tri . distance a)
+        f x = solve distance [ minimum x .. maximum x ] x
+        g x | length x >= 100 = solve triangle [ sum x `div` length x ] x     
+            | 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]
 parse = map read . wordsBy (== ',')
 
-solve :: (Int -> Int -> Int) -> [Int] -> Int
-solve f x = minimum $ map g [minimum x .. maximum x]
-    where g t = sum $ map (f t) x
+-- | Solve the problem by finding the minimum of a list of integers, where
+-- | 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
+    where
+        g t = sum $ map (f t) x
 
-tri :: Int -> Int
-tri x = x * (x + 1) `div` 2
+-- | Calculate the fuel usage based on the triangle number of the distance
+-- | between the two numbers.
+triangle :: Int -> Int -> Int
+triangle a b = tri $ distance a b
+    where
+        tri n = n * (n + 1) `div` 2 -- Calculate nth triangle number
 
+-- | Calculate the absolute distance between two numbers
 distance :: Int -> Int -> Int
-distance n a = abs (n - a)
+distance a b = abs (a - b)