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lib/utils/fast_gps_packet.dart

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+import 'dart:typed_data';
+
+class FastGpsPacket {
+  static const int magic = 0x47; // 'G'
+  static const int _payloadLength = 19;
+  // Store coordinates in microdegrees. This preserves sub-meter precision,
+  // which comfortably satisfies the meter-accuracy requirement.
+  static const double coordinateScale = 1e6;
+
+  final String senderKey6;
+  final double latitude;
+  final double longitude;
+  final int timestampSeconds;
+
+  const FastGpsPacket({
+    required this.senderKey6,
+    required this.latitude,
+    required this.longitude,
+    required this.timestampSeconds,
+  });
+
+  static bool isFastGpsBinary(Uint8List payload) =>
+      payload.length == _payloadLength && payload[0] == magic;
+
+  static FastGpsPacket? tryParseBinary(Uint8List payload) {
+    if (!isFastGpsBinary(payload)) return null;
+
+    final key6 = payload
+        .sublist(1, 7)
+        .map((b) => b.toRadixString(16).padLeft(2, '0'))
+        .join();
+    final data = ByteData.sublistView(payload);
+    final latitude = data.getInt32(7, Endian.little) / coordinateScale;
+    final longitude = data.getInt32(11, Endian.little) / coordinateScale;
+    final timestampSeconds = data.getUint32(15, Endian.little);
+
+    if (!_isValidCoordinate(latitude, longitude)) {
+      return null;
+    }
+
+    return FastGpsPacket(
+      senderKey6: key6,
+      latitude: latitude,
+      longitude: longitude,
+      timestampSeconds: timestampSeconds,
+    );
+  }
+
+  Uint8List encodeBinary() {
+    final out = Uint8List(_payloadLength);
+    final data = ByteData.sublistView(out);
+    out[0] = magic;
+    for (var i = 0; i < 6; i++) {
+      out[1 + i] = int.parse(senderKey6.substring(i * 2, i * 2 + 2), radix: 16);
+    }
+    data.setInt32(7, (latitude * coordinateScale).round(), Endian.little);
+    data.setInt32(11, (longitude * coordinateScale).round(), Endian.little);
+    data.setUint32(15, timestampSeconds, Endian.little);
+    return out;
+  }
+
+  static bool _isValidCoordinate(double latitude, double longitude) {
+    if (!latitude.isFinite || !longitude.isFinite) return false;
+    return latitude >= -90.0 &&
+        latitude <= 90.0 &&
+        longitude >= -180.0 &&
+        longitude <= 180.0;
+  }
+}