Add len() for VertexMap and EdgeMap

src/maps.rs

@@ -13,6 +13,11 @@ impl<V: Copy, T: Clone> VertexMap<V, T> {
         }
     }
 
+    // Reads beyond len() are valid and return the default value.
+    pub fn len(&self) -> usize {
+        self.inner.len()
+    }
+
     pub fn sync<G: GraphTopology<Vertex = V>>(&mut self, graph: &G) {
         self.inner.resize(graph.vertex_capacity());
     }
@@ -43,6 +48,11 @@ impl<E: Copy, T: Clone> EdgeMap<E, T> {
         }
     }
 
+    // Reads beyond len() are valid and return the default value.
+    pub fn len(&self) -> usize {
+        self.inner.len()
+    }
+
     pub fn sync<G: GraphTopology<Edge = E>>(&mut self, graph: &G) {
         self.inner.resize(graph.edge_capacity());
     }
@@ -77,6 +87,10 @@ impl<E: Copy, T: Clone> EntityMap<E, T> {
         }
     }
 
+    pub fn len(&self) -> usize {
+        self.data.len()
+    }
+
     pub fn resize(&mut self, capacity: usize) {
         if capacity > self.data.len() {
             self.data.resize(capacity, self.default.clone());