Consider available capacity while routing

Prioritize sufficient channels, also consider fees.

Author: naumenkogs

lightning/src/routing/router.rs

@@ -193,6 +193,7 @@ pub fn get_route<L: Deref>(our_node_id: &PublicKey, network: &NetworkGraph, targ
 
 	let mut targets = BinaryHeap::new(); //TODO: Do we care about switching to eg Fibbonaci heap?
 	let mut dist = HashMap::with_capacity(network.get_nodes().len());
+	let recommended_available_msat = final_value_msat * 4;
 
 	let mut first_hop_targets = HashMap::with_capacity(if first_hops.is_some() { first_hops.as_ref().unwrap().len() } else { 0 });
 	if let Some(hops) = first_hops {
@@ -278,7 +279,45 @@ pub fn get_route<L: Deref>(our_node_id: &PublicKey, network: &NetworkGraph, targ
 						lowest_fee_to_peer_through_node: total_fee,
 						lowest_fee_to_node: $starting_fee_msat as u64 + new_fee,
 					};
-					if old_entry.0 > total_fee {
+
+					let mut replace_hop = false;
+
+					// Prioritize sufficient channels over insufficient (low capacity) and over those with unknown capacity.
+					// If both channels are sufficient, prioritize by fee.
+					// If at least one channel is insufficient, prioritize by capacity (if possible) and then fee.
+					if let Some(new_available_msat) = available_msat {
+						// TODO: use weightened formula to consider both available capacity and fees.
+						if new_available_msat > recommended_available_msat {
+							if let Some(old_available_msat) = old_entry.4 {
+								if old_available_msat < recommended_available_msat {
+									replace_hop = true;
+								}
+								else if old_available_msat < new_available_msat {
+									replace_hop = old_entry.0 > total_fee;
+								}
+							} else {
+								replace_hop = true;
+							}
+						} else {
+							if let Some(old_available_msat) = old_entry.4 {
+								if old_available_msat < new_available_msat {
+									replace_hop = true;
+								}
+							} else {
+								replace_hop = old_entry.0 > total_fee;
+							}
+						}
+					} else {
+						if let Some(old_available_msat) = old_entry.4 {
+							if old_available_msat < recommended_available_msat {
+								replace_hop = old_entry.0 > total_fee;
+							}
+						} else {
+							replace_hop = old_entry.0 > total_fee;
+						}
+					}
+
+					if replace_hop {
 						targets.push(new_graph_node);
 						old_entry.0 = total_fee;
 						old_entry.3 = RouteHop {