// ExampleLowLevelTransaction creates and signs a simple transaction, and then
// encodes it into a hex string capable of being submitted to stellar-core.
//
// It uses the low-level xdr facilities to create the transaction.
func ExampleLowLevelTransaction() {
	spub, spriv, err := GenerateKeyFromSeed("s3Fy8h5LEcYVE8aofthKWHeJpygbntw5HgcekFw93K6XqTW4gEx")

	if err != nil {
		log.Fatal(err)
	}

	dpub, _, err := GenerateKeyFromSeed("sfkPCKA6XgaeHZH3NE57i3QrjVcw61c1noWQCgnHa6KJP2BrbXD")

	if err != nil {
		log.Fatal(err)
	}

	op := xdr.PaymentOp{
		Destination: dpub.KeyData(),
		Currency:    xdr.NewCurrencyCurrencyTypeNative(),
		Amount:      50 * 10000000,
	}

	tx := xdr.Transaction{
		SourceAccount: spub.KeyData(),
		Fee:           10,
		SeqNum:        xdr.SequenceNumber(1),
		Memo:          xdr.NewMemoMemoNone(),
		Operations: []xdr.Operation{
			{Body: xdr.NewOperationBodyPayment(op)},
		},
	}

	var txBytes bytes.Buffer
	_, err = xdr.Marshal(&txBytes, tx)
	if err != nil {
		log.Fatal(err)
	}

	txHash := Hash(txBytes.Bytes())
	signature := spriv.Sign(txHash[:])

	ds := xdr.DecoratedSignature{
		Hint:      spriv.Hint(),
		Signature: xdr.Uint512(signature),
	}

	txe := xdr.TransactionEnvelope{
		Tx:         tx,
		Signatures: []xdr.DecoratedSignature{ds},
	}

	var txeBytes bytes.Buffer
	_, err = xdr.Marshal(&txeBytes, txe)
	if err != nil {
		log.Fatal(err)
	}

	txeHex := hex.EncodeToString(txeBytes.Bytes())

	fmt.Printf("tx hex: %s", txeHex)
	// Output: tx hex: 3658fe7598d20c7a6de3297f84bc52bd2a1ec8c0f1f6c5b41cc1c7571b4331f00000000a000000000000000100000000000000000000000100000000000000012d24692ed08bbf679ba199448870d2191e876fecd92fdd9f6d274da4e6de134100000000000000001dcd650000000001dd302d0c0cee527cf02f6a0aec6916966298712914c63e3c57de74a6e27c29ea234a555fcc36533417afe4e1147815a42529fbca3429bc7caf0a06dc6b383ca6e9d4d80f
}
// MutateTransactionEnvelope adds a signature to the provided envelope
func (m Sign) MutateTransactionEnvelope(txe *xdr.TransactionEnvelope) error {
	tb := TransactionBuilder{TX: txe.Tx}
	hash, err := tb.Hash()

	if err != nil {
		return err
	}

	if m.Key == nil {
		return errors.New("Invalid key")
	}

	sig := m.Key.Sign(hash[:])

	ds := xdr.DecoratedSignature{
		Hint:      m.Key.Hint(),
		Signature: xdr.Uint512(sig),
	}

	txe.Signatures = append(txe.Signatures, ds)
	return nil
}