Move transaction types in dedicated modules

To allow the related timelock to be defined with the
transaction that uses it. This will allow the access to the
timelock's struct inner field with defining `From` impl.

swap/src/bitcoin/lock.rs

@@ -0,0 +1,98 @@
+use crate::bitcoin::{
+    build_shared_output_descriptor, Address, Amount, BuildTxLockPsbt, GetNetwork, PublicKey,
+    Transaction, TX_FEE,
+};
+use ::bitcoin::{util::psbt::PartiallySignedTransaction, OutPoint, TxIn, TxOut, Txid};
+use anyhow::Result;
+use miniscript::{Descriptor, NullCtx};
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
+pub struct TxLock {
+    inner: Transaction,
+    pub(in crate::bitcoin) output_descriptor: Descriptor<::bitcoin::PublicKey>,
+}
+
+impl TxLock {
+    pub async fn new<W>(wallet: &W, amount: Amount, A: PublicKey, B: PublicKey) -> Result<Self>
+    where
+        W: BuildTxLockPsbt + GetNetwork,
+    {
+        let lock_output_descriptor = build_shared_output_descriptor(A.0, B.0);
+        let address = lock_output_descriptor
+            .address(wallet.get_network(), NullCtx)
+            .expect("can derive address from descriptor");
+
+        // We construct a psbt for convenience
+        let psbt = wallet.build_tx_lock_psbt(address, amount).await?;
+
+        // We don't take advantage of psbt functionality yet, instead we convert to a
+        // raw transaction
+        let inner = psbt.extract_tx();
+
+        Ok(Self {
+            inner,
+            output_descriptor: lock_output_descriptor,
+        })
+    }
+
+    pub fn lock_amount(&self) -> Amount {
+        Amount::from_sat(self.inner.output[self.lock_output_vout()].value)
+    }
+
+    pub fn txid(&self) -> Txid {
+        self.inner.txid()
+    }
+
+    pub fn as_outpoint(&self) -> OutPoint {
+        // This is fine because a transaction that has that many outputs is not
+        // realistic
+        #[allow(clippy::cast_possible_truncation)]
+        OutPoint::new(self.inner.txid(), self.lock_output_vout() as u32)
+    }
+
+    /// Retreive the index of the locked output in the transaction outputs
+    /// vector
+    fn lock_output_vout(&self) -> usize {
+        self.inner
+            .output
+            .iter()
+            .position(|output| {
+                output.script_pubkey == self.output_descriptor.script_pubkey(NullCtx)
+            })
+            .expect("transaction contains lock output")
+    }
+
+    pub fn build_spend_transaction(
+        &self,
+        spend_address: &Address,
+        sequence: Option<u32>,
+    ) -> Transaction {
+        let previous_output = self.as_outpoint();
+
+        let tx_in = TxIn {
+            previous_output,
+            script_sig: Default::default(),
+            sequence: sequence.unwrap_or(0xFFFF_FFFF),
+            witness: Vec::new(),
+        };
+
+        let tx_out = TxOut {
+            value: self.inner.output[self.lock_output_vout()].value - TX_FEE,
+            script_pubkey: spend_address.script_pubkey(),
+        };
+
+        Transaction {
+            version: 2,
+            lock_time: 0,
+            input: vec![tx_in],
+            output: vec![tx_out],
+        }
+    }
+}
+
+impl From<TxLock> for PartiallySignedTransaction {
+    fn from(from: TxLock) -> Self {
+        PartiallySignedTransaction::from_unsigned_tx(from.inner).expect("to be unsigned")
+    }
+}