Add generators::cycle, without tests

src/generators.rs

@@ -61,6 +61,24 @@ pub fn path<G: GraphTopologyAddition>(graph: &mut G, n: usize) -> (Vec<G::Vertex
     (vertices, edges)
 }
 
+/// Generates a cycle graph.
+///
+/// Generates a cycle with `n` edges in `graph` as a new connected component and returns the new
+/// vertices *v0, v1, ..., v(n - 1)* and edges *e0, e1, ..., e(n - 1)* in order, i.e.
+/// *e0 = (v0, v1), e1 = (v1, v2), ..., e(n - 1) = (v(n - 1), v0)*.
+///
+/// For `n == 0`, nothing is added. For `n == 1`, a single vertex with a loop is added. For
+/// `n == 2`, two vertices and two edges between them (multi-edge) are added.
+pub fn cycle<G: GraphTopologyAddition>(graph: &mut G, n: usize) -> (Vec<G::Vertex>, Vec<G::Edge>) {
+    graph.reserve_vertices(n);
+    graph.reserve_edges(n);
+    let vertices: Vec<_> = (0..n).map(|_| graph.add_vertex()).collect();
+    let edges = (0..n)
+        .map(|i| graph.add_edge(vertices[i], vertices[if i < n - 1 { i + 1 } else { 0 }]))
+        .collect();
+    (vertices, edges)
+}
+
 /// Generates a Petersen graph.
 ///
 /// Generates a [Petersen graph] in `graph` as a new connected component and returns the new