// TODO: Add functions to reserve memory for vertices and edges.
// TODO: Add iterator of edges.
// TODO: Split out GraphTopologyAddition trait.
pub trait GraphTopology {
    type Vertex: Copy + Eq;
    type Edge;

    fn vertex_count(&self) -> usize;
    fn edge_count(&self) -> usize;
    fn degree(&self, v: Self::Vertex) -> usize;
    fn are_adjacent(&self, v1: Self::Vertex, v2: Self::Vertex) -> bool;
    fn vertices(&self) -> impl Iterator<Item = Self::Vertex>;
    fn neighbors(&self, v: Self::Vertex) -> impl Iterator<Item = Self::Vertex>;
    fn add_vertex(&mut self) -> Self::Vertex;
    fn add_edge(&mut self, v1: Self::Vertex, v2: Self::Vertex) -> Self::Edge;
}

pub trait GraphTopologyDeletion: GraphTopology {
    fn delete_vertex(&mut self, v: Self::Vertex);
    fn delete_edge(&mut self, e: Self::Edge);
}