Add function to create subgraph

Author: flowtoolz

Code/Graph/Graph+SubGraph.swift

@@ -0,0 +1,23 @@
+import OrderedCollections
+
+public extension Graph
+{
+    func subGraph(nodes: OrderedSet<Node>) -> Graph where NodeValue: Identifiable, NodeValue.ID == NodeID
+    {
+        let nodeIDs = Set(nodes.map { $0.id })
+        
+        var subGraph = Graph(values: nodes.map { $0.value })
+        
+        for edge in edges
+        {
+            if nodeIDs.contains(edge.originID) && nodeIDs.contains(edge.destinationID)
+            {
+                subGraph.addEdge(from: edge.originID,
+                                 to: edge.destinationID,
+                                 count: edge.count)
+            }
+        }
+        
+        return subGraph
+    }
+}