rfq: replace AssetSalePolicy.AskPrice with asset-to-BTC rate

This commit modifies the AssetSalePolicy struct to carry the
asset-to-BTC rate instead of a "price". It also updates the policy check
equations to use the new rate field.

rfq/order.go

@@ -4,13 +4,15 @@ import (
 	"bytes"
 	"context"
 	"fmt"
+	"math/big"
 	"sync"
 	"time"
 
 	"github.com/davecgh/go-spew/spew"
 	"github.com/lightninglabs/lndclient"
 	"github.com/lightninglabs/taproot-assets/asset"
 	"github.com/lightninglabs/taproot-assets/fn"
+	"github.com/lightninglabs/taproot-assets/rfqmath"
 	"github.com/lightninglabs/taproot-assets/rfqmsg"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/lnutils"
@@ -87,9 +89,8 @@ type AssetSalePolicy struct {
 	// requested.
 	MaxAssetAmount uint64
 
-	// AskPrice is the asking price of the quote in milli-satoshis per asset
-	// unit.
-	AskPrice lnwire.MilliSatoshi
+	// AskAssetRate is the quote's asking asset unit to BTC conversion rate.
+	AskAssetRate rfqmsg.Uint64FixedPoint
 
 	// expiry is the policy's expiry unix timestamp after which the policy
 	// is no longer valid.
@@ -104,12 +105,9 @@ func NewAssetSalePolicy(quote rfqmsg.BuyAccept) *AssetSalePolicy {
 	return &AssetSalePolicy{
 		ID:             quote.ID,
 		MaxAssetAmount: quote.Request.AssetAmount,
-		// TODO(ffranr): Temp solution.
-		AskPrice: lnwire.MilliSatoshi(
-			quote.AssetRate.Coefficient.ToUint64(),
-		),
-		expiry:  quote.Expiry,
-		assetID: quote.Request.AssetID,
+		AskAssetRate:   quote.AssetRate,
+		expiry:         quote.Expiry,
+		assetID:        quote.Request.AssetID,
 	}
 }
 
@@ -128,7 +126,16 @@ func (c *AssetSalePolicy) CheckHtlcCompliance(
 
 	// Check that the HTLC amount is not greater than the negotiated maximum
 	// amount.
-	maxOutboundAmount := lnwire.MilliSatoshi(c.MaxAssetAmount) * c.AskPrice
+	askAssetRate := c.AskAssetRate.ToBigIntFixedPoint()
+
+	maxAssetAmount := rfqmsg.NewBigIntFixedPoint(
+		new(big.Int).SetUint64(c.MaxAssetAmount), 0,
+	)
+
+	maxOutboundAmount := rfqmath.UnitsToMilliSatoshi(
+		maxAssetAmount, askAssetRate,
+	)
+
 	if htlc.AmountOutMsat > maxOutboundAmount {
 		return fmt.Errorf("htlc out amount is greater than the policy "+
 			"maximum (htlc_out_msat=%d, policy_max_out_msat=%d)",
@@ -175,8 +182,19 @@ func (c *AssetSalePolicy) GenerateInterceptorResponse(
 		return nil, fmt.Errorf("policy has no asset ID")
 	}
 
-	outgoingAssetAmount := uint64(htlc.AmountOutMsat / c.AskPrice)
-	htlcBalance := rfqmsg.NewAssetBalance(*c.assetID, outgoingAssetAmount)
+	// Compute the outgoing asset amount given the msat outgoing amount and
+	// the asset to BTC rate.
+	askAssetRate := rfqmath.FixedPoint[rfqmath.GoInt[uint64]]{
+		Coefficient: c.AskAssetRate.Coefficient,
+		Scale:       c.AskAssetRate.Scale,
+	}
+	outgoingAssetAmount := rfqmath.MilliSatoshiToUnits(
+		htlc.AmountOutMsat, askAssetRate,
+	)
+	amt := outgoingAssetAmount.ScaleTo(0).ToUint64()
+
+	// Include the asset balance in the HTLC record.
+	htlcBalance := rfqmsg.NewAssetBalance(*c.assetID, amt)
 	htlcRecord := rfqmsg.NewHtlc(
 		[]*rfqmsg.AssetBalance{htlcBalance}, fn.Some(c.ID),
 	)

rfqmath/convert.go

@@ -62,6 +62,10 @@ func MilliSatoshiToUnits[N Int[N]](milliSat lnwire.MilliSatoshi,
 // compute the total amount of mSAT (X) as follows:
 //   - X = (U / Y) * M
 //   - where M is the number of mSAT in a BTC (100,000,000,000).
+//
+// TODO(ffranr): This function only works with BigInt as the underlying
+// integer type. For built-in integer types, oneBtcInMilliSat overflows.
+// We should remove the type generic or reformulate.
 func UnitsToMilliSatoshi[N Int[N]](assetUnits,
 	unitsPerBtc FixedPoint[N]) lnwire.MilliSatoshi {