tapfreighter: add PrepareAnchoringTemplate function

tapfreighter/wallet.go

@@ -1764,3 +1764,101 @@ func copyPsbt(packet *psbt.Packet) (*psbt.Packet, error) {
 
 	return psbt.NewFromRawBytes(&buf, false)
 }
+
+// PrepareAnchoringTemplate creates a BTC level PSBT packet that can be used to
+// anchor the given virtual packets. The PSBT packet is created with the
+// necessary inputs and outputs to anchor the virtual packets, but without any
+// signatures.
+func PrepareAnchoringTemplate(
+	vPackets []*tappsbt.VPacket) (*psbt.Packet, error) {
+
+	// Create output skeleton from the allocations.
+	var maxOutputIndex uint32
+	for _, vPkt := range vPackets {
+		for _, vOut := range vPkt.Outputs {
+			if vOut.AnchorOutputIndex > maxOutputIndex {
+				maxOutputIndex = vOut.AnchorOutputIndex
+			}
+		}
+	}
+
+	txTemplate := wire.NewMsgTx(2)
+	for i := uint32(0); i <= maxOutputIndex; i++ {
+		txTemplate.AddTxOut(createDummyOutput())
+	}
+
+	btcPacket, err := psbt.NewFromUnsignedTx(txTemplate)
+	if err != nil {
+		return nil, fmt.Errorf("unable to make psbt packet: %w", err)
+	}
+
+	// Populate output keys and values.
+	for _, vPkt := range vPackets {
+		for _, vOut := range vPkt.Outputs {
+			btcOut := &btcPacket.Outputs[vOut.AnchorOutputIndex]
+
+			btcOut.TaprootInternalKey = schnorr.SerializePubKey(
+				vOut.AnchorOutputInternalKey,
+			)
+
+			// TODO: Using the TaprootTapTree for storing the
+			// encode sibling preimage is wrong! We should require
+			// the full Tapscript tree to be provided and encode it
+			// in the BIP-0174 manner.
+			siblingBytes, _, err := commitment.MaybeEncodeTapscriptPreimage(
+				vOut.AnchorOutputTapscriptSibling,
+			)
+			if err != nil {
+				return nil, fmt.Errorf("unable to encode "+
+					"sibling preimage: %w", err)
+			}
+			btcOut.TaprootTapTree = siblingBytes
+		}
+	}
+
+	// Populate inputs.
+	for pIdx := range vPackets {
+		for iIdx := range vPackets[pIdx].Inputs {
+			vIn := vPackets[pIdx].Inputs[iIdx]
+			anchor := vIn.Anchor
+
+			if HasInput(btcPacket.UnsignedTx, vIn.PrevID.OutPoint) {
+				continue
+			}
+
+			btcPacket.Inputs = append(btcPacket.Inputs, psbt.PInput{
+				WitnessUtxo: &wire.TxOut{
+					Value:    int64(anchor.Value),
+					PkScript: anchor.PkScript,
+				},
+				SighashType:            anchor.SigHashType,
+				Bip32Derivation:        anchor.Bip32Derivation,
+				TaprootBip32Derivation: anchor.TrBip32Derivation,
+				TaprootInternalKey: schnorr.SerializePubKey(
+					anchor.InternalKey,
+				),
+				TaprootMerkleRoot: anchor.MerkleRoot,
+			})
+			btcPacket.UnsignedTx.TxIn = append(
+				btcPacket.UnsignedTx.TxIn, &wire.TxIn{
+					PreviousOutPoint: vIn.PrevID.OutPoint,
+				},
+			)
+
+		}
+	}
+
+	return btcPacket, nil
+}
+
+// HasInput returns true if the given transaction has an input that spends the
+// given outpoint.
+func HasInput(tx *wire.MsgTx, outpoint wire.OutPoint) bool {
+	for _, in := range tx.TxIn {
+		if in.PreviousOutPoint == outpoint {
+			return true
+		}
+	}
+
+	return false
+}