add spark send fee estimate function

lib/flutter_libsparkmobile.dart

@@ -745,6 +745,81 @@ abstract final class LibSpark {
 
     return hashes;
   }
+
+  static int estimateSparkFee({
+    required String privateKeyHex,
+    int index = 1,
+    required int sendAmount,
+    required bool subtractFeeFromAmount,
+    required List<
+            ({
+              String serializedCoin,
+              String serializedCoinContext,
+              int groupId,
+              int height,
+            })>
+        serializedCoins,
+    required int privateRecipientsCount,
+  }) {
+
+    final privateKeyPtr =
+    privateKeyHex.to32BytesFromHex().unsignedCharPointer();
+
+    final serializedCoinsPtr = malloc.allocate<DartSpendCoinData>(
+        sizeOf<DartSpendCoinData>() * serializedCoins.length);
+    for (int i = 0; i < serializedCoins.length; i++) {
+      final b64CoinDecoded = base64Decode(serializedCoins[i].serializedCoin);
+      serializedCoinsPtr[i].serializedCoin =
+          malloc.allocate<CCDataStream>(sizeOf<CCDataStream>());
+      serializedCoinsPtr[i].serializedCoin.ref.data =
+          b64CoinDecoded.unsignedCharPointer();
+      serializedCoinsPtr[i].serializedCoin.ref.length = b64CoinDecoded.length;
+
+      final b64ContextDecoded =
+          base64Decode(serializedCoins[i].serializedCoinContext);
+      serializedCoinsPtr[i].serializedCoinContext =
+          malloc.allocate<CCDataStream>(sizeOf<CCDataStream>());
+      serializedCoinsPtr[i].serializedCoinContext.ref.data =
+          b64ContextDecoded.unsignedCharPointer();
+      serializedCoinsPtr[i].serializedCoinContext.ref.length =
+          b64ContextDecoded.length;
+
+      serializedCoinsPtr[i].groupId = serializedCoins[i].groupId;
+      serializedCoinsPtr[i].height = serializedCoins[i].height;
+    }
+
+    final result = _bindings.estimateSparkFee(
+      privateKeyPtr,
+      index,
+      sendAmount,
+      subtractFeeFromAmount ? 1 : 0,
+      serializedCoinsPtr,
+      serializedCoins.length,
+      privateRecipientsCount,
+    );
+
+    for (int i = 0; i < serializedCoins.length; i++) {
+      malloc.free(serializedCoinsPtr[i].serializedCoinContext);
+      malloc.free(serializedCoinsPtr[i].serializedCoin);
+    }
+    malloc.free(serializedCoinsPtr);
+    malloc.free(privateKeyPtr);
+
+    if (result.ref.error.address != nullptr.address) {
+      final ex = Exception(
+        result.ref.error.cast<Utf8>().toDartString(),
+      );
+      malloc.free(result.ref.error);
+      malloc.free(result);
+      throw ex;
+    }
+
+    final fee = result.ref.fee;
+
+    malloc.free(result);
+
+    return fee;
+  }
 }
 
 extension on Pointer<UnsignedChar> {

lib/src/flutter_libsparkmobile_bindings_generated.dart

@@ -294,6 +294,40 @@ class FlutterLibsparkmobileBindings {
               ffi.Pointer<ffi.UnsignedChar>, ffi.Int)>>('hashTags');
   late final _hashTags = _hashTagsPtr.asFunction<
       ffi.Pointer<ffi.Char> Function(ffi.Pointer<ffi.UnsignedChar>, int)>();
+
+  ffi.Pointer<SparkFeeResult> estimateSparkFee(
+    ffi.Pointer<ffi.UnsignedChar> keyData,
+    int index,
+    int sendAmount,
+    int subtractFeeFromAmount,
+    ffi.Pointer<DartSpendCoinData> coins,
+    int coinsLength,
+    int privateRecipientsLength,
+  ) {
+    return _estimateSparkFee(
+      keyData,
+      index,
+      sendAmount,
+      subtractFeeFromAmount,
+      coins,
+      coinsLength,
+      privateRecipientsLength,
+    );
+  }
+
+  late final _estimateSparkFeePtr = _lookup<
+      ffi.NativeFunction<
+          ffi.Pointer<SparkFeeResult> Function(
+              ffi.Pointer<ffi.UnsignedChar>,
+              ffi.Int,
+              ffi.Int,
+              ffi.Int,
+              ffi.Pointer<DartSpendCoinData>,
+              ffi.Int,
+              ffi.Int)>>('estimateSparkFee');
+  late final _estimateSparkFee = _estimateSparkFeePtr.asFunction<
+      ffi.Pointer<SparkFeeResult> Function(ffi.Pointer<ffi.UnsignedChar>, int,
+          int, int, ffi.Pointer<DartSpendCoinData>, int, int)>();
 }
 
 /// FFI-friendly wrapper for a spark::Coin.
@@ -647,3 +681,10 @@ final class DartSpendCoinData extends ffi.Struct {
   @ffi.Int()
   external int height;
 }
+
+final class SparkFeeResult extends ffi.Struct {
+  external ffi.Pointer<ffi.Char> error;
+
+  @ffi.Int()
+  external int fee;
+}