add closest-preceding-node operation

chord/topology.go

@@ -32,6 +32,38 @@ func (n *Node) IsLocal(key string) bool {
 	return contains(n.self.id, n.Successor().id, id)
 }
 
+// Closest finds the locally known peer which is the closest predecessor of key.
+func (n *Node) Closest(key string) Peer {
+	self := n.self.id
+	id := keyID(key)
+	l := n.fingers
+	for i := len(l) - 1; i >= 0; i-- {
+		f := l[i]
+		if contains(self, id, f.id) {
+			// contains reports true if id == f.id, which is
+			// technically incorrect for this operation.
+			// We could reasonably skip double-checking and rely on the hash,
+			// but we can also not skip it and just be correct.
+			if id == f.id {
+				continue
+			}
+			return f
+		}
+	}
+	// Also try successors.
+	l = n.succ
+	for i := len(l) - 1; i >= 0; i-- {
+		f := l[i]
+		if contains(self, id, f.id) {
+			if id == f.id {
+				continue
+			}
+			return f
+		}
+	}
+	return n.self
+}
+
 // Peer is the ID and address of a node.
 type Peer struct {
 	id   ID
@@ -56,7 +88,7 @@ func Create(addr netip.AddrPort) (*Node, error) {
 	n := &Node{
 		self:    self,
 		succ:    []Peer{self}[:1:1], // extra cautious about capacity
-		fingers: make([]Peer, len(ID{})*8),
+		fingers: make([]Peer, 0, len(ID{})*8),
 	}
 	return n, nil
 }