move helper function clearLowest from global to local,

repair forw 0, remove work-around in powerSet

Author: jwaldmann

containers/src/Data/Set/Internal.hs

@@ -1828,7 +1828,6 @@ splitRoot orig =
 -- @since 0.5.11
 
 powerSet :: Set a -> Set (Set a)
-powerSet xs | null xs = singleton xs
 powerSet xs =
   let !w = length xs
       !u = A.listArray (0, w-1) $ toList xs
@@ -1850,6 +1849,7 @@ generateA :: A.Ix i => (i,i) -> (i -> a) -> A.Array i a
 generateA bnd f = A.listArray bnd $ fmap f $ A.range bnd
 
 forw :: Int -> Int -> Int
+forw 0 _ = 0
 forw !width !n = fr_go_branch (bit $ width-1) n 0
 
 fr_go_branch :: Int -> Int -> Int -> Int
@@ -1867,16 +1867,14 @@ data StrictTriple = ST !Int !Int !Int
 -- index of middle bit, bitmask for lower half
 splitBits :: Int -> StrictTriple
 splitBits m =
-  let go 0 !x = x; go k !x = go (k-1) (clearLowest x)
-      !up_med = go (div (popCount m) 2) m
-      !lo = xor m up_med
-      !up = clearLowest up_med
-      !med = xor up_med up
+  let clearLowest !x = x .&. (x-1)
+      go 0 !x = x; go k !x = go (k-1) (clearLowest x)
+      up_med = go (div (popCount m) 2) m
+      lo = xor m up_med
+      up = clearLowest up_med
+      med = xor up_med up
   in  ST up (countTrailingZeros med) lo
 
-clearLowest :: Int -> Int
-clearLowest m = m .&. (m-1)
-
 -- Proof of complexity: step executes n times. At the ith step,
 -- "insertMin x `mapMonotonic` pxs" takes O(2^i log i) time since pxs has size
 -- 2^i - 1 and we insertMin into its elements which are sets of size <= i.