Add FromGraph trait, and add implementation for FrozenGraph

src/traits.rs

@@ -237,6 +237,39 @@ pub trait GraphTopologyDeletion: GraphTopology {
     fn delete_edge(&mut self, e: Self::Edge);
 }
 
+/// A trait for constructing an undirected graph topology by converting another graph
+/// implementation to this one.
+pub trait FromGraph: GraphTopology {
+    /// Creates a new graph instance that is isomorphic to `source`.
+    ///
+    /// Returns the copied graph instance and two mappings from the vertices and edges of `source`
+    /// to the corresponding vertices and edges of the returned graph.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use grapherity::prelude::*;
+    /// use grapherity::models::{AppendGraph, FrozenGraph};
+    ///
+    /// let mut source = AppendGraph::new();
+    /// let u = source.add_vertex();
+    /// let v = source.add_vertex();
+    /// let e = source.add_edge(u, v);
+    /// let (graph, vertex_map, edge_map) = FrozenGraph::from_graph(&source);
+    ///
+    /// assert!(graph.are_adjacent(vertex_map[u], vertex_map[v]));
+    /// ```
+    fn from_graph<G: GraphTopology>(
+        source: &G,
+    ) -> (
+        Self,
+        ElementMap<G::Vertex, Self::Vertex>,
+        ElementMap<G::Edge, Self::Edge>,
+    )
+    where
+        Self: Sized;
+}
+
 /// A cursor for traversing the incidences of a vertex one step at a time.
 ///
 /// Because the cursor is [`Copy`], its state can be saved and restored to replay or branch a