Factor out the code for choosing the bucket count, and which bucket that hash values map to make it easier to experiment with alternative policies.

[SVN r44697]

include/boost/unordered/detail/hash_table.hpp

@@ -50,7 +50,6 @@ namespace boost {
 
         static const std::size_t default_initial_bucket_count = 50;
         static const float minimum_max_load_factor = 1e-3f;
-        inline std::size_t next_prime(std::size_t n);
 
         template <class T>
         inline void hash_swap(T& x, T& y)
@@ -101,6 +100,32 @@ namespace boost {
                 bound--;
             return *bound;
         }
+        
+        // Controls how many buckets are allocated and which buckets hash
+        // values map to. Does not contain the buckets themselves, or ever
+        // deal with them directly.
+
+        struct bucket_manager {
+            std::size_t bucket_count_;
+            
+            bucket_manager()
+                : bucket_count_(0) {}
+            
+            explicit bucket_manager(std::size_t n)
+                : bucket_count_(next_prime(n)) {}
+            
+            std::size_t bucket_count() const {
+                return bucket_count_;
+            }
+            
+            std::size_t bucket_from_hash(std::size_t hashed) const {
+                return hashed % bucket_count_;
+            }
+            
+            std::size_t max_bucket_count(std::size_t max_size) const {
+                return prev_prime(max_size);
+            }
+        };
 
         // pair_cast - used to convert between pair types.