autopilot/interface+prefattach: scale node scores to range [0.0, 1.0]

To prepare for combinning scores from multiple heuristics, we require the
scores returned from the NodeSores API to be in the range [0.0, 1.0].

The prefAttach heuristic is altered to scale the returned scores such
that the most connected node in the grpah is given a score of 1.0.

autopilot/interface.go

@@ -125,11 +125,12 @@ type AttachmentHeuristic interface {
 	// returned channel candidates maps the NodeID to a NodeScore for the
 	// node.
 	//
-	// The scores will be in the range [0, M], where 0 indicates no
+	// The returned scores will be in the range [0, 1.0], where 0 indicates
-	// improvement in connectivity if a channel is opened to this node,
+	// no improvement in connectivity if a channel is opened to this node,
-	// while M is the maximum possible improvement in connectivity. The
+	// while 1.0 is the maximum possible improvement in connectivity. The
-	// size of M is up to the implementation of this interface, so scores
+	// implementation of this interface must return scores in this range to
-	// must be normalized if compared against other implementations.
+	// properly allow the autopilot agent to make a reasonable choice based
+	// on the score from multiple heuristics.
 	//
 	// NOTE: A NodeID not found in the returned map is implicitly given a
 	// score of 0.

autopilot/prefattach.go

@@ -43,9 +43,10 @@ func NewNodeID(pub *btcec.PublicKey) NodeID {
 	return n
 }
 
-// NodeScores is a method that given the current channel graph and
+// NodeScores is a method that given the current channel graph and current set
-// current set of local channels, scores the given nodes according to
+// of local channels, scores the given nodes according to the preference of
-// the preference of opening a channel of the given size with them.
+// opening a channel of the given size with them. The returned channel
+// candidates maps the NodeID to a NodeScore for the node.
 //
 // The heuristic employed by this method is one that attempts to promote a
 // scale-free network globally, via local attachment preferences for new nodes
@@ -64,21 +65,25 @@ func (p *PrefAttachment) NodeScores(g ChannelGraph, chans []Channel,
 	chanSize btcutil.Amount, nodes map[NodeID]struct{}) (
 	map[NodeID]*NodeScore, error) {
 
-	// Count the number of channels in the graph. We'll also count the
+	// Count the number of channels for each particular node in the graph.
-	// number of channels as we go for the nodes we are interested in.
+	var maxChans int
-	var graphChans int
 	nodeChanNum := make(map[NodeID]int)
 	if err := g.ForEachNode(func(n Node) error {
 		var nodeChans int
 		err := n.ForEachChannel(func(_ ChannelEdge) error {
 			nodeChans++
-			graphChans++
 			return nil
 		})
 		if err != nil {
 			return err
 		}
 
+		// We keep track of the highest-degree node we've seen, as this
+		// will be given the max score.
+		if nodeChans > maxChans {
+			maxChans = nodeChans
+		}
+
 		// If this node is not among our nodes to score, we can return
 		// early.
 		nID := NodeID(n.PubKey())
@@ -97,7 +102,7 @@ func (p *PrefAttachment) NodeScores(g ChannelGraph, chans []Channel,
 	// If there are no channels in the graph we cannot determine any
 	// preferences, so we return, indicating all candidates get a score of
 	// zero.
-	if graphChans == 0 {
+	if maxChans == 0 {
 		return nil, nil
 	}
 
@@ -127,8 +132,9 @@ func (p *PrefAttachment) NodeScores(g ChannelGraph, chans []Channel,
 		}
 
 		// Otherwise we score the node according to its fraction of
-		// channels in the graph.
+		// channels in the graph, scaled such that the highest-degree
-		score := float64(nodeChans) / float64(graphChans)
+		// node will be given a score of 1.0.
+		score := float64(nodeChans) / float64(maxChans)
 		candidates[nID] = &NodeScore{
 			NodeID: nID,
 			Score:  score,

autopilot/prefattach_test.go

@@ -249,8 +249,8 @@ func TestPrefAttachmentSelectTwoVertexes(t *testing.T) {
 
 				// Since each of the nodes has 1 channel, out
 				// of only one channel in the graph, we expect
-				// their score to be 0.5.
+				// their score to be 1.0.
-				expScore := float64(0.5)
+				expScore := float64(1.0)
 				if candidate.Score != expScore {
 					t1.Fatalf("expected candidate score "+
 						"to be %v, instead was %v",