expose estimate_fee api

bdk::blockchain::Blockchain contains an `estimate_fee` api. This commit adds the bindings for estimate_fee.

This will fix https://github.com/bitcoindevkit/bdk-ffi/issues/287

Tested this by adding a unit test in lib.rs. Also generated the python code and verified that it is able to invoke estimate_fee and get the fee rate

api-docs/kotlin/src/main/kotlin/org/bitcoindevkit/bdk.kt

@@ -250,6 +250,9 @@ class Blockchain(
     /** Broadcast a transaction. */
     fun broadcast(psbt: PartiallySignedBitcoinTransaction) {}
 
+    /** Estimate the fee rate required to confirm a transaction in a given target of blocks. */
+    fun estimateFee(target: ULong): FeeRate {}
+
     /** Get the current height of the blockchain. */
     fun getHeight(): UInt {}
 

api-docs/kotlin/src/test/kotlin/org/bitcoindevkit/Samples.kt

@@ -107,7 +107,16 @@ fun blockchainSample() {
 
     val blockchain: Blockchain = Blockchain(blockchainConfig)
 
-    blockchain.broadcast(signedPsbt)
+    val feeRate: FeeRate = blockchain.estimateFee(3u)
+
+    val (psbt, txDetails) = TxBuilder()
+        .addRecipient(faucetAddress.scriptPubkey(), 1000uL)
+        .feeRate(feeRate.asSatPerVb())
+        .finish(wallet)
+
+    wallet.sign(psbt)
+
+    blockchain.broadcast(psbt)
 }
 
 

bdk-ffi/src/bdk.udl

@@ -164,6 +164,9 @@ interface Blockchain {
   [Throws=BdkError]
   void broadcast([ByRef] PartiallySignedTransaction psbt);
 
+  [Throws=BdkError]
+  FeeRate estimate_fee(u64 target);
+
   [Throws=BdkError]
   u32 get_height();
 

bdk-ffi/src/lib.rs

@@ -305,6 +305,12 @@ impl Blockchain {
         self.get_blockchain().broadcast(&tx)
     }
 
+    fn estimate_fee(&self, target: u64) -> Result<Arc<FeeRate>, BdkError> {
+        let result: Result<FeeRate, bdk::Error> =
+            self.get_blockchain().estimate_fee(target as usize);
+        result.map(Arc::new)
+    }
+
     fn get_height(&self) -> Result<u32, BdkError> {
         self.get_blockchain().get_height()
     }