WIP 3

lib/pure/bitops.nim

@@ -355,6 +355,15 @@ proc firstSetBit*(x: SomeInteger): int {.inline, noSideEffect.} =
       when sizeof(x) <= 4: result = firstSetBitNim(x.uint32)
       else: result = firstSetBitNim(x.uint64)
 
+proc lastSetBit*(x: SomeUnsignedInt): uint {.inline, noSideEffect.} =
+  ## Returns the 1-based index of the most significant set bit of x.
+  ## If x == 0, the result will also be 0
+  var temp = x
+  result = 0
+  while temp != 0:
+    temp = temp shr 1
+    result += 1
+
 proc fastLog2*(x: SomeInteger): int {.inline, noSideEffect.} =
   ## Quickly find the log base 2 of an integer.
   ## If `x` is zero, when ``noUndefinedBitOpts`` is set, result is -1,

lib/pure/collections/hashcommon.nim

@@ -9,6 +9,7 @@
 
 # An ``include`` file which contains common code for
 # hash sets and tables.
+from bitops import lastSetBit
 
 const
   growthFactor = 2
@@ -50,16 +51,8 @@ template rawGetKnownHCImpl() {.dirty.} =
 proc rawGetKnownHC[X, A](t: X, key: A, hc: Hash): int {.inline.} =
   rawGetKnownHCImpl()
 
-template bits(n): uint32 =
-  var temp = n
-  var bits = 0.uint32
-  while temp != 0:
-    temp = temp shr 1
-    bits += 1
-  bits
-
 template genHashImpl(key, hc: typed) =
-  hc = hash(key, targetBits=bits(maxHash(t)))
+  hc = hash(key, targetBits=lastSetBit(maxHash(t)))
   if hc == 0: # This almost never taken branch should be very predictable.
     hc = 314159265 # Value doesn't matter; Any non-zero favorite is fine.