routing: add index map to distanceHeap

This commit adds the pubkeyIndices map to the distanceHeap to avoid
duplicate entries on the heap. This happened in the earlier iteration
of the findPath algorithm and would cause the driving loop to
evaluate already evaluated entries when there was no need.

routing/pathfind.go

@@ -300,7 +300,7 @@ func findPath(g *graphParams, r *RestrictParams, source, target route.Vertex,
 	// First we'll initialize an empty heap which'll help us to quickly
 	// locate the next edge we should visit next during our graph
 	// traversal.
-	var nodeHeap distanceHeap
+	nodeHeap := newDistanceHeap()
 
 	// For each node in the graph, we create an entry in the distance map
 	// for the node set with a distance of "infinity". graph.ForEachNode
@@ -527,9 +527,10 @@ func findPath(g *graphParams, r *RestrictParams, source, target route.Vertex,
 
 		next[fromVertex] = edge
 
-		// Add this new node to our heap as we'd like to further
-		// explore backwards through this edge.
-		heap.Push(&nodeHeap, distance[fromVertex])
+		// Either push distance[fromVertex] onto the heap if the node
+		// represented by fromVertex is not already on the heap OR adjust
+		// its position within the heap via heap.Fix.
+		nodeHeap.PushOrFix(distance[fromVertex])
 	}
 
 	// TODO(roasbeef): also add path caching