Avoid range checks due to bit, clearBit, testBit

…and bring back array indices.
haskell-unordered-containers · Mar 25, 2022 · 0f4f623 · 0f4f623
1 parent e88d075
commit 0f4f623
Showing 1 changed file with 14 additions and 11 deletions.
diff --git a/Data/HashMap/Internal.hs b/Data/HashMap/Internal.hs
@@ -144,7 +144,7 @@ import Control.DeepSeq            (NFData (..), NFData1 (..), NFData2 (..))
 import Control.Monad.ST           (ST, runST)
 import Data.Bifoldable            (Bifoldable (..))
 import Data.Bits                  (complement, popCount, unsafeShiftL,
-                                   unsafeShiftR, (.&.), (.|.))
+                                   unsafeShiftR, (.&.), (.|.), countTrailingZeros)
 import Data.Coerce                (coerce)
 import Data.Data                  (Constr, Data (..), DataType)
 import Data.Functor.Classes       (Eq1 (..), Eq2 (..), Ord1 (..), Ord2 (..),
@@ -1627,21 +1627,24 @@ unionArrayBy f !b1 !b2 !ary1 !ary2 = A.run $ do
     -- iterate over nonzero bits of b1 .|. b2
     -- it would be nice if we could shift m by more than 1 each time
     let ba = b1 .&. b2
-        go !i !i1 !i2 !m
-            | m > b'        = return ()
-            | b' .&. m == 0 = go i i1 i2 (m `unsafeShiftL` 1)
-            | ba .&. m /= 0 = do
+        go !i !i1 !i2 !b
+            | b == 0        = return ()
+            | testBit ba = do
                 x1 <- A.indexM ary1 i1
                 x2 <- A.indexM ary2 i2
                 A.write mary i $! f x1 x2
-                go (i+1) (i1+1) (i2+1) (m `unsafeShiftL` 1)
-            | b1 .&. m /= 0 = do
+                go (i+1) (i1+1) (i2+1) b''
+            | testBit b1 = do
                 A.write mary i =<< A.indexM ary1 i1
-                go (i+1) (i1+1)  i2    (m `unsafeShiftL` 1)
-            | otherwise     = do
+                go (i+1) (i1+1)  i2    b''
+            | otherwise  = do
                 A.write mary i =<< A.indexM ary2 i2
-                go (i+1)  i1    (i2+1) (m `unsafeShiftL` 1)
-    go 0 0 0 (b' .&. negate b') -- XXX: b' must be non-zero
+                go (i+1)  i1    (i2+1) b''
+          where
+            m = 1 `unsafeShiftL` (countTrailingZeros b)
+            testBit x = x .&. m /= 0
+            b'' = b .&. complement m
+    go 0 0 0 b'
     return mary
     -- TODO: For the case where b1 .&. b2 == b1, i.e. when one is a
     -- subset of the other, we could use a slightly simpler algorithm,