lnd/sweep/aggregator.go

package sweep

import (
	"sort"

	"github.com/btcsuite/btcd/btcutil"
	"github.com/btcsuite/btcd/wire"
	"github.com/lightningnetwork/lnd/lnwallet"
	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
)

const (
	// DefaultFeeRateBucketSize is the default size of fee rate buckets
	// we'll use when clustering inputs into buckets with similar fee rates
	// within the SimpleAggregator.
	//
	// Given a minimum relay fee rate of 1 sat/vbyte, a multiplier of 10
	// would result in the following fee rate buckets up to the maximum fee
	// rate:
	//
	//   #1: min = 1 sat/vbyte, max = 10 sat/vbyte
	//   #2: min = 11 sat/vbyte, max = 20 sat/vbyte...
	DefaultFeeRateBucketSize = 10
)

// inputCluster is a helper struct to gather a set of pending inputs that
// should be swept with the specified fee rate.
type inputCluster struct {
	lockTime     *uint32
	sweepFeeRate chainfee.SatPerKWeight
	inputs       InputsMap
}

// createInputSets goes through the cluster's inputs and constructs sets of
// inputs that can be used to generate a sweeping transaction. Each set
// contains up to the configured maximum number of inputs. Negative yield
// inputs are skipped.  No input sets with a total value after fees below the
// dust limit are returned.
func (c *inputCluster) createInputSets(maxFeeRate chainfee.SatPerKWeight,
	maxInputs uint32) []InputSet {

	// Turn the inputs into a slice so we can sort them.
	inputList := make([]*SweeperInput, 0, len(c.inputs))
	for _, input := range c.inputs {
		inputList = append(inputList, input)
	}

	// Yield is calculated as the difference between value and added fee
	// for this input. The fee calculation excludes fee components that are
	// common to all inputs, as those wouldn't influence the order. The
	// single component that is differentiating is witness size.
	//
	// For witness size, the upper limit is taken. The actual size depends
	// on the signature length, which is not known yet at this point.
	calcYield := func(input *SweeperInput) int64 {
		size, _, err := input.WitnessType().SizeUpperBound()
		if err != nil {
			log.Errorf("Failed to get input weight: %v", err)

			return 0
		}

		yield := input.SignDesc().Output.Value -
			int64(c.sweepFeeRate.FeeForWeight(int64(size)))

		return yield
	}

	// Sort input by yield. We will start constructing input sets starting
	// with the highest yield inputs. This is to prevent the construction
	// of a set with an output below the dust limit, causing the sweep
	// process to stop, while there are still higher value inputs
	// available. It also allows us to stop evaluating more inputs when the
	// first input in this ordering is encountered with a negative yield.
	sort.Slice(inputList, func(i, j int) bool {
		// Because of the specific ordering and termination condition
		// that is described above, we place force sweeps at the start
		// of the list. Otherwise we can't be sure that they will be
		// included in an input set.
		if inputList[i].parameters().Immediate {
			return true
		}

		return calcYield(inputList[i]) > calcYield(inputList[j])
	})

	// Select blocks of inputs up to the configured maximum number.
	var sets []InputSet
	for len(inputList) > 0 {
		// Start building a set of positive-yield tx inputs under the
		// condition that the tx will be published with the specified
		// fee rate.
		txInputs := newTxInputSet(c.sweepFeeRate, maxFeeRate, maxInputs)

		// From the set of sweepable inputs, keep adding inputs to the
		// input set until the tx output value no longer goes up or the
		// maximum number of inputs is reached.
		txInputs.addPositiveYieldInputs(inputList)

		// If there are no positive yield inputs, we can stop here.
		inputCount := len(txInputs.inputs)
		if inputCount == 0 {
			return sets
		}

		log.Infof("Candidate sweep set of size=%v (+%v wallet inputs),"+
			" has yield=%v, weight=%v",
			inputCount, len(txInputs.inputs)-inputCount,
			txInputs.totalOutput()-txInputs.walletInputTotal,
			txInputs.weightEstimate(true).weight())

		sets = append(sets, txInputs)
		inputList = inputList[inputCount:]
	}

	return sets
}

// UtxoAggregator defines an interface that takes a list of inputs and
// aggregate them into groups. Each group is used as the inputs to create a
// sweeping transaction.
type UtxoAggregator interface {
	// ClusterInputs takes a list of inputs and groups them into input
	// sets. Each input set will be used to create a sweeping transaction.
	ClusterInputs(inputs InputsMap) []InputSet
}

// SimpleAggregator aggregates inputs known by the Sweeper based on each
// input's locktime and feerate.
type SimpleAggregator struct {
	// FeeEstimator is used when crafting sweep transactions to estimate
	// the necessary fee relative to the expected size of the sweep
	// transaction.
	FeeEstimator chainfee.Estimator

	// MaxFeeRate is the maximum fee rate allowed within the
	// SimpleAggregator.
	MaxFeeRate chainfee.SatPerKWeight

	// MaxInputsPerTx specifies the default maximum number of inputs allowed
	// in a single sweep tx. If more need to be swept, multiple txes are
	// created and published.
	MaxInputsPerTx uint32

	// FeeRateBucketSize is the default size of fee rate buckets we'll use
	// when clustering inputs into buckets with similar fee rates within
	// the SimpleAggregator.
	//
	// Given a minimum relay fee rate of 1 sat/vbyte, a fee rate bucket
	// size of 10 would result in the following fee rate buckets up to the
	// maximum fee rate:
	//
	//   #1: min = 1 sat/vbyte, max (exclusive) = 11 sat/vbyte
	//   #2: min = 11 sat/vbyte, max (exclusive) = 21 sat/vbyte...
	FeeRateBucketSize int
}

// Compile-time constraint to ensure SimpleAggregator implements UtxoAggregator.
var _ UtxoAggregator = (*SimpleAggregator)(nil)

// NewSimpleUtxoAggregator creates a new instance of a SimpleAggregator.
func NewSimpleUtxoAggregator(estimator chainfee.Estimator,
	max chainfee.SatPerKWeight, maxTx uint32) *SimpleAggregator {

	return &SimpleAggregator{
		FeeEstimator:      estimator,
		MaxFeeRate:        max,
		MaxInputsPerTx:    maxTx,
		FeeRateBucketSize: DefaultFeeRateBucketSize,
	}
}

// ClusterInputs creates a list of input clusters from the set of pending
// inputs known by the UtxoSweeper. It clusters inputs by
// 1) Required tx locktime
// 2) Similar fee rates.
func (s *SimpleAggregator) ClusterInputs(inputs InputsMap) []InputSet {
	// We start by getting the inputs clusters by locktime. Since the
	// inputs commit to the locktime, they can only be clustered together
	// if the locktime is equal.
	lockTimeClusters, nonLockTimeInputs := s.clusterByLockTime(inputs)

	// Cluster the remaining inputs by sweep fee rate.
	feeClusters := s.clusterBySweepFeeRate(nonLockTimeInputs)

	// Since the inputs that we clustered by fee rate don't commit to a
	// specific locktime, we can try to merge a locktime cluster with a fee
	// cluster.
	clusters := zipClusters(lockTimeClusters, feeClusters)

	sort.Slice(clusters, func(i, j int) bool {
		return clusters[i].sweepFeeRate >
			clusters[j].sweepFeeRate
	})

	// Now that we have the clusters, we can create the input sets.
	var inputSets []InputSet
	for _, cluster := range clusters {
		sets := cluster.createInputSets(
			s.MaxFeeRate, s.MaxInputsPerTx,
		)
		inputSets = append(inputSets, sets...)
	}

	return inputSets
}

// clusterByLockTime takes the given set of pending inputs and clusters those
// with equal locktime together. Each cluster contains a sweep fee rate, which
// is determined by calculating the average fee rate of all inputs within that
// cluster. In addition to the created clusters, inputs that did not specify a
// required locktime are returned.
func (s *SimpleAggregator) clusterByLockTime(
	inputs InputsMap) ([]inputCluster, InputsMap) {

	locktimes := make(map[uint32]InputsMap)
	rem := make(InputsMap)

	// Go through all inputs and check if they require a certain locktime.
	for op, input := range inputs {
		lt, ok := input.RequiredLockTime()
		if !ok {
			rem[op] = input
			continue
		}

		// Check if we already have inputs with this locktime.
		cluster, ok := locktimes[lt]
		if !ok {
			cluster = make(InputsMap)
		}

		// Get the fee rate based on the fee preference. If an error is
		// returned, we'll skip sweeping this input for this round of
		// cluster creation and retry it when we create the clusters
		// from the pending inputs again.
		feeRate, err := input.params.Fee.Estimate(
			s.FeeEstimator, s.MaxFeeRate,
		)
		if err != nil {
			log.Warnf("Skipping input %v: %v", op, err)
			continue
		}

		log.Debugf("Adding input %v to cluster with locktime=%v, "+
			"feeRate=%v", op, lt, feeRate)

		// Attach the fee rate to the input.
		input.lastFeeRate = feeRate

		// Update the cluster about the updated input.
		cluster[op] = input
		locktimes[lt] = cluster
	}

	// We'll then determine the sweep fee rate for each set of inputs by
	// calculating the average fee rate of the inputs within each set.
	inputClusters := make([]inputCluster, 0, len(locktimes))
	for lt, cluster := range locktimes {
		lt := lt

		var sweepFeeRate chainfee.SatPerKWeight
		for _, input := range cluster {
			sweepFeeRate += input.lastFeeRate
		}

		sweepFeeRate /= chainfee.SatPerKWeight(len(cluster))
		inputClusters = append(inputClusters, inputCluster{
			lockTime:     &lt,
			sweepFeeRate: sweepFeeRate,
			inputs:       cluster,
		})
	}

	return inputClusters, rem
}

// clusterBySweepFeeRate takes the set of pending inputs within the UtxoSweeper
// and clusters those together with similar fee rates. Each cluster contains a
// sweep fee rate, which is determined by calculating the average fee rate of
// all inputs within that cluster.
func (s *SimpleAggregator) clusterBySweepFeeRate(
	inputs InputsMap) []inputCluster {

	bucketInputs := make(map[int]*bucketList)
	inputFeeRates := make(map[wire.OutPoint]chainfee.SatPerKWeight)

	// First, we'll group together all inputs with similar fee rates. This
	// is done by determining the fee rate bucket they should belong in.
	for op, input := range inputs {
		feeRate, err := input.params.Fee.Estimate(
			s.FeeEstimator, s.MaxFeeRate,
		)
		if err != nil {
			log.Warnf("Skipping input %v: %v", op, err)
			continue
		}

		// Only try to sweep inputs with an unconfirmed parent if the
		// current sweep fee rate exceeds the parent tx fee rate. This
		// assumes that such inputs are offered to the sweeper solely
		// for the purpose of anchoring down the parent tx using cpfp.
		parentTx := input.UnconfParent()
		if parentTx != nil {
			parentFeeRate :=
				chainfee.SatPerKWeight(parentTx.Fee*1000) /
					chainfee.SatPerKWeight(parentTx.Weight)

			if parentFeeRate >= feeRate {
				log.Debugf("Skipping cpfp input %v: "+
					"fee_rate=%v, parent_fee_rate=%v", op,
					feeRate, parentFeeRate)

				continue
			}
		}

		feeGroup := s.bucketForFeeRate(feeRate)

		// Create a bucket list for this fee rate if there isn't one
		// yet.
		buckets, ok := bucketInputs[feeGroup]
		if !ok {
			buckets = &bucketList{}
			bucketInputs[feeGroup] = buckets
		}

		// Request the bucket list to add this input. The bucket list
		// will take into account exclusive group constraints.
		buckets.add(input)

		input.lastFeeRate = feeRate
		inputFeeRates[op] = feeRate
	}

	// We'll then determine the sweep fee rate for each set of inputs by
	// calculating the average fee rate of the inputs within each set.
	inputClusters := make([]inputCluster, 0, len(bucketInputs))
	for _, buckets := range bucketInputs {
		for _, inputs := range buckets.buckets {
			var sweepFeeRate chainfee.SatPerKWeight
			for op := range inputs {
				sweepFeeRate += inputFeeRates[op]
			}
			sweepFeeRate /= chainfee.SatPerKWeight(len(inputs))
			inputClusters = append(inputClusters, inputCluster{
				sweepFeeRate: sweepFeeRate,
				inputs:       inputs,
			})
		}
	}

	return inputClusters
}

// bucketForFeeReate determines the proper bucket for a fee rate. This is done
// in order to batch inputs with similar fee rates together.
func (s *SimpleAggregator) bucketForFeeRate(
	feeRate chainfee.SatPerKWeight) int {

	relayFeeRate := s.FeeEstimator.RelayFeePerKW()

	// Create an isolated bucket for sweeps at the minimum fee rate. This
	// is to prevent very small outputs (anchors) from becoming
	// uneconomical if their fee rate would be averaged with higher fee
	// rate inputs in a regular bucket.
	if feeRate == relayFeeRate {
		return 0
	}

	return 1 + int(feeRate-relayFeeRate)/s.FeeRateBucketSize
}

// mergeClusters attempts to merge cluster a and b if they are compatible. The
// new cluster will have the locktime set if a or b had a locktime set, and a
// sweep fee rate that is the maximum of a and b's. If the two clusters are not
// compatible, they will be returned unchanged.
func mergeClusters(a, b inputCluster) []inputCluster {
	newCluster := inputCluster{}

	switch {
	// Incompatible locktimes, return the sets without merging them.
	case a.lockTime != nil && b.lockTime != nil &&
		*a.lockTime != *b.lockTime:

		return []inputCluster{a, b}

	case a.lockTime != nil:
		newCluster.lockTime = a.lockTime

	case b.lockTime != nil:
		newCluster.lockTime = b.lockTime
	}

	if a.sweepFeeRate > b.sweepFeeRate {
		newCluster.sweepFeeRate = a.sweepFeeRate
	} else {
		newCluster.sweepFeeRate = b.sweepFeeRate
	}

	newCluster.inputs = make(InputsMap)

	for op, in := range a.inputs {
		newCluster.inputs[op] = in
	}

	for op, in := range b.inputs {
		newCluster.inputs[op] = in
	}

	return []inputCluster{newCluster}
}

// zipClusters merges pairwise clusters from as and bs such that cluster a from
// as is merged with a cluster from bs that has at least the fee rate of a.
// This to ensure we don't delay confirmation by decreasing the fee rate (the
// lock time inputs are typically second level HTLC transactions, that are time
// sensitive).
func zipClusters(as, bs []inputCluster) []inputCluster {
	// Sort the clusters by decreasing fee rates.
	sort.Slice(as, func(i, j int) bool {
		return as[i].sweepFeeRate >
			as[j].sweepFeeRate
	})
	sort.Slice(bs, func(i, j int) bool {
		return bs[i].sweepFeeRate >
			bs[j].sweepFeeRate
	})

	var (
		finalClusters []inputCluster
		j             int
	)

	// Go through each cluster in as, and merge with the next one from bs
	// if it has at least the fee rate needed.
	for i := range as {
		a := as[i]

		switch {
		// If the fee rate for the next one from bs is at least a's, we
		// merge.
		case j < len(bs) && bs[j].sweepFeeRate >= a.sweepFeeRate:
			merged := mergeClusters(a, bs[j])
			finalClusters = append(finalClusters, merged...)

			// Increment j for the next round.
			j++

		// We did not merge, meaning all the remaining clusters from bs
		// have lower fee rate. Instead we add a directly to the final
		// clusters.
		default:
			finalClusters = append(finalClusters, a)
		}
	}

	// Add any remaining clusters from bs.
	for ; j < len(bs); j++ {
		b := bs[j]
		finalClusters = append(finalClusters, b)
	}

	return finalClusters
}

// BudgetAggregator is a budget-based aggregator that creates clusters based on
// deadlines and budgets of inputs.
type BudgetAggregator struct {
	// estimator is used when crafting sweep transactions to estimate the
	// necessary fee relative to the expected size of the sweep
	// transaction.
	estimator chainfee.Estimator

	// maxInputs specifies the maximum number of inputs allowed in a single
	// sweep tx.
	maxInputs uint32
}

// Compile-time constraint to ensure BudgetAggregator implements UtxoAggregator.
var _ UtxoAggregator = (*BudgetAggregator)(nil)

// NewBudgetAggregator creates a new instance of a BudgetAggregator.
func NewBudgetAggregator(estimator chainfee.Estimator,
	maxInputs uint32) *BudgetAggregator {

	return &BudgetAggregator{
		estimator: estimator,
		maxInputs: maxInputs,
	}
}

// clusterGroup defines an alias for a set of inputs that are to be grouped.
type clusterGroup map[int32][]SweeperInput

// ClusterInputs creates a list of input sets from pending inputs.
// 1. filter out inputs whose budget cannot cover min relay fee.
// 2. filter a list of exclusive inputs.
// 3. group the inputs into clusters based on their deadline height.
// 4. sort the inputs in each cluster by their budget.
// 5. optionally split a cluster if it exceeds the max input limit.
// 6. create input sets from each of the clusters.
// 7. create input sets for each of the exclusive inputs.
func (b *BudgetAggregator) ClusterInputs(inputs InputsMap) []InputSet {
	// Filter out inputs that have a budget below min relay fee.
	filteredInputs := b.filterInputs(inputs)

	// Create clusters to group inputs based on their deadline height.
	clusters := make(clusterGroup, len(filteredInputs))

	// exclusiveInputs is a set of inputs that are not to be included in
	// any cluster. These inputs can only be swept independently as there's
	// no guarantee which input will be confirmed first, which means
	// grouping exclusive inputs may jeopardize non-exclusive inputs.
	exclusiveInputs := make(map[wire.OutPoint]clusterGroup)

	// Iterate all the inputs and group them based on their specified
	// deadline heights.
	for _, input := range filteredInputs {
		// Get deadline height, and use the specified default deadline
		// height if it's not set.
		height := input.DeadlineHeight

		// Put exclusive inputs in their own set.
		if input.params.ExclusiveGroup != nil {
			log.Tracef("Input %v is exclusive", input.OutPoint())
			exclusiveInputs[input.OutPoint()] = clusterGroup{
				height: []SweeperInput{*input},
			}

			continue
		}

		cluster, ok := clusters[height]
		if !ok {
			cluster = make([]SweeperInput, 0)
		}

		cluster = append(cluster, *input)
		clusters[height] = cluster
	}

	// Now that we have the clusters, we can create the input sets.
	//
	// NOTE: cannot pre-allocate the slice since we don't know the number
	// of input sets in advance.
	inputSets := make([]InputSet, 0)
	for height, cluster := range clusters {
		// Sort the inputs by their economical value.
		sortedInputs := b.sortInputs(cluster)

		// Split on locktimes if they are different.
		splitClusters := splitOnLocktime(sortedInputs)

		// Create input sets from the cluster.
		for _, cluster := range splitClusters {
			sets := b.createInputSets(cluster, height)
			inputSets = append(inputSets, sets...)
		}
	}

	// Create input sets from the exclusive inputs.
	for _, cluster := range exclusiveInputs {
		for height, input := range cluster {
			sets := b.createInputSets(input, height)
			inputSets = append(inputSets, sets...)
		}
	}

	return inputSets
}

// createInputSet takes a set of inputs which share the same deadline height
// and turns them into a list of `InputSet`, each set is then used to create a
// sweep transaction.
//
// TODO(yy): by the time we call this method, all the invalid/uneconomical
// inputs have been filtered out, all the inputs have been sorted based on
// their budgets, and we are about to create input sets. The only thing missing
// here is, we need to group the inputs here even further based on whether
// their budgets can cover the starting fee rate used for this input set.
func (b *BudgetAggregator) createInputSets(inputs []SweeperInput,
	deadlineHeight int32) []InputSet {

	// sets holds the InputSets that we will return.
	sets := make([]InputSet, 0)

	// Copy the inputs to a new slice so we can modify it.
	remainingInputs := make([]SweeperInput, len(inputs))
	copy(remainingInputs, inputs)

	// If the number of inputs is greater than the max inputs allowed, we
	// will split them into smaller clusters.
	for uint32(len(remainingInputs)) > b.maxInputs {
		log.Tracef("Cluster has %v inputs, max is %v, dividing...",
			len(inputs), b.maxInputs)

		// Copy the inputs to be put into the new set, and update the
		// remaining inputs by removing currentInputs.
		currentInputs := make([]SweeperInput, b.maxInputs)
		copy(currentInputs, remainingInputs[:b.maxInputs])
		remainingInputs = remainingInputs[b.maxInputs:]

		// Create an InputSet using the max allowed number of inputs.
		set, err := NewBudgetInputSet(
			currentInputs, deadlineHeight,
		)
		if err != nil {
			log.Errorf("unable to create input set: %v", err)

			continue
		}

		sets = append(sets, set)
	}

	// Create an InputSet from the remaining inputs.
	if len(remainingInputs) > 0 {
		set, err := NewBudgetInputSet(
			remainingInputs, deadlineHeight,
		)
		if err != nil {
			log.Errorf("unable to create input set: %v", err)
			return nil
		}

		sets = append(sets, set)
	}

	return sets
}

// filterInputs filters out inputs that have,
// - a budget below the min relay fee.
// - a budget below its requested starting fee.
// - a required output that's below the dust.
func (b *BudgetAggregator) filterInputs(inputs InputsMap) InputsMap {
	// Get the current min relay fee for this round.
	minFeeRate := b.estimator.RelayFeePerKW()

	// filterInputs stores a map of inputs that has a budget that at least
	// can pay the minimal fee.
	filteredInputs := make(InputsMap, len(inputs))

	// Iterate all the inputs and filter out the ones whose budget cannot
	// cover the min fee.
	for _, pi := range inputs {
		op := pi.OutPoint()

		// Get the size and skip if there's an error.
		size, _, err := pi.WitnessType().SizeUpperBound()
		if err != nil {
			log.Warnf("Skipped input=%v: cannot get its size: %v",
				op, err)

			continue
		}

		// Skip inputs that has too little budget.
		minFee := minFeeRate.FeeForWeight(int64(size))
		if pi.params.Budget < minFee {
			log.Warnf("Skipped input=%v: has budget=%v, but the "+
				"min fee requires %v", op, pi.params.Budget,
				minFee)

			continue
		}

		// Skip inputs that has cannot cover its starting fees.
		startingFeeRate := pi.params.StartingFeeRate.UnwrapOr(
			chainfee.SatPerKWeight(0),
		)
		startingFee := startingFeeRate.FeeForWeight(int64(size))
		if pi.params.Budget < startingFee {
			log.Errorf("Skipped input=%v: has budget=%v, but the "+
				"starting fee requires %v", op,
				pi.params.Budget, minFee)

			continue
		}

		// If the input comes with a required tx out that is below
		// dust, we won't add it.
		//
		// NOTE: only HtlcSecondLevelAnchorInput returns non-nil
		// RequiredTxOut.
		reqOut := pi.RequiredTxOut()
		if reqOut != nil {
			if isDustOutput(reqOut) {
				log.Errorf("Rejected input=%v due to dust "+
					"required output=%v", op, reqOut.Value)

				continue
			}
		}

		filteredInputs[op] = pi
	}

	return filteredInputs
}

// sortInputs sorts the inputs based on their economical value.
//
// NOTE: besides the forced inputs, the sorting won't make any difference
// because all the inputs are added to the same set. The exception is when the
// number of inputs exceeds the maxInputs limit, it requires us to split them
// into smaller clusters. In that case, the sorting will make a difference as
// the budgets of the clusters will be different.
func (b *BudgetAggregator) sortInputs(inputs []SweeperInput) []SweeperInput {
	// sortedInputs is the final list of inputs sorted by their economical
	// value.
	sortedInputs := make([]SweeperInput, 0, len(inputs))

	// Copy the inputs.
	sortedInputs = append(sortedInputs, inputs...)

	// Sort the inputs based on their budgets.
	//
	// NOTE: We can implement more sophisticated algorithm as the budget
	// left is a function f(minFeeRate, size) = b1 - s1 * r > b2 - s2 * r,
	// where b1 and b2 are budgets, s1 and s2 are sizes of the inputs.
	sort.Slice(sortedInputs, func(i, j int) bool {
		left := sortedInputs[i].params.Budget
		right := sortedInputs[j].params.Budget

		// Make sure forced inputs are always put in the front.
		leftForce := sortedInputs[i].params.Immediate
		rightForce := sortedInputs[j].params.Immediate

		// If both are forced inputs, we return the one with the higher
		// budget. If neither are forced inputs, we also return the one
		// with the higher budget.
		if leftForce == rightForce {
			return left > right
		}

		// Otherwise, it's either the left or the right is forced. We
		// can simply return `leftForce` here as, if it's true, the
		// left is forced and should be put in the front. Otherwise,
		// the right is forced and should be put in the front.
		return leftForce
	})

	return sortedInputs
}

// splitOnLocktime splits the list of inputs based on their locktime.
//
// TODO(yy): this is a temporary hack as the blocks are not synced among the
// contractcourt and the sweeper.
func splitOnLocktime(inputs []SweeperInput) map[uint32][]SweeperInput {
	result := make(map[uint32][]SweeperInput)
	noLocktimeInputs := make([]SweeperInput, 0, len(inputs))

	// mergeLocktime is the locktime that we use to merge all the
	// nolocktime inputs into.
	var mergeLocktime uint32

	// Iterate all inputs and split them based on their locktimes.
	for _, inp := range inputs {
		locktime, required := inp.RequiredLockTime()
		if !required {
			log.Tracef("No locktime required for input=%v",
				inp.OutPoint())

			noLocktimeInputs = append(noLocktimeInputs, inp)

			continue
		}

		log.Tracef("Split input=%v on locktime=%v", inp.OutPoint(),
			locktime)

		// Get the slice - the slice will be initialized if not found.
		inputList := result[locktime]

		// Add the input to the list.
		inputList = append(inputList, inp)

		// Update the map.
		result[locktime] = inputList

		// Update the merge locktime.
		mergeLocktime = locktime
	}

	// If there are locktime inputs, we will merge the no locktime inputs
	// to the last locktime group found.
	if len(result) > 0 {
		log.Tracef("No locktime inputs has been merged to locktime=%v",
			mergeLocktime)
		result[mergeLocktime] = append(
			result[mergeLocktime], noLocktimeInputs...,
		)
	} else {
		// Otherwise just return the no locktime inputs.
		result[mergeLocktime] = noLocktimeInputs
	}

	return result
}

// isDustOutput checks if the given output is considered as dust.
func isDustOutput(output *wire.TxOut) bool {
	// Fetch the dust limit for this output.
	dustLimit := lnwallet.DustLimitForSize(len(output.PkScript))

	// If the output is below the dust limit, we consider it dust.
	return btcutil.Amount(output.Value) < dustLimit
}