Graph protocol

Graph.swift

public enum EdgeType {
  case directed
  case undirected
}

public protocol Graph {
  
  associatedtype Element
  
  func createVertex(data: Element) -> Vertex<Element>
  func addDirectedEdge(from source: Vertex<Element>,
                       to destination: Vertex<Element>,
                       weight: Double?)
  func addUndirectedEdge(between source: Vertex<Element>,
                         and destination: Vertex<Element>,
                         weight: Double?)
  func add(_ edge: EdgeType, from source: Vertex<Element>,
                             to destination: Vertex<Element>,
                             weight: Double?)
  func edges(from source: Vertex<Element>) -> [Edge<Element>]
  func weight(from source: Vertex<Element>,
              to destination: Vertex<Element>) -> Double?
}

public struct Vertex<T> {
  
  public let index: Int
  public let data: T
}

extension Vertex: Hashable where T: Hashable {}
extension Vertex: Equatable where T: Equatable {}

public struct Edge<T> {
  
  public let source: Vertex<T>
  public let destination: Vertex<T>
  public let weight: Double?
}