Add num_edges(g) and has_edge(g) CPOs

include/graph/container/compressed_graph.hpp

@@ -752,6 +752,13 @@ class compressed_graph_base
       return const_vertices_type(g.row_index_.begin(), g.row_index_.end() - 1); // don't include terminating row
   }
 
+  friend constexpr auto num_edges(const compressed_graph_base& g) {
+      return static_cast<size_type>(g.col_index_.size());
+  }
+  friend constexpr bool has_edge(const compressed_graph_base& g) {
+      return g.col_index_.size() > 0;
+  }
+
   friend vertex_id_type vertex_id(const compressed_graph_base& g, const_iterator ui) {
     return static_cast<vertex_id_type>(ui - g.row_index_.begin());
   }

include/graph/container/dynamic_graph.hpp

@@ -1102,6 +1102,7 @@ class dynamic_graph_base {
           edge_adder(edge_type(std::move(e.target_id), std::move(e.value)));
         }
       }
+      edge_count_ += 1;
     }
   }
 
@@ -1169,6 +1170,7 @@ class dynamic_graph_base {
           edge_adder(edge_type(std::move(e.target_id), std::move(e.value)));
         }
       }
+      edge_count_ += 1;
     }
   }
 
@@ -1226,11 +1228,17 @@ class dynamic_graph_base {
 
 private: // Member Variables
   vertices_type vertices_;
+  size_t        edge_count_ = 0;
 
 private: // CPO properties
   friend constexpr vertices_type&       vertices(dynamic_graph_base& g) { return g.vertices_; }
   friend constexpr const vertices_type& vertices(const dynamic_graph_base& g) { return g.vertices_; }
 
+  friend auto num_edges(const dynamic_graph_base& g) {
+    return g.edge_count_;
+  }
+  friend bool has_edge(const dynamic_graph_base& g) { return g.edge_count_ > 0; }
+
   friend vertex_id_type vertex_id(const dynamic_graph_base& g, typename vertices_type::const_iterator ui) {
     return static_cast<vertex_id_type>(ui - g.vertices_.begin());
   }