f make things clearer

Author: TheBlueMatt

lightning/src/routing/router.rs

@@ -1113,13 +1113,16 @@ where L::Target: Logger {
 						fees: hop.fees,
 					};
 
-					let reqd_channel_cap = if let Some (val) = final_value_msat
-						.checked_mul(ROUTE_CAPACITY_PROVISION_FACTOR).unwrap_or(u64::max_value())
-						.checked_add(match idx {
-							0 => 999,
-							_ => aggregate_next_hops_fee_msat.checked_add(999).unwrap_or(u64::max_value())
-						})
-					{ Some( val / 1000 ) } else { break; }; // converting from msat or breaking if max ~ infinity
+					// We want a value of final_value_msat * ROUTE_CAPACITY_PROVISION_FACTOR but we
+					// need it to increment at each hop by the fee charged at later hops. Further,
+					// we need to ensure we round up when we divide to get satoshis.
+					let channel_cap_msat = final_value_msat
+						.checked_mul(ROUTE_CAPACITY_PROVISION_FACTOR).map(|v| v.checked_add(aggregate_next_hops_fee_msat))
+						.flatten().unwrap_or(u64::max_value());
+					let channel_cap_sat = match channel_cap_msat.checked_add(999) {
+						None => break, // We overflowed above, just ignore this route hint
+						Some(val) => Some(val / 1000),
+					};
 
 					let src_node_id = NodeId::from_pubkey(&hop.src_node_id);
 					let dest_node_id = NodeId::from_pubkey(&prev_hop_id);
@@ -1131,7 +1134,7 @@ where L::Target: Logger {
 					// sufficient value to route `final_value_msat`. Note that in the case of "0-value"
 					// invoices where the invoice does not specify value this may not be the case, but
 					// better to include the hints than not.
-					if !add_entry!(hop.short_channel_id, src_node_id, dest_node_id, directional_info, reqd_channel_cap, &empty_channel_features, aggregate_next_hops_fee_msat, path_value_msat, aggregate_next_hops_path_htlc_minimum_msat, aggregate_next_hops_path_penalty_msat) {
+					if !add_entry!(hop.short_channel_id, src_node_id, dest_node_id, directional_info, channel_cap_sat, &empty_channel_features, aggregate_next_hops_fee_msat, path_value_msat, aggregate_next_hops_path_htlc_minimum_msat, aggregate_next_hops_path_penalty_msat) {
 						// If this hop was not used then there is no use checking the preceding hops
 						// in the RouteHint. We can break by just searching for a direct channel between
 						// last checked hop and first_hop_targets