package routing import ( "bytes" "encoding/binary" "fmt" "math" "container/heap" "github.com/coreos/bbolt" "github.com/lightningnetwork/lightning-onion" "github.com/lightningnetwork/lnd/channeldb" "github.com/lightningnetwork/lnd/lnwire" "github.com/roasbeef/btcd/btcec" "github.com/roasbeef/btcd/chaincfg/chainhash" "github.com/roasbeef/btcutil" ) const ( // HopLimit is the maximum number hops that is permissible as a route. // Any potential paths found that lie above this limit will be rejected // with an error. This value is computed using the current fixed-size // packet length of the Sphinx construction. HopLimit = 20 // infinity is used as a starting distance in our shortest path search. infinity = math.MaxInt64 // RiskFactorBillionths controls the influence of time lock delta // of a channel on route selection. It is expressed as billionths // of msat per msat sent through the channel per time lock delta // block. See edgeWeight function below for more details. // The chosen value is based on the previous incorrect weight function // 1 + timelock + fee * fee. In this function, the fee penalty // diminishes the time lock penalty for all but the smallest amounts. // To not change the behaviour of path finding too drastically, a // relatively small value is chosen which is still big enough to give // some effect with smaller time lock values. The value may need // tweaking and/or be made configurable in the future. RiskFactorBillionths = 15 ) // HopHint is a routing hint that contains the minimum information of a channel // required for an intermediate hop in a route to forward the payment to the // next. This should be ideally used for private channels, since they are not // publicly advertised to the network for routing. type HopHint struct { // NodeID is the public key of the node at the start of the channel. NodeID *btcec.PublicKey // ChannelID is the unique identifier of the channel. ChannelID uint64 // FeeBaseMSat is the base fee of the channel in millisatoshis. FeeBaseMSat uint32 // FeeProportionalMillionths is the fee rate, in millionths of a // satoshi, for every satoshi sent through the channel. FeeProportionalMillionths uint32 // CLTVExpiryDelta is the time-lock delta of the channel. CLTVExpiryDelta uint16 } // ChannelHop describes the channel through which an intermediate or final // hop can be reached. This struct contains the relevant routing policy of // the particular edge (which is a property of the source node of the channel // edge), as well as the total capacity. It also includes the origin chain of // the channel itself. type ChannelHop struct { // Capacity is the total capacity of the channel being traversed. This // value is expressed for stability in satoshis. Capacity btcutil.Amount // Chain is a 32-byte has that denotes the base blockchain network of // the channel. The 32-byte hash is the "genesis" block of the // blockchain, or the very first block in the chain. // // TODO(roasbeef): store chain within edge info/policy in database. Chain chainhash.Hash *channeldb.ChannelEdgePolicy } // Hop represents an intermediate or final node of the route. This naming // is in line with the definition given in BOLT #4: Onion Routing Protocol. // The struct houses the channel along which this hop can be reached and // the values necessary to create the HTLC that needs to be sent to the // next hop. It is also used to encode the per-hop payload included within // the Sphinx packet. type Hop struct { // Channel is the active payment channel edge along which the packet // travels to reach this hop. This is the _incoming_ channel to this hop. Channel *ChannelHop // OutgoingTimeLock is the timelock value that should be used when // crafting the _outgoing_ HTLC from this hop. OutgoingTimeLock uint32 // AmtToForward is the amount that this hop will forward to the next // hop. This value is less than the value that the incoming HTLC // carries as a fee will be subtracted by the hop. AmtToForward lnwire.MilliSatoshi // Fee is the total fee that this hop will subtract from the incoming // payment, this difference nets the hop fees for forwarding the // payment. Fee lnwire.MilliSatoshi } // computeFee computes the fee to forward an HTLC of `amt` milli-satoshis over // the passed active payment channel. This value is currently computed as // specified in BOLT07, but will likely change in the near future. func computeFee(amt lnwire.MilliSatoshi, edge *channeldb.ChannelEdgePolicy) lnwire.MilliSatoshi { return edge.FeeBaseMSat + (amt*edge.FeeProportionalMillionths)/1000000 } // isSamePath returns true if path1 and path2 travel through the exact same // edges, and false otherwise. func isSamePath(path1, path2 []*ChannelHop) bool { if len(path1) != len(path2) { return false } for i := 0; i < len(path1); i++ { if path1[i].ChannelID != path2[i].ChannelID { return false } } return true } // Route represents a path through the channel graph which runs over one or // more channels in succession. This struct carries all the information // required to craft the Sphinx onion packet, and send the payment along the // first hop in the path. A route is only selected as valid if all the channels // have sufficient capacity to carry the initial payment amount after fees are // accounted for. type Route struct { // TotalTimeLock is the cumulative (final) time lock across the entire // route. This is the CLTV value that should be extended to the first // hop in the route. All other hops will decrement the time-lock as // advertised, leaving enough time for all hops to wait for or present // the payment preimage to complete the payment. TotalTimeLock uint32 // TotalFees is the sum of the fees paid at each hop within the final // route. In the case of a one-hop payment, this value will be zero as // we don't need to pay a fee to ourself. TotalFees lnwire.MilliSatoshi // TotalAmount is the total amount of funds required to complete a // payment over this route. This value includes the cumulative fees at // each hop. As a result, the HTLC extended to the first-hop in the // route will need to have at least this many satoshis, otherwise the // route will fail at an intermediate node due to an insufficient // amount of fees. TotalAmount lnwire.MilliSatoshi // Hops contains details concerning the specific forwarding details at // each hop. Hops []*Hop // nodeIndex is a map that allows callers to quickly look up if a node // is present in this computed route or not. nodeIndex map[Vertex]struct{} // chanIndex is an index that allows callers to determine if a channel // is present in this route or not. Channels are identified by the // uint64 version of the short channel ID. chanIndex map[uint64]struct{} // nextHop maps a node, to the next channel that it will pass the HTLC // off to. With this map, we can easily look up the next outgoing // channel or node for pruning purposes. nextHopMap map[Vertex]*ChannelHop // prevHop maps a node, to the channel that was directly before it // within the route. With this map, we can easily look up the previous // channel or node for pruning purposes. prevHopMap map[Vertex]*ChannelHop } // nextHopVertex returns the next hop (by Vertex) after the target node. If the // target node is not found in the route, then false is returned. func (r *Route) nextHopVertex(n *btcec.PublicKey) (Vertex, bool) { hop, ok := r.nextHopMap[NewVertex(n)] return Vertex(hop.Node.PubKeyBytes), ok } // nextHopChannel returns the uint64 channel ID of the next hop after the // target node. If the target node is not found in the route, then false is // returned. func (r *Route) nextHopChannel(n *btcec.PublicKey) (*ChannelHop, bool) { hop, ok := r.nextHopMap[NewVertex(n)] return hop, ok } // prevHopChannel returns the uint64 channel ID of the before hop after the // target node. If the target node is not found in the route, then false is // returned. func (r *Route) prevHopChannel(n *btcec.PublicKey) (*ChannelHop, bool) { hop, ok := r.prevHopMap[NewVertex(n)] return hop, ok } // containsNode returns true if a node is present in the target route, and // false otherwise. func (r *Route) containsNode(v Vertex) bool { _, ok := r.nodeIndex[v] return ok } // containsChannel returns true if a channel is present in the target route, // and false otherwise. The passed chanID should be the converted uint64 form // of lnwire.ShortChannelID. func (r *Route) containsChannel(chanID uint64) bool { _, ok := r.chanIndex[chanID] return ok } // ToHopPayloads converts a complete route into the series of per-hop payloads // that is to be encoded within each HTLC using an opaque Sphinx packet. func (r *Route) ToHopPayloads() []sphinx.HopData { hopPayloads := make([]sphinx.HopData, len(r.Hops)) // For each hop encoded within the route, we'll convert the hop struct // to the matching per-hop payload struct as used by the sphinx // package. for i, hop := range r.Hops { hopPayloads[i] = sphinx.HopData{ // TODO(roasbeef): properly set realm, make sphinx type // an enum actually? Realm: 0, ForwardAmount: uint64(hop.AmtToForward), OutgoingCltv: hop.OutgoingTimeLock, } // As a base case, the next hop is set to all zeroes in order // to indicate that the "last hop" as no further hops after it. nextHop := uint64(0) // If we aren't on the last hop, then we set the "next address" // field to be the channel that directly follows it. if i != len(r.Hops)-1 { nextHop = r.Hops[i+1].Channel.ChannelID } binary.BigEndian.PutUint64(hopPayloads[i].NextAddress[:], nextHop) } return hopPayloads } // newRoute returns a fully valid route between the source and target that's // capable of supporting a payment of `amtToSend` after fees are fully // computed. If the route is too long, or the selected path cannot support the // fully payment including fees, then a non-nil error is returned. // // NOTE: The passed slice of ChannelHops MUST be sorted in forward order: from // the source to the target node of the path finding attempt. func newRoute(amtToSend, feeLimit lnwire.MilliSatoshi, sourceVertex Vertex, pathEdges []*ChannelHop, currentHeight uint32, finalCLTVDelta uint16) (*Route, error) { // First, we'll create a new empty route with enough hops to match the // amount of path edges. We set the TotalTimeLock to the current block // height, as this is the basis that all of the time locks will be // calculated from. route := &Route{ Hops: make([]*Hop, len(pathEdges)), TotalTimeLock: currentHeight, nodeIndex: make(map[Vertex]struct{}), chanIndex: make(map[uint64]struct{}), nextHopMap: make(map[Vertex]*ChannelHop), prevHopMap: make(map[Vertex]*ChannelHop), } // We'll populate the next hop map for the _source_ node with the // information for the first hop so the mapping is sound. route.nextHopMap[sourceVertex] = pathEdges[0] pathLength := len(pathEdges) for i := pathLength - 1; i >= 0; i-- { edge := pathEdges[i] // First, we'll update both the node and channel index, to // indicate that this Vertex, and outgoing channel link are // present within this route. v := Vertex(edge.Node.PubKeyBytes) route.nodeIndex[v] = struct{}{} route.chanIndex[edge.ChannelID] = struct{}{} // If this isn't a direct payment, and this isn't the edge to // the last hop in the route, then we'll also populate the // nextHop map to allow easy route traversal by callers. if len(pathEdges) > 1 && i != len(pathEdges)-1 { route.nextHopMap[v] = route.Hops[i+1].Channel } // Now we'll start to calculate the items within the per-hop // payload for the hop this edge is leading to. This hop will // be called the 'current hop'. // If it is the last hop, then the hop payload will contain // the exact amount. In BOLT #4: Onion Routing // Protocol / "Payload for the Last Node", this is detailed. amtToForward := amtToSend // Fee is not part of the hop payload, but only used for // reporting through RPC. Set to zero for the final hop. fee := lnwire.MilliSatoshi(0) // If the current hop isn't the last hop, then add enough funds // to pay for transit over the next link. if i != len(pathEdges)-1 { // We'll grab the per-hop payload of the next hop (the // hop _after_ the hop this edge leads to) in the // route so we can calculate fees properly. nextHop := route.Hops[i+1] // The amount that the current hop needs to forward is // based on how much the next hop forwards plus the fee // that needs to be paid to the next hop. amtToForward = nextHop.AmtToForward + nextHop.Fee // The fee that needs to be paid to the current hop is // based on the amount that this hop needs to forward // and its policy for the outgoing channel. This policy // is stored as part of the incoming channel of // the next hop. fee = computeFee(amtToForward, nextHop.Channel.ChannelEdgePolicy) } // Now we create the hop struct for the current hop. currentHop := &Hop{ Channel: edge, AmtToForward: amtToForward, Fee: fee, } // Accumulate all fees. route.TotalFees += currentHop.Fee // Invalidate this route if its total fees exceed our fee limit. if route.TotalFees > feeLimit { err := fmt.Sprintf("total route fees exceeded fee "+ "limit of %v", feeLimit) return nil, newErrf(ErrFeeLimitExceeded, err) } // As a sanity check, we ensure that the incoming channel has // enough capacity to carry the required amount which // includes the fee dictated at each hop. Make the comparison // in msat to prevent rounding errors. if currentHop.AmtToForward + fee > lnwire.NewMSatFromSatoshis( currentHop.Channel.Capacity) { err := fmt.Sprintf("channel graph has insufficient "+ "capacity for the payment: need %v, have %v", currentHop.AmtToForward.ToSatoshis(), currentHop.Channel.Capacity) return nil, newErrf(ErrInsufficientCapacity, err) } // If this is the last hop, then for verification purposes, the // value of the outgoing time-lock should be _exactly_ the // absolute time out they'd expect in the HTLC. if i == len(pathEdges)-1 { // As this is the last hop, we'll use the specified // final CLTV delta value instead of the value from the // last link in the route. route.TotalTimeLock += uint32(finalCLTVDelta) currentHop.OutgoingTimeLock = currentHeight + uint32(finalCLTVDelta) } else { // Next, increment the total timelock of the entire // route such that each hops time lock increases as we // walk backwards in the route, using the delta of the // previous hop. delta := uint32(pathEdges[i+1].TimeLockDelta) route.TotalTimeLock += delta // Otherwise, the value of the outgoing time-lock will // be the value of the time-lock for the _outgoing_ // HTLC, so we factor in their specified grace period // (time lock delta). currentHop.OutgoingTimeLock = route.TotalTimeLock - delta } route.Hops[i] = currentHop } // We'll then make a second run through our route in order to set up // our prev hop mapping. for _, hop := range route.Hops { vertex := Vertex(hop.Channel.Node.PubKeyBytes) route.prevHopMap[vertex] = hop.Channel } // The total amount required for this route will be the value // that the first hop needs to forward plus the fee that // the first hop charges for this. Note that the sender of the // payment is not a hop in the route. route.TotalAmount = route.Hops[0].AmtToForward + route.Hops[0].Fee return route, nil } // Vertex is a simple alias for the serialization of a compressed Bitcoin // public key. type Vertex [33]byte // NewVertex returns a new Vertex given a public key. func NewVertex(pub *btcec.PublicKey) Vertex { var v Vertex copy(v[:], pub.SerializeCompressed()) return v } // String returns a human readable version of the Vertex which is the // hex-encoding of the serialized compressed public key. func (v Vertex) String() string { return fmt.Sprintf("%x", v[:]) } // edgeWithPrev is a helper struct used in path finding that couples an // directional edge with the node's ID in the opposite direction. type edgeWithPrev struct { edge *ChannelHop prevNode [33]byte } // edgeWeight computes the weight of an edge. This value is used when searching // for the shortest path within the channel graph between two nodes. Weight is // is the fee itself plus a time lock penalty added to it. This benefits // channels with shorter time lock deltas and shorter (hops) routes in general. // RiskFactor controls the influence of time lock on route selection. This is // currently a fixed value, but might be configurable in the future. func edgeWeight(amt lnwire.MilliSatoshi, e *channeldb.ChannelEdgePolicy) int64 { // First, we'll compute the "pure" fee through this hop. We say pure, // as this may not be what's ultimately paid as fees are properly // calculated backwards, while we're going in the reverse direction. pureFee := int64(computeFee(amt, e)) // timeLockPenalty is the penalty for the time lock delta of this channel. // It is controlled by RiskFactorBillionths and scales proportional // to the amount that will pass through channel. Rationale is that it if // a twice as large amount gets locked up, it is twice as bad. timeLockPenalty := int64(amt) * int64(e.TimeLockDelta) * RiskFactorBillionths / 1000000000 return pureFee + timeLockPenalty } // findPath attempts to find a path from the source node within the // ChannelGraph to the target node that's capable of supporting a payment of // `amt` value. The current approach implemented is modified version of // Dijkstra's algorithm to find a single shortest path between the source node // and the destination. The distance metric used for edges is related to the // time-lock+fee costs along a particular edge. If a path is found, this // function returns a slice of ChannelHop structs which encoded the chosen path // from the target to the source. func findPath(tx *bolt.Tx, graph *channeldb.ChannelGraph, additionalEdges map[Vertex][]*channeldb.ChannelEdgePolicy, sourceNode *channeldb.LightningNode, target *btcec.PublicKey, ignoredNodes map[Vertex]struct{}, ignoredEdges map[uint64]struct{}, amt lnwire.MilliSatoshi, bandwidthHints map[uint64]lnwire.MilliSatoshi) ([]*ChannelHop, error) { var err error if tx == nil { tx, err = graph.Database().Begin(false) if err != nil { return nil, err } defer tx.Rollback() } // First we'll initialize an empty heap which'll help us to quickly // locate the next edge we should visit next during our graph // traversal. var nodeHeap distanceHeap // For each node in the graph, we create an entry in the distance // map for the node set with a distance of "infinity". distance := make(map[Vertex]nodeWithDist) if err := graph.ForEachNode(tx, func(_ *bolt.Tx, node *channeldb.LightningNode) error { // TODO(roasbeef): with larger graph can just use disk seeks // with a visited map distance[Vertex(node.PubKeyBytes)] = nodeWithDist{ dist: infinity, node: node, } return nil }); err != nil { return nil, err } // We'll also include all the nodes found within the additional edges // that are not known to us yet in the distance map. for vertex := range additionalEdges { node := &channeldb.LightningNode{PubKeyBytes: vertex} distance[vertex] = nodeWithDist{ dist: infinity, node: node, } } // We can't always assume that the end destination is publicly // advertised to the network and included in the graph.ForEachNode call // above, so we'll manually include the target node. targetVertex := NewVertex(target) targetNode := &channeldb.LightningNode{PubKeyBytes: targetVertex} distance[targetVertex] = nodeWithDist{ dist: infinity, node: targetNode, } // We'll use this map as a series of "previous" hop pointers. So to get // to `Vertex` we'll take the edge that it's mapped to within `prev`. prev := make(map[Vertex]edgeWithPrev) // processEdge is a helper closure that will be used to make sure edges // satisfy our specific requirements. processEdge := func(edge *channeldb.ChannelEdgePolicy, bandwidth lnwire.MilliSatoshi, pivot Vertex) { v := Vertex(edge.Node.PubKeyBytes) // If the edge is currently disabled, then we'll stop here, as // we shouldn't attempt to route through it. edgeFlags := lnwire.ChanUpdateFlag(edge.Flags) if edgeFlags&lnwire.ChanUpdateDisabled != 0 { return } // If this vertex or edge has been black listed, then we'll skip // exploring this edge. if _, ok := ignoredNodes[v]; ok { return } if _, ok := ignoredEdges[edge.ChannelID]; ok { return } // Compute the tentative distance to this new channel/edge which // is the distance to our pivot node plus the weight of this // edge. tempDist := distance[pivot].dist + edgeWeight(amt, edge) // If this new tentative distance is better than the current // best known distance to this node, then we record the new // better distance, and also populate our "next hop" map with // this edge. We'll also shave off irrelevant edges by adding // the sufficient capacity of an edge and clearing their // min-htlc amount to our relaxation condition. if tempDist < distance[v].dist && bandwidth >= amt && amt >= edge.MinHTLC && edge.TimeLockDelta != 0 { distance[v] = nodeWithDist{ dist: tempDist, node: edge.Node, } prev[v] = edgeWithPrev{ edge: &ChannelHop{ ChannelEdgePolicy: edge, Capacity: bandwidth.ToSatoshis(), }, prevNode: pivot, } // Add this new node to our heap as we'd like to further // explore down this edge. heap.Push(&nodeHeap, distance[v]) } } // TODO(roasbeef): also add path caching // * similar to route caching, but doesn't factor in the amount // To start, we add the source of our path finding attempt to the // distance map with a distance of 0. This indicates our starting // point in the graph traversal. sourceVertex := Vertex(sourceNode.PubKeyBytes) distance[sourceVertex] = nodeWithDist{ dist: 0, node: sourceNode, } // To start, our source node will the sole item within our distance // heap. heap.Push(&nodeHeap, distance[sourceVertex]) for nodeHeap.Len() != 0 { // Fetch the node within the smallest distance from our source // from the heap. partialPath := heap.Pop(&nodeHeap).(nodeWithDist) bestNode := partialPath.node // If we've reached our target (or we don't have any outgoing // edges), then we're done here and can exit the graph // traversal early. if bytes.Equal(bestNode.PubKeyBytes[:], targetVertex[:]) { break } // Now that we've found the next potential step to take we'll // examine all the outgoing edge (channels) from this node to // further our graph traversal. pivot := Vertex(bestNode.PubKeyBytes) err := bestNode.ForEachChannel(tx, func(tx *bolt.Tx, edgeInfo *channeldb.ChannelEdgeInfo, outEdge, _ *channeldb.ChannelEdgePolicy) error { // We'll query the lower layer to see if we can obtain // any more up to date information concerning the // bandwidth of this edge. edgeBandwidth, ok := bandwidthHints[edgeInfo.ChannelID] if !ok { // If we don't have a hint for this edge, then // we'll just use the known Capacity as the // available bandwidth. edgeBandwidth = lnwire.NewMSatFromSatoshis( edgeInfo.Capacity, ) } processEdge(outEdge, edgeBandwidth, pivot) // TODO(roasbeef): return min HTLC as error in end? return nil }) if err != nil { return nil, err } // Then, we'll examine all the additional edges from the node // we're currently visiting. Since we don't know the capacity // of the private channel, we'll assume it was selected as a // routing hint due to having enough capacity for the payment // and use the payment amount as its capacity. for _, edge := range additionalEdges[bestNode.PubKeyBytes] { processEdge(edge, amt, pivot) } } // If the target node isn't found in the prev hop map, then a path // doesn't exist, so we terminate in an error. if _, ok := prev[NewVertex(target)]; !ok { return nil, newErrf(ErrNoPathFound, "unable to find a path to "+ "destination") } // If the potential route if below the max hop limit, then we'll use // the prevHop map to unravel the path. We end up with a list of edges // in the reverse direction which we'll use to properly calculate the // timelock and fee values. pathEdges := make([]*ChannelHop, 0, len(prev)) prevNode := NewVertex(target) for prevNode != sourceVertex { // TODO(roasbeef): assumes no cycles // Add the current hop to the limit of path edges then walk // backwards from this hop via the prev pointer for this hop // within the prevHop map. pathEdges = append(pathEdges, prev[prevNode].edge) prevNode = Vertex(prev[prevNode].prevNode) } // The route is invalid if it spans more than 20 hops. The current // Sphinx (onion routing) implementation can only encode up to 20 hops // as the entire packet is fixed size. If this route is more than 20 // hops, then it's invalid. numEdges := len(pathEdges) if numEdges > HopLimit { return nil, newErr(ErrMaxHopsExceeded, "potential path has "+ "too many hops") } // As our traversal of the prev map above walked backwards from the // target to the source in the route, we need to reverse it before // returning the final route. for i := 0; i < numEdges/2; i++ { pathEdges[i], pathEdges[numEdges-i-1] = pathEdges[numEdges-i-1], pathEdges[i] } return pathEdges, nil } // findPaths implements a k-shortest paths algorithm to find all the reachable // paths between the passed source and target. The algorithm will continue to // traverse the graph until all possible candidate paths have been depleted. // This function implements a modified version of Yen's. To find each path // itself, we utilize our modified version of Dijkstra's found above. When // examining possible spur and root paths, rather than removing edges or // Vertexes from the graph, we instead utilize a Vertex+edge black-list that // will be ignored by our modified Dijkstra's algorithm. With this approach, we // make our inner path finding algorithm aware of our k-shortest paths // algorithm, rather than attempting to use an unmodified path finding // algorithm in a block box manner. func findPaths(tx *bolt.Tx, graph *channeldb.ChannelGraph, source *channeldb.LightningNode, target *btcec.PublicKey, amt lnwire.MilliSatoshi, numPaths uint32, bandwidthHints map[uint64]lnwire.MilliSatoshi) ([][]*ChannelHop, error) { ignoredEdges := make(map[uint64]struct{}) ignoredVertexes := make(map[Vertex]struct{}) // TODO(roasbeef): modifying ordering within heap to eliminate final // sorting step? var ( shortestPaths [][]*ChannelHop candidatePaths pathHeap ) // First we'll find a single shortest path from the source (our // selfNode) to the target destination that's capable of carrying amt // satoshis along the path before fees are calculated. startingPath, err := findPath( tx, graph, nil, source, target, ignoredVertexes, ignoredEdges, amt, bandwidthHints, ) if err != nil { log.Errorf("Unable to find path: %v", err) return nil, err } // Manually insert a "self" edge emanating from ourselves. This // self-edge is required in order for the path finding algorithm to // function properly. firstPath := make([]*ChannelHop, 0, len(startingPath)+1) firstPath = append(firstPath, &ChannelHop{ ChannelEdgePolicy: &channeldb.ChannelEdgePolicy{ Node: source, }, }) firstPath = append(firstPath, startingPath...) shortestPaths = append(shortestPaths, firstPath) // While we still have candidate paths to explore we'll keep exploring // the sub-graphs created to find the next k-th shortest path. for k := uint32(1); k < numPaths; k++ { prevShortest := shortestPaths[k-1] // We'll examine each edge in the previous iteration's shortest // path in order to find path deviations from each node in the // path. for i := 0; i < len(prevShortest)-1; i++ { // These two maps will mark the edges and Vertexes // we'll exclude from the next path finding attempt. // These are required to ensure the paths are unique // and loopless. ignoredEdges = make(map[uint64]struct{}) ignoredVertexes = make(map[Vertex]struct{}) // Our spur node is the i-th node in the prior shortest // path, and our root path will be all nodes in the // path leading up to our spurNode. spurNode := prevShortest[i].Node rootPath := prevShortest[:i+1] // Before we kickoff our next path finding iteration, // we'll find all the edges we need to ignore in this // next round. This ensures that we create a new unique // path. for _, path := range shortestPaths { // If our current rootPath is a prefix of this // shortest path, then we'll remove the edge // directly _after_ our spur node from the // graph so we don't repeat paths. if len(path) > i+1 && isSamePath(rootPath, path[:i+1]) { ignoredEdges[path[i+1].ChannelID] = struct{}{} } } // Next we'll remove all entries in the root path that // aren't the current spur node from the graph. This // ensures we don't create a path with loops. for _, hop := range rootPath { node := hop.Node.PubKeyBytes if node == spurNode.PubKeyBytes { continue } ignoredVertexes[Vertex(node)] = struct{}{} } // With the edges that are part of our root path, and // the Vertexes (other than the spur path) within the // root path removed, we'll attempt to find another // shortest path from the spur node to the destination. spurPath, err := findPath( tx, graph, nil, spurNode, target, ignoredVertexes, ignoredEdges, amt, bandwidthHints, ) // If we weren't able to find a path, we'll continue to // the next round. if IsError(err, ErrNoPathFound) { continue } else if err != nil { return nil, err } // Create the new combined path by concatenating the // rootPath to the spurPath. newPathLen := len(rootPath) + len(spurPath) newPath := path{ hops: make([]*ChannelHop, 0, newPathLen), dist: newPathLen, } newPath.hops = append(newPath.hops, rootPath...) newPath.hops = append(newPath.hops, spurPath...) // TODO(roasbeef): add and consult path finger print // We'll now add this newPath to the heap of candidate // shortest paths. heap.Push(&candidatePaths, newPath) } // If our min-heap of candidate paths is empty, then we can // exit early. if candidatePaths.Len() == 0 { break } // To conclude this latest iteration, we'll take the shortest // path in our set of candidate paths and add it to our // shortestPaths list as the *next* shortest path. nextShortestPath := heap.Pop(&candidatePaths).(path).hops shortestPaths = append(shortestPaths, nextShortestPath) } return shortestPaths, nil }