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TxBuilder example

Bitcoin-S features a transaction builder that constructs and signs Bitcoin transactions. Here's an example of how to use it

implicit val ec: ExecutionContext = ExecutionContext.Implicits.global
// ec: ExecutionContext = scala.concurrent.impl.ExecutionContextImpl$$anon$3@4ac7afe0[Running, parallelism = 2, size = 1, active = 0, running = 0, steals = 13, tasks = 0, submissions = 0]

// generate a fresh private key that we are going to use in the scriptpubkey
val privKey = ECPrivateKey.freshPrivateKey
// privKey: ECPrivateKey = Masked(ECPrivateKeyImpl)
val pubKey = privKey.publicKey
// pubKey: ECPublicKey = ECPublicKey(03f0ea9c4c81fa579fbbc3f4821e5e34feaca65824f46f4fefbb802e813e67c5d8)

// this is the script that the TxBuilder is going to create a
// script signature that validly spends this scriptPubKey
val creditingSpk = P2PKHScriptPubKey(pubKey = privKey.publicKey)
// creditingSpk: P2PKHScriptPubKey = pkh(482dfc5a500ce6dfc593cc46679d9767304b795d)
val amount = 10000.satoshis
// amount: Satoshis = 10000 sats

// this is the UTXO we are going to be spending
val utxo =
  TransactionOutput(value = amount, scriptPubKey = creditingSpk)
// utxo: TransactionOutput = TransactionOutput(10000 sats,pkh(482dfc5a500ce6dfc593cc46679d9767304b795d))

// the private key that locks the funds for the script we are spending too
val destinationPrivKey = ECPrivateKey.freshPrivateKey
// destinationPrivKey: ECPrivateKey = Masked(ECPrivateKeyImpl)

// the amount we are sending -- 5000 satoshis -- to the destinationSPK
val destinationAmount = 5000.satoshis
// destinationAmount: Satoshis = 5000 sats

// the script that corresponds to destination private key, this is what is protecting the money
val destinationSPK =
  P2PKHScriptPubKey(pubKey = destinationPrivKey.publicKey)
// destinationSPK: P2PKHScriptPubKey = pkh(fef1885207477dd36d6bd985d91b35437fa8c287)

// this is where we are sending money too
// we could add more destinations here if we
// wanted to batch transactions
val destinations = {
    val destination1 = TransactionOutput(value = destinationAmount,
                                         scriptPubKey = destinationSPK)

    Vector(destination1)
}
// destinations: Vector[TransactionOutput] = Vector(TransactionOutput(5000 sats,pkh(fef1885207477dd36d6bd985d91b35437fa8c287)))

// we have to fabricate a transaction that contains the
// UTXO we are trying to spend. If this were a real blockchain
// we would need to reference the UTXO set
val creditingTx = BaseTransaction(version = Int32.one,
                                  inputs = Vector.empty,
                                  outputs = Vector(utxo),
                                  lockTime = UInt32.zero)
// creditingTx: BaseTransaction = BaseTransactionImpl(Int32Impl(1),Vector(),Vector(TransactionOutput(10000 sats,pkh(482dfc5a500ce6dfc593cc46679d9767304b795d))),UInt32Impl(0))

// this is the information we need from the crediting TX
// to properly "link" it in the transaction we are creating
val outPoint = TransactionOutPoint(creditingTx.txId, UInt32.zero)
// outPoint: TransactionOutPoint = TransactionOutPoint(b7f80840a7fc331b206b3ac6fec3c3701916402f42b0fb79c69d7e8272b54875:0)

// this contains all the information we need to
// validly sign the UTXO above
val utxoSpendingInfo = BitcoinUTXOSpendingInfoFull(outPoint = outPoint,
                                                   output = utxo,
                                                   signers = Vector(privKey),
                                                   redeemScriptOpt = None,
                                                   scriptWitnessOpt = None,
                                                   hashType =
                                                       HashType.sigHashAll,
                                                   conditionalPath =
                                                       ConditionalPath.NoConditionsLeft)
// utxoSpendingInfo: BitcoinUTXOSpendingInfoFull = P2PKHSpendingInfo(TransactionOutPoint(b7f80840a7fc331b206b3ac6fec3c3701916402f42b0fb79c69d7e8272b54875:0),10000 sats,pkh(482dfc5a500ce6dfc593cc46679d9767304b795d),Masked(ECPrivateKeyImpl),SIGHASH_ALL(Int32Impl(1)))

// all of the UTXO spending information, since we are only
//spending one UTXO, this is just one element
val utxos: Vector[BitcoinUTXOSpendingInfoFull] = Vector(utxoSpendingInfo)
// utxos: Vector[BitcoinUTXOSpendingInfoFull] = Vector(P2PKHSpendingInfo(TransactionOutPoint(b7f80840a7fc331b206b3ac6fec3c3701916402f42b0fb79c69d7e8272b54875:0),10000 sats,pkh(482dfc5a500ce6dfc593cc46679d9767304b795d),Masked(ECPrivateKeyImpl),SIGHASH_ALL(Int32Impl(1))))

// this is how much we are going to pay as a fee to the network
// for this example, we are going to pay 1 satoshi per byte
val feeRate = SatoshisPerByte(1.satoshi)
// feeRate: SatoshisPerByte = SatoshisPerByte(1 sat)

val changePrivKey = ECPrivateKey.freshPrivateKey
// changePrivKey: ECPrivateKey = Masked(ECPrivateKeyImpl)
val changeSPK = P2PKHScriptPubKey(pubKey = changePrivKey.publicKey)
// changeSPK: P2PKHScriptPubKey = pkh(224d26b2e9575824908db84c1af1e15832d6d435)

// the network we are on, for this example we are using
// the regression test network. This is a network you control
// on your own machine
val networkParams = RegTest
// networkParams: RegTest.type = RegTest

// Yay! Now we have a TxBuilder object that we can use
// to sign the TX.
val txBuilder: BitcoinTxBuilder = {
  val builderF = BitcoinTxBuilder(
    destinations = destinations,
    utxos = utxos,
    feeRate = feeRate,
    changeSPK = changeSPK,
    network = networkParams)
  Await.result(builderF, 30.seconds)
}
// txBuilder: BitcoinTxBuilder = BitcoinTxBuilderImpl(Vector(TransactionOutput(5000 sats,pkh(fef1885207477dd36d6bd985d91b35437fa8c287))),Map(TransactionOutPoint(b7f80840a7fc331b206b3ac6fec3c3701916402f42b0fb79c69d7e8272b54875:0) -> P2PKHSpendingInfo(TransactionOutPoint(b7f80840a7fc331b206b3ac6fec3c3701916402f42b0fb79c69d7e8272b54875:0),10000 sats,pkh(482dfc5a500ce6dfc593cc46679d9767304b795d),Masked(ECPrivateKeyImpl),SIGHASH_ALL(Int32Impl(1)))),SatoshisPerByte(1 sat),pkh(224d26b2e9575824908db84c1af1e15832d6d435),RegTest)

// Let's finally produce a validly signed tx!
// The 'sign' method is going produce a validly signed transaction
// This is going to iterate through each of the UTXOs and use
// the corresponding UTXOSpendingInfo to produce a validly
// signed input. This UTXO has:
//   1: one input
//   2: outputs (destination and change outputs)
//   3: a fee rate of 1 satoshi/byte
val signedTx: Transaction = {
  val signF = txBuilder.sign
  Await.result(signF, 30.seconds)
}
// signedTx: Transaction = BaseTransactionImpl(Int32Impl(2),List(TransactionInputImpl(TransactionOutPoint(b7f80840a7fc331b206b3ac6fec3c3701916402f42b0fb79c69d7e8272b54875:0),P2PKHScriptSignature(ECPublicKey(03f0ea9c4c81fa579fbbc3f4821e5e34feaca65824f46f4fefbb802e813e67c5d8), ECDigitalSignature(30440220728b48b7f17eca17998b0e2b70caa3d9f465301437a9e48b33768ea6b665a32602202715574bb9a9a96c77dfbc818c39cdcd8d10e89c6e4db8965155b9af2353634901)),UInt32Impl(0))),Vector(TransactionOutput(5000 sats,pkh(fef1885207477dd36d6bd985d91b35437fa8c287)), TransactionOutput(4774 sats,pkh(224d26b2e9575824908db84c1af1e15832d6d435))),UInt32Impl(0))

signedTx.inputs.length
// res0: Int = 1

signedTx.outputs.length
// res1: Int = 2

//remember, you can call .hex on any bitcoin-s data structure to get the hex representation!
signedTx.hex
// res2: String = 02000000017548b572827e9dc679fbb0422f40161970c3c3fec63a6b201b33fca74008f8b7000000006a4730440220728b48b7f17eca17998b0e2b70caa3d9f465301437a9e48b33768ea6b665a32602202715574bb9a9a96c77dfbc818c39cdcd8d10e89c6e4db8965155b9af23536349012103f0ea9c4c81fa579fbbc3f4821e5e34feaca65824f46f4fefbb802e813e67c5d8000000000288130000000000001976a914fef1885207477dd36d6bd985d91b35437fa8c28788aca6120000000000001976a914224d26b2e9575824908db84c1af1e15832d6d43588ac00000000