Fix issue #23. The 'distance_type' used to store the distance of a va…

…lue in a bucket from its ideal bucket could overflow when a lot of collisions happened and the load_factor() stayed below REHASH_ON_HIGH_NB_PROBES__MIN_LOAD_FACTOR. We now rehash no matter the load factor if the distance becomes too high.

include/tsl/robin_hash.h

@@ -293,6 +293,11 @@ class bucket_entry: public bucket_entry_hash<StoreHash> {
         tsl_rh_assert(!empty());
         value().~value_type();
     }
+
+public:
+    static const distance_type DIST_FROM_IDEAL_BUCKET_LIMIT = 512;
+    static_assert(DIST_FROM_IDEAL_BUCKET_LIMIT <= std::numeric_limits<distance_type>::max() - 1,
+                "DIST_FROM_IDEAL_BUCKET_LIMIT must be <= std::numeric_limits<distance_type>::max() - 1.");
 
 private:
     using storage = typename std::aligned_storage<sizeof(value_type), alignof(value_type)>::type;
@@ -1266,9 +1271,7 @@ class robin_hash: private Hash, private KeyEqual, private GrowthPolicy {
 
         while(!m_buckets[ibucket].empty()) {
             if(dist_from_ideal_bucket > m_buckets[ibucket].dist_from_ideal_bucket()) {
-                if(dist_from_ideal_bucket >= REHASH_ON_HIGH_NB_PROBES__NPROBES && 
-                   load_factor() >= REHASH_ON_HIGH_NB_PROBES__MIN_LOAD_FACTOR) 
-                {
+                if(dist_from_ideal_bucket >= bucket_entry::DIST_FROM_IDEAL_BUCKET_LIMIT) {
                     /**
                      * The number of probes is really high, rehash the map on the next insert.
                      * Difficult to do now as rehash may throw an exception.
@@ -1376,10 +1379,6 @@ class robin_hash: private Hash, private KeyEqual, private GrowthPolicy {
                   "MAXIMUM_MIN_LOAD_FACTOR should be < MINIMUM_MAX_LOAD_FACTOR");
 
 private:
-    static const distance_type REHASH_ON_HIGH_NB_PROBES__NPROBES = 128;
-    static constexpr float REHASH_ON_HIGH_NB_PROBES__MIN_LOAD_FACTOR = 0.15f;
-
-
     /**
      * Return an always valid pointer to an static empty bucket_entry with last_bucket() == true.
      */