Make begin() to return end() on empty tables. (#297)

Eliminate performance overhead in constructing begin() iterators on empty hash tables.

examples/bench.cc

@@ -337,6 +337,21 @@ Timer _lookup(vector<T> &v, HT &hash, size_t &num_present)
     return timer;
 }
 
+// --------------------------------------------------------------------------
+template <class HT>
+Timer _traverse_empty(int64_t capacity, HT& hash)
+{
+    hash.clear();
+    hash.reserve(capacity);
+    Timer timer(true);
+
+    for (auto& pair : hash)
+        if (pair.second)
+            fprintf(stderr, "should not be here");
+
+    return timer;
+}
+
 // --------------------------------------------------------------------------
 template <class T, class HT>
 Timer _delete(vector<T> &v, HT &hash)
@@ -441,6 +456,10 @@ int main(int argc, char ** argv)
             timer = _lookup(v, hash, num_present);
             //fprintf(stderr, "found %zu\n", num_present);
         }
+        else if (!strcmp(bench_name, "traverse_empty"))
+        {
+            timer = _traverse_empty(num_keys, hash);
+        }
         else if(!strcmp(bench_name, "delete"))
         {
             vector<int64_t> v(num_keys);

parallel_hashmap/phmap.h

@@ -1245,6 +1245,7 @@ public:
     ~raw_hash_set() { destroy_slots(); }
 
     iterator begin() {
+        if (empty()) return end();
         auto it = iterator_at(0);
         it.skip_empty_or_deleted();
         return it;