loop/sweepbatcher/greedy_batch_selection.go

package sweepbatcher

import (
	"context"
	"errors"
	"fmt"
	"sort"

	"github.com/btcsuite/btcd/btcutil"
	"github.com/btcsuite/btcd/txscript"
	sweeppkg "github.com/lightninglabs/loop/sweep"
	"github.com/lightningnetwork/lnd/input"
	"github.com/lightningnetwork/lnd/lntypes"
	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
)

// greedyAddSweep selects a batch for the sweep using the greedy algorithm,
// which minimizes costs, and adds the sweep to the batch. To accomplish this,
// it first collects fee details about the sweep being added, about a potential
// new batch composed of this sweep only, and about all existing batches. It
// skips batches with at least MaxSweepsPerBatch swaps to keep tx standard. Then
// it passes the data to selectBatches() function, which emulates adding the
// sweep to each batch and creating new batch for the sweep, and calculates the
// costs of each alternative. Based on the estimates of selectBatches(), this
// method adds the sweep to the batch that results in the least overall fee
// increase, or creates new batch for it. If the sweep is not accepted by an
// existing batch (may happen because of too distant timeouts), next batch is
// tried in the list returned by selectBatches(). If adding fails or new batch
// creation fails, this method returns an error. If this method fails for any
// reason, the caller falls back to the simple algorithm (method handleSweep).
func (b *Batcher) greedyAddSweep(ctx context.Context, sweep *sweep) error {
	// Collect weight and fee rate info about the sweep and new batch.
	sweepFeeDetails, newBatchFeeDetails, err := estimateSweepFeeIncrement(
		sweep,
	)
	if err != nil {
		return fmt.Errorf("failed to estimate tx weight for "+
			"sweep %x: %w", sweep.swapHash[:6], err)
	}

	// Collect weight and fee rate info about existing batches.
	batches := make([]feeDetails, 0, len(b.batches))
	for _, existingBatch := range b.batches {
		// Enforce MaxSweepsPerBatch. If there are already too many
		// sweeps in the batch, do not add another sweep to prevent the
		// tx from becoming non-standard.
		if len(existingBatch.sweeps) >= MaxSweepsPerBatch {
			continue
		}

		batchFeeDetails, err := estimateBatchWeight(existingBatch)
		if err != nil {
			return fmt.Errorf("failed to estimate tx weight for "+
				"batch %d: %w", existingBatch.id, err)
		}
		batches = append(batches, batchFeeDetails)
	}

	// Run the algorithm. Get batchId of possible batches, sorted from best
	// to worst.
	batchesIds, err := selectBatches(
		batches, sweepFeeDetails, newBatchFeeDetails, b.mixedBatch,
	)
	if err != nil {
		return fmt.Errorf("batch selection algorithm failed for sweep "+
			"%x: %w", sweep.swapHash[:6], err)
	}

	// Try batches, starting with the best.
	for _, batchId := range batchesIds {
		// If the best option is to start new batch, do it.
		if batchId == newBatchSignal {
			return b.spinUpNewBatch(ctx, sweep)
		}

		// Locate the batch to add the sweep to.
		bestBatch, has := b.batches[batchId]
		if !has {
			return fmt.Errorf("batch selection algorithm returned "+
				"batch id %d which doesn't exist, for sweep %x",
				batchId, sweep.swapHash[:6])
		}

		// Add the sweep to the batch.
		accepted, err := bestBatch.addSweep(ctx, sweep)
		if err != nil {
			return fmt.Errorf("batch selection algorithm returned "+
				"batch id %d for sweep %x, but adding failed: "+
				"%w", batchId, sweep.swapHash[:6], err)
		}
		if accepted {
			return nil
		}

		log.Debugf("Batch selection algorithm returned batch id %d for"+
			" sweep %x, but acceptance failed.", batchId,
			sweep.swapHash[:6])
	}

	return fmt.Errorf("no batch accepted sweep %x", sweep.swapHash[:6])
}

// estimateSweepFeeIncrement returns fee details for adding the sweep to a batch
// and for creating new batch with this sweep only.
func estimateSweepFeeIncrement(s *sweep) (feeDetails, feeDetails, error) {
	// Create a fake batch with this sweep.
	batch := &batch{
		rbfCache: rbfCache{
			FeeRate: s.minFeeRate,
		},
		sweeps: map[lntypes.Hash]sweep{
			s.swapHash: *s,
		},
	}

	// Estimate new batch.
	fd1, err := estimateBatchWeight(batch)
	if err != nil {
		return feeDetails{}, feeDetails{}, err
	}

	// Add the same sweep again to measure weight increments.
	swapHash2 := s.swapHash
	swapHash2[0]++
	batch.sweeps[swapHash2] = *s

	// Estimate weight of a batch with two sweeps.
	fd2, err := estimateBatchWeight(batch)
	if err != nil {
		return feeDetails{}, feeDetails{}, err
	}

	// Create feeDetails for sweep.
	sweepFeeDetails := feeDetails{
		FeeRate:        s.minFeeRate,
		NonCoopHint:    s.nonCoopHint || s.coopFailed,
		IsExternalAddr: s.isExternalAddr,

		// Calculate sweep weight as a difference.
		MixedWeight:   fd2.MixedWeight - fd1.MixedWeight,
		CoopWeight:    fd2.CoopWeight - fd1.CoopWeight,
		NonCoopWeight: fd2.NonCoopWeight - fd1.NonCoopWeight,
	}

	return sweepFeeDetails, fd1, nil
}

// estimateBatchWeight estimates batch weight and returns its fee details.
func estimateBatchWeight(batch *batch) (feeDetails, error) {
	// Make sure the batch is not empty.
	if len(batch.sweeps) == 0 {
		return feeDetails{}, errors.New("empty batch")
	}

	// Make sure fee rate is valid.
	if batch.rbfCache.FeeRate < chainfee.AbsoluteFeePerKwFloor {
		return feeDetails{}, fmt.Errorf("feeRate is too low: %v",
			batch.rbfCache.FeeRate)
	}

	// Find if the batch has at least one non-cooperative sweep.
	hasNonCoop := false
	for _, sweep := range batch.sweeps {
		if sweep.nonCoopHint || sweep.coopFailed {
			hasNonCoop = true
		}
	}

	// Find some sweep of the batch. It is used if there is just one sweep.
	var theSweep sweep
	for _, sweep := range batch.sweeps {
		theSweep = sweep
		break
	}

	// Find sweep destination address (type) for weight estimations.
	var destAddr btcutil.Address
	if theSweep.isExternalAddr {
		if theSweep.destAddr == nil {
			return feeDetails{}, errors.New("isExternalAddr=true," +
				" but destAddr is nil")
		}
		destAddr = theSweep.destAddr
	} else {
		// Assume it is taproot by default.
		destAddr = (*btcutil.AddressTaproot)(nil)
	}

	// Make three estimators: for mixed, coop and non-coop cases.
	var mixedWeight, coopWeight, nonCoopWeight input.TxWeightEstimator

	// Add output weight to the estimator.
	err := sweeppkg.AddOutputEstimate(&mixedWeight, destAddr)
	if err != nil {
		return feeDetails{}, fmt.Errorf("sweep.AddOutputEstimate: %w",
			err)
	}
	err = sweeppkg.AddOutputEstimate(&coopWeight, destAddr)
	if err != nil {
		return feeDetails{}, fmt.Errorf("sweep.AddOutputEstimate: %w",
			err)
	}
	err = sweeppkg.AddOutputEstimate(&nonCoopWeight, destAddr)
	if err != nil {
		return feeDetails{}, fmt.Errorf("sweep.AddOutputEstimate: %w",
			err)
	}

	// Add inputs.
	for _, sweep := range batch.sweeps {
		if sweep.nonCoopHint || sweep.coopFailed {
			err = sweep.htlcSuccessEstimator(&mixedWeight)
			if err != nil {
				return feeDetails{}, fmt.Errorf(
					"htlcSuccessEstimator failed: %w", err,
				)
			}
		} else {
			mixedWeight.AddTaprootKeySpendInput(
				txscript.SigHashDefault,
			)
		}

		coopWeight.AddTaprootKeySpendInput(txscript.SigHashDefault)

		err = sweep.htlcSuccessEstimator(&nonCoopWeight)
		if err != nil {
			return feeDetails{}, fmt.Errorf("htlcSuccessEstimator "+
				"failed: %w", err)
		}
	}

	return feeDetails{
		BatchId:        batch.id,
		FeeRate:        batch.rbfCache.FeeRate,
		MixedWeight:    mixedWeight.Weight(),
		CoopWeight:     coopWeight.Weight(),
		NonCoopWeight:  nonCoopWeight.Weight(),
		NonCoopHint:    hasNonCoop,
		IsExternalAddr: theSweep.isExternalAddr,
	}, nil
}

// newBatchSignal is the value that indicates a new batch. It is returned by
// selectBatches to encode new batch creation.
const newBatchSignal = -1

// feeDetails is either a batch or a sweep and it holds data important for
// selection of a batch to add the sweep to (or new batch creation).
type feeDetails struct {
	BatchId        int32
	FeeRate        chainfee.SatPerKWeight
	MixedWeight    lntypes.WeightUnit
	CoopWeight     lntypes.WeightUnit
	NonCoopWeight  lntypes.WeightUnit
	NonCoopHint    bool
	IsExternalAddr bool
}

// fee returns fee of onchain transaction representing this instance.
func (e feeDetails) fee(mixedBatch bool) btcutil.Amount {
	var weight lntypes.WeightUnit
	switch {
	case mixedBatch:
		weight = e.MixedWeight
	case e.NonCoopHint:
		weight = e.NonCoopWeight
	default:
		weight = e.CoopWeight
	}

	return e.FeeRate.FeeForWeight(weight)
}

// combine returns new feeDetails, combining properties.
func (e1 feeDetails) combine(e2 feeDetails) feeDetails {
	// The fee rate is max of two fee rates.
	feeRate := e1.FeeRate
	if feeRate < e2.FeeRate {
		feeRate = e2.FeeRate
	}

	return feeDetails{
		FeeRate:        feeRate,
		MixedWeight:    e1.MixedWeight + e2.MixedWeight,
		CoopWeight:     e1.CoopWeight + e2.CoopWeight,
		NonCoopWeight:  e1.NonCoopWeight + e2.NonCoopWeight,
		NonCoopHint:    e1.NonCoopHint || e2.NonCoopHint,
		IsExternalAddr: e1.IsExternalAddr || e2.IsExternalAddr,
	}
}

// selectBatches returns the list of id of batches sorted from best to worst.
// Creation a new batch is encoded as newBatchSignal. For each batch its fee
// rate and a set of weights are provided: weight in case of a mixed batch,
// weight in case of cooperative spending and weight in case non-cooperative
// spending. Also, a hint is provided to signal what spending path will be used
// by the batch.
//
// The same data is also provided for the sweep for which we are selecting a
// batch to add. In case of the sweep weights are weight deltas resulted from
// adding the sweep. Finally, the same data is provided for new batch having
// this sweep only.
//
// The algorithm compares costs of adding the sweep to each existing batch, and
// costs of new batch creation for this sweep and returns BatchId of the winning
// batch. If the best option is to create a new batch, return newBatchSignal.
//
// Each fee details has also IsExternalAddr flag. There is a rule that sweeps
// having flag IsExternalAddr must go in individual batches. Cooperative
// spending is only available if all the sweeps support cooperative spending
// path of in a mixed batch.
func selectBatches(batches []feeDetails, sweep, oneSweepBatch feeDetails,
	mixedBatch bool) ([]int32, error) {

	// If the sweep has IsExternalAddr flag, the sweep can't be added to
	// a batch, so create new batch for it.
	if sweep.IsExternalAddr {
		return []int32{newBatchSignal}, nil
	}

	// alternative holds batch ID and its cost.
	type alternative struct {
		batchId int32
		cost    btcutil.Amount
	}

	// Create the list of possible actions and their costs.
	alternatives := make([]alternative, 0, len(batches)+1)

	// Track the best batch to add a sweep to. The default case is new batch
	// creation with this sweep only in it. The cost is its full fee.
	alternatives = append(alternatives, alternative{
		batchId: newBatchSignal,
		cost:    oneSweepBatch.fee(mixedBatch),
	})

	// Try to add the sweep to every batch, calculate the costs and
	// find the batch adding to which results in minimum costs.
	for _, batch := range batches {
		// If the batch has IsExternalAddr flag, the sweep can't be
		// added to it, so skip the batch.
		if batch.IsExternalAddr {
			continue
		}

		// Add the sweep to the batch virtually.
		combinedBatch := batch.combine(sweep)

		// The cost is the fee increase.
		cost := combinedBatch.fee(mixedBatch) - batch.fee(mixedBatch)

		// The cost must be positive, because we added a sweep.
		if cost <= 0 {
			return nil, fmt.Errorf("got non-positive cost of "+
				"adding sweep to batch %d: %d", batch.BatchId,
				cost)
		}

		// Track the best batch, according to the costs.
		alternatives = append(alternatives, alternative{
			batchId: batch.BatchId,
			cost:    cost,
		})
	}

	// Sort the alternatives by cost. The lower the cost, the better.
	sort.Slice(alternatives, func(i, j int) bool {
		return alternatives[i].cost < alternatives[j].cost
	})

	// Collect batches IDs.
	batchesIds := make([]int32, len(alternatives))
	for i, alternative := range alternatives {
		batchesIds[i] = alternative.batchId
	}

	return batchesIds, nil
}