Refactor size

    $ cabal run fine-grained -- -p size --stdev 1
    All
      HashMap.Strict
        size
          Int
            0:      OK
              2.04 ns ±  22 ps
            1:      OK
              3.01 ns ±  40 ps
            5:      OK
              20.0 ns ± 128 ps
            10:     OK
              45.3 ns ± 568 ps
            50:     OK
              186  ns ± 1.1 ns
            100:    OK
              396  ns ± 4.1 ns
            500:    OK
              1.88 μs ± 5.8 ns
            1000:   OK
              4.59 μs ±  84 ns
            5000:   OK
              28.3 μs ± 483 ns
            10000:  OK
              56.2 μs ± 885 ns
            50000:  OK
              387  μs ± 4.4 μs
            100000: OK
              735  μs ± 4.9 μs
            500000: OK
              5.36 ms ±  51 μs

Author: sjakobi

Data/HashMap/Internal.hs

@@ -615,12 +615,19 @@ null _ = False
 
 -- | \(O(n)\) Return the number of key-value mappings in this map.
 size :: HashMap k v -> Int
-size t = go t 0
-  where
-    go (Leaf _ _)           !n = n + 1
-    go (BitmapIndexed _ ary) n = A.foldl' (flip go) n ary
-    go (Full ary)            n = A.foldl' (flip go) n ary
-    go (Collision _ ary)     n = n + A.length ary
+size = \case
+  BitmapIndexed b ary
+    | b == 0    -> 0
+    | otherwise -> A.foldl' size_ 0 ary
+  m -> size_ 0 m
+{-# INLINE size #-}
+
+size_ :: Int -> HashMap k v -> Int
+size_ !n = \case
+  Leaf _ _            -> n + 1
+  BitmapIndexed _ ary -> A.foldl' size_ n ary
+  Full ary            -> A.foldl' size_ n ary
+  Collision _ ary     -> n + A.length ary
 
 -- | \(O(\log n)\) Return 'True' if the specified key is present in the
 -- map, 'False' otherwise.