Disconnect unreachable nodes in a graph

This function removes all edges between unreachable nodes in a graph. This can then be used to compute the cone of influence between 2 points, A and B:
1 -  disconnect all nodes unreachable from A
2 - do backwards reachability from B.

Author: polgreen

src/util/graph.h

@@ -252,6 +252,9 @@ class grapht
   std::vector<node_indext>
   get_reachable(const std::vector<node_indext> &src, bool forwards) const;
 
+  void disconnect_unreachable(node_indext src);
+  void disconnect_unreachable(std::vector<node_indext> src);
+
   void make_chordal();
 
   // return value: number of connected subgraphs
@@ -461,6 +464,48 @@ void grapht<N>::visit_reachable(node_indext src)
     nodes[index].visited = true;
 }
 
+/// Removes any edges between nodes in a graph that are unreachable from
+/// a given start node. Used for computing cone of influence,
+/// by disconnecting unreachable nodes and then performing backwards
+/// reachability.
+/// Note nodes are not actually removed from the vector nodes, because
+/// this requires renumbering node indices. This copies the way nodes
+/// are "removed" in make_chordal.
+/// \param src: start node
+template <class N>
+void grapht<N>::disconnect_unreachable(node_indext src)
+{
+  std::vector<node_indext> source_nodes;
+  source_nodes.push_back(src);
+  disconnect_unreachable(source_nodes);
+}
+
+/// Removes any edges between nodes in a graph that are unreachable
+/// from a vector of start nodes.
+/// \param src: vector of indices of start nodes
+template <class N>
+void grapht<N>::disconnect_unreachable(std::vector<node_indext> src)
+{
+  std::vector<node_indext> reachable = get_reachable(src, true);
+  std::sort(reachable.begin(), reachable.end());
+  std::size_t reachable_idx = 0;
+  for(std::size_t i = 0; i < nodes.size(); i++)
+  {
+    if(reachable_idx >= reachable.size())
+      return;
+    PRECONDITION(i <= reachable[reachable_idx]);
+    if(i > reachable[reachable_idx])
+      throw "error disconnecting unreachable nodes";
+
+    if(i == reachable[reachable_idx])
+    {
+      reachable_idx++;
+    }
+    else
+      remove_edges(i);
+  }
+}
+
 /// Add to `set`, nodes that are reachable from `set`.
 ///
 /// This implements a depth first search using a stack: at each step we pop a