add substitution (rough)

node.go

@@ -131,3 +131,7 @@ func (n *Node) Rate() float64 {
 	last := n.Samples.Peek()
 	return float64(len) / float64(time.Since(last.Start))
 }
+
+func (n *Node) String() string {
+	return n.URL
+}

node_ring.go

@@ -1,6 +1,8 @@
 package caboose
 
 import (
+	"fmt"
+	"strings"
 	"sync"
 
 	"github.com/willscott/hashring"
@@ -34,6 +36,40 @@ func (nr *NodeRing) updateRing() error {
 	return nil
 }
 
+// A score of '0' ==> overall experience is the same as the current state
+// A positive score ==> overall experience is better than the current state
+// A negative score ==> overall experience is worse than the current state
+func (nr *NodeRing) getScoreForUpdate(candidate string, priority float64, weight int) float64 {
+	changes := nr.ring.ConsiderUpdateWeightedNode(candidate, weight)
+	delta := float64(0)
+	var neighbor *Node
+
+	for n, v := range changes {
+		neighbor = nr.Nodes[n]
+		neighborVolume := neighbor.Rate()
+		if neighborVolume < 1 {
+			neighborVolume = 1
+		}
+
+		amntChanged := v
+		// for now, add some bounds
+		if amntChanged < -1 {
+			amntChanged = -1
+		} else if amntChanged > 1 {
+			amntChanged = 1
+		}
+		// a negative amntChanged means that we're replacing the neighbor with the candidate.
+		amntChanged *= -1
+
+		// how much worse is candidate?
+		diff := priority - neighbor.Priority()
+		cs := diff * neighborVolume * float64(amntChanged)
+		delta += cs
+		// fmt.Printf("+%f (n %s: diff %f=(n %f - candidate %f) * volume %f * v = %f)", cs, neighbor.URL, diff, neighbor.Priority(), priority, neighborVolume, amntChanged)
+	}
+	return delta
+}
+
 func (nr *NodeRing) MaybeSubstituteOrAdd(candidate *Node, activationThreshold int64) (bool, error) {
 	nr.lk.Lock()
 	defer nr.lk.Unlock()
@@ -46,27 +82,32 @@ func (nr *NodeRing) MaybeSubstituteOrAdd(candidate *Node, activationThreshold in
 	}
 
 	// how much space is being claimed?
-	overlapEstimate := nr.ring.ConsiderUpdateWeightedNode(candidate.URL, 1)
+	delta := nr.getScoreForUpdate(candidate.URL, candidate.Priority(), 1)
 
-	var neighbor *Node
-	delta := float64(0)
+	if delta >= float64(activationThreshold) {
+		nr.Nodes[candidate.URL] = candidate
+		return true, nr.updateRing()
+	}
 
-	for n, v := range overlapEstimate {
-		neighbor = nr.Nodes[n]
-		neighborVolume := neighbor.Rate()
-		if neighborVolume < 1 {
-			neighborVolume = 1
+	// not a clear benefit to add, but maybe acceptable for substitution:
+	worst := candidate.Priority()
+	worstN := ""
+	for _, n := range nr.Nodes {
+		if n.Priority() < worst {
+			worst = n.Priority()
+			worstN = n.URL
 		}
-
-		// how much worse is candidate?
-		diff := neighbor.Priority() - candidate.Priority()
-		delta += diff * neighborVolume * float64(v)
 	}
 
-	if delta > float64(activationThreshold) {
+	// todo: the '+1' is an arbitrary threshold to prevent thrashing. it should be configurable.
+	if worstN != "" && candidate.Priority()-worst > float64(activationThreshold)+1 {
 		nr.Nodes[candidate.URL] = candidate
+		delete(nr.Nodes, worstN)
 		return true, nr.updateRing()
+
 	}
+
+	// fmt.Printf("did not add - delta %f activation %d, node priority %f\n", delta, activationThreshold, candidate.Priority())
 	return false, nil
 }
 
@@ -121,3 +162,15 @@ func (nr *NodeRing) Len() int {
 	defer nr.lk.RUnlock()
 	return nr.ring.Size()
 }
+
+func (nr *NodeRing) String() string {
+	nr.lk.RLock()
+	defer nr.lk.RUnlock()
+
+	ns := make([]string, 0, len(nr.Nodes))
+	for _, n := range nr.Nodes {
+		ns = append(ns, n.String())
+	}
+
+	return fmt.Sprintf("NodeRing[len %d]{%s}", nr.ring.Size(), strings.Join(ns, ","))
+}