routing: add ToHopPayloads method to routing.Route

This commit adds a new method to the routing.Route struct:
ToHopPayloads. This function will converts a complete route into the
series of per-hop payloads that is to be encoded within each HTLC using
an opaque Sphinx packet.

We can now use this function when creating the sphinx packet to
properly encoded the hop payload for each hop in the route.

routing/pathfind.go

@@ -1,11 +1,13 @@
 package routing
 
 import (
+	"encoding/binary"
 	"math"
 
 	"container/heap"
 
 	"github.com/boltdb/bolt"
+	"github.com/lightningnetwork/lightning-onion"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/roasbeef/btcd/btcec"
 	"github.com/roasbeef/btcd/chaincfg/chainhash"
@@ -121,6 +123,40 @@ type Route struct {
 	Hops []*Hop
 }
 
+// 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
+}
+
 // sortableRoutes is a slice of routes that can be sorted. Routes are typically
 // sorted according to their total cumulative fee within the route. In the case
 // that two routes require and identical amount of fees, then the total