Allow non-power-of-2 signature cache sizes

Author: sipa

src/cuckoocache.h

@@ -154,7 +154,7 @@ class bit_packed_atomic_flags
  * @tparam Element should be a movable and copyable type
  * @tparam Hash should be a function/callable which takes a template parameter
  * hash_select and an Element and extracts a hash from it. Should return
- * high-entropy hashes for `Hash h; h<0>(e) ... h<7>(e)`.
+ * high-entropy uint32_t hashes for `Hash h; h<0>(e) ... h<7>(e)`.
  */
 template <typename Element, typename Hash>
 class cache
@@ -193,12 +193,6 @@ class cache
      */
     uint32_t epoch_size;
 
-    /** hash_mask should be set to appropriately mask out a hash such that every
-     * masked hash is [0,size), eg, if floor(log2(size)) == 20, then hash_mask
-     * should be (1<<20)-1
-     */
-    uint32_t hash_mask;
-
     /** depth_limit determines how many elements insert should try to replace.
      * Should be set to log2(n)*/
     uint8_t depth_limit;
@@ -217,14 +211,14 @@ class cache
      */
     inline std::array<uint32_t, 8> compute_hashes(const Element& e) const
     {
-        return {{hash_function.template operator()<0>(e) & hash_mask,
-                 hash_function.template operator()<1>(e) & hash_mask,
-                 hash_function.template operator()<2>(e) & hash_mask,
-                 hash_function.template operator()<3>(e) & hash_mask,
-                 hash_function.template operator()<4>(e) & hash_mask,
-                 hash_function.template operator()<5>(e) & hash_mask,
-                 hash_function.template operator()<6>(e) & hash_mask,
-                 hash_function.template operator()<7>(e) & hash_mask}};
+        return {{(uint32_t)((hash_function.template operator()<0>(e) * (uint64_t)size) >> 32),
+                 (uint32_t)((hash_function.template operator()<1>(e) * (uint64_t)size) >> 32),
+                 (uint32_t)((hash_function.template operator()<2>(e) * (uint64_t)size) >> 32),
+                 (uint32_t)((hash_function.template operator()<3>(e) * (uint64_t)size) >> 32),
+                 (uint32_t)((hash_function.template operator()<4>(e) * (uint64_t)size) >> 32),
+                 (uint32_t)((hash_function.template operator()<5>(e) * (uint64_t)size) >> 32),
+                 (uint32_t)((hash_function.template operator()<6>(e) * (uint64_t)size) >> 32),
+                 (uint32_t)((hash_function.template operator()<7>(e) * (uint64_t)size) >> 32)}};
     }
 
     /* end
@@ -305,7 +299,7 @@ class cache
     }
 
     /** setup initializes the container to store no more than new_size
-     * elements. setup rounds down to a power of two size.
+     * elements.
      *
      * setup should only be called once.
      *
@@ -316,8 +310,7 @@ class cache
     {
         // depth_limit must be at least one otherwise errors can occur.
         depth_limit = static_cast<uint8_t>(std::log2(static_cast<float>(std::max((uint32_t)2, new_size))));
-        size = 1 << depth_limit;
-        hash_mask = size-1;
+        size = std::max<uint32_t>(2, new_size);
         table.resize(size);
         collection_flags.setup(size);
         epoch_flags.resize(size);