reserve and capacity not matching #18

reserve(), especially for parallel_*_hash_*, was reserving excessive sizes. Now the size is closer to the one requested, even though the "power-of-two" capacity requirement still creates a capacity() greater than what is asked in reserve().

parallel_hashmap/phmap.h

@@ -1541,7 +1541,7 @@ public:
         }
         // bitor is a faster way of doing `max` here. We will round up to the next
         // power-of-2-minus-1, so bitor is good enough.
-        auto m = NormalizeCapacity(n | GrowthToLowerboundCapacity(size()));
+        auto m = NormalizeCapacity(std::max(n, size()));
         // n == 0 unconditionally rehashes as per the standard.
         if (n == 0 || m > capacity_) {
             resize(m);
@@ -3008,7 +3008,12 @@ public:
         }
     }
 
-    void reserve(size_t n) { rehash(GrowthToLowerboundCapacity(n)); }
+    void reserve(size_t n) 
+    {
+        size_t target = GrowthToLowerboundCapacity(n);
+        size_t normalized = 16 * NormalizeCapacity(n / num_tables);
+        rehash(normalized > target ? normalized : target); 
+    }
 
     // Extension API: support for heterogeneous keys.
     //