Avoid loops for only the last three nodes, don't alloc.

Author: tnull

lightning/src/routing/router.rs

@@ -1564,9 +1564,13 @@ fn add_random_cltv_offset(route: &mut Route, payment_params: &PaymentParameters,
 	for path in route.paths.iter_mut() {
 		let mut shadow_ctlv_expiry_delta_offset: u32 = 0;
 
-		// Mark all nodes on the actual path as visited to avoid looping random walks.
-		let mut visited_nodes: Vec<NodeId> = path.iter()
-			.map(|h| NodeId::from_pubkey(&h.pubkey)).collect();
+		// Remember the last three nodes of the random walk and avoid looping back on them.
+		// Init with the last three nodes from the actual path, if possible.
+		let last_id = NodeId::from_pubkey(&path.last().unwrap().pubkey);
+		let second_last_id = NodeId::from_pubkey(&path.get(path.len().saturating_sub(2)).unwrap().pubkey);
+		let third_last_id = NodeId::from_pubkey(&path.get(path.len().saturating_sub(3)).unwrap().pubkey);
+		let mut nodes_to_avoid: [NodeId; 3] = [third_last_id, second_last_id, last_id];
+		let mut nodes_ring_index = 0;
 
 		// Choose the last publicly known node as the starting point for the random walk.
 		let mut cur_hop: Option<NodeId> = None;
@@ -1599,8 +1603,9 @@ fn add_random_cltv_offset(route: &mut Route, payment_params: &PaymentParameters,
 						.and_then(|index| cur_node.channels.get(index))
 						.and_then(|id| network_channels.get(id)) {
 							random_channel.as_directed_from(&cur_node_id).map(|(dir_info, next_id)| {
-								if !visited_nodes.contains(next_id) {
-									visited_nodes.push(*next_id);
+								if !nodes_to_avoid.iter().any(|x| x == next_id) {
+									nodes_to_avoid[nodes_ring_index] = *next_id;
+									nodes_ring_index = (nodes_ring_index + 1) % 3;
 									dir_info.direction().map(|channel_update_info| {
 										random_hop_offset = channel_update_info.cltv_expiry_delta.into();
 										cur_hop = Some(*next_id);