Simplify lib api and clean up duplicate code

lib/extensions.dart

@@ -0,0 +1,23 @@
+import 'dart:typed_data';
+
+extension Uint8ListExt on Uint8List {
+  String toHexString() {
+    return map((byte) => byte.toRadixString(16).padLeft(2, '0')).join();
+  }
+}
+
+/// Convert a hex string to a list of bytes, padded to 32 bytes if necessary.
+extension StringExt on String {
+  Uint8List toBytes() {
+    // Pad the string to 64 characters with zeros if it's shorter.
+    String hexString = padLeft(64, '0');
+
+    List<int> bytes = [];
+    for (int i = 0; i < hexString.length; i += 2) {
+      var byteString = hexString.substring(i, i + 2);
+      var byteValue = int.parse(byteString, radix: 16);
+      bytes.add(byteValue);
+    }
+    return Uint8List.fromList(bytes);
+  }
+}

lib/flutter_libsparkmobile.dart

@@ -1,37 +1,86 @@
 import 'dart:ffi';
+import 'dart:io';
+import 'dart:typed_data';
 
 import 'package:ffi/ffi.dart';
+import 'package:flutter_libsparkmobile/extensions.dart';
 
 import 'flutter_libsparkmobile_bindings.dart';
-import 'flutter_libsparkmobile_platform_interface.dart';
 
-class FlutterLibsparkmobile {
-  final SparkMobileBindings _bindings;
+const kSparkChain = 6;
+const kSparkBaseDerivationPath = "m/44'/136'/0'/$kSparkChain/";
 
-  FlutterLibsparkmobile(DynamicLibrary dynamicLibrary)
-      : _bindings = SparkMobileBindings(dynamicLibrary);
+abstract final class LibSpark {
+  static SparkMobileBindings? _bindings;
 
-  Future<String?> getPlatformVersion() {
-    return FlutterLibsparkmobilePlatform.instance.getPlatformVersion();
+  static void _checkLoaded() {
+    _bindings ??= SparkMobileBindings(_loadLibrary());
+  }
+
+  static DynamicLibrary _loadLibrary() {
+    // hack in prefix for test env
+    String testPrefix = "";
+    if (Platform.environment.containsKey('FLUTTER_TEST')) {
+      if (Platform.isLinux) {
+        testPrefix = 'scripts/linux/build/';
+      } else if (Platform.isMacOS) {
+        testPrefix = 'scripts/macos/build/';
+      } else if (Platform.isWindows) {
+        testPrefix = 'scripts/windows/build/';
+      } else {
+        throw UnsupportedError('This platform is not supported');
+      }
+    }
+
+    if (Platform.isLinux) {
+      return DynamicLibrary.open('${testPrefix}libsparkmobile.so');
+    } else if (Platform.isAndroid) {
+      // return DynamicLibrary.open('${testPrefix}libsparkmobile.so');
+    } else if (Platform.isIOS) {
+      // return DynamicLibrary.open('${testPrefix}libsparkmobile.dylib');
+    } else if (Platform.isMacOS) {
+      // return DynamicLibrary.open('${testPrefix}libsparkmobile.dylib');
+    } else if (Platform.isWindows) {
+      // return DynamicLibrary.open('${testPrefix}sparkmobile.dll');
+    }
+    throw UnsupportedError('This platform is not supported');
   }
 
   // SparkMobileBindings methods:
 
   /// Derive an address from the keyData (mnemonic).
-  Future<String> getAddress(
-      List<int> keyData, int index, int diversifier, bool isTestNet) async {
-    // Validate that the keyData is 32 bytes.
-    if (keyData.length != 32) {
-      throw 'Key data must be 32 bytes.';
+  static Future<String> getAddress({
+    required Uint8List privateKey,
+    required int index,
+    required int diversifier,
+    bool isTestNet = false,
+  }) async {
+    _checkLoaded();
+
+    if (index < 0) {
+      throw Exception("Index must not be negative.");
+    }
+
+    if (diversifier < 0) {
+      throw Exception("Diversifier must not be negative.");
+    }
+
+    if (privateKey.length != 32) {
+      throw Exception(
+        "Invalid private key length: ${privateKey.length}. Must be 32 bytes.",
+      );
     }
 
     // Allocate memory for the hex string on the native heap.
-    final keyDataHex = keyData.toHexString();
-    final keyDataPointer = keyDataHex.toNativeUtf8().cast<Char>();
+    final keyDataPointer = privateKey.toHexString().toNativeUtf8().cast<Char>();
 
     // Call the native method with the pointer.
-    final addressPointer = _bindings.getAddress(
-        keyDataPointer, index, diversifier, isTestNet ? 1 : 0);
+    final addressPointer = _bindings!.getAddress(
+      keyDataPointer,
+      index,
+      diversifier,
+      isTestNet ? 1 : 0,
+    );
 
     // Convert the Pointer<Char> to a Dart String.
     final addressString = addressPointer.cast<Utf8>().toDartString();
@@ -43,10 +92,3 @@ class FlutterLibsparkmobile {
     return addressString;
   }
 }
-
-/// Convert List<int> keyData to a hex string.
-extension on List<int> {
-  String toHexString() {
-    return map((byte) => byte.toRadixString(16).padLeft(2, '0')).join();
-  }
-}