loop/utils/fees.go

package utils

import (
	"fmt"

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

const (
	// MaxFeeToAmountRatio is the maximum fee to total amount ratio allowed
	// for a sweep transaction.
	MaxFeeToAmountRatio = 0.2
)

// ClampSweepFee caps a fee to a percentage of the provided total amount and
// verifies the resulting fee rate is not below the minimum relay fee. It
// returns the clamped fee, whether it was clamped, or an error if the clamped
// fee would fall below the minimum relay fee.
func ClampSweepFee(fee btcutil.Amount, totalAmount btcutil.Amount,
	ratio float64, minRelayFeeRate chainfee.SatPerKWeight,
	weight lntypes.WeightUnit) (btcutil.Amount, bool, error) {

	maxFeeAmount := btcutil.Amount(float64(totalAmount) * ratio)

	clampedFee := fee
	clamped := false
	if fee > maxFeeAmount {
		clampedFee = maxFeeAmount
		clamped = true
	}

	if minRelayFeeRate > 0 {
		clampedFeeRate := chainfee.NewSatPerKWeight(clampedFee, weight)
		if clampedFeeRate < minRelayFeeRate {
			return 0, clamped, fmt.Errorf("clamped fee rate %v is "+
				"less than minimum relay fee %v",
				clampedFeeRate, minRelayFeeRate)
		}
	}

	return clampedFee, clamped, nil
}