Even wilder performance increases

src/projectEuler/question14.hs

@@ -16,33 +16,24 @@ NOTE: Once the chain starts the terms are allowed to go above one million.
 -}
 module Main where
 
--- import           Data.Set    (Set)
--- import qualified Data.Set    as Set
-
-import           Data.IntSet (IntSet)
-import qualified Data.IntSet as IntSet
+import           Data.List (sortOn)
+import           Data.Ord  (Down (Down))
 
 main :: IO ()
 main = print ans
 
-ans :: Integer
-ans = 42
+ans :: Int
+ans = fst $ head $ sortOn (Down . snd) $ map collatzChainStartingAt [1..limit]
 
+nextCollatz :: Int -> Int
 nextCollatz n
   | n == 1 = 0
   | even n = n `div` 2
   | odd  n = 3*n + 1
 
-collatzChainStartingAt n = takeWhile (/= 0) $ iterate nextCollatz n
+collatzChainStartingAt :: Int -> (Int, Int)
+collatzChainStartingAt n = (n, s')
+  where s' = length $ takeWhile (/= 0) $ iterate nextCollatz n
 
--- solve limit = (n, chain)
---   where
---     s = maximum
---       $ map length
---       $ map collatzChainStartingAt [1..limit]
+limit :: Int
 limit = 1_000_000
-
-wholePositiveNumbers :: IntSet
-wholePositiveNumbers = IntSet.fromDistinctAscList [1..limit]
-
-f = IntSet.foldl (\acc key -> collatzChainStartingAt key : acc) []