Handle S300 alternate tail RPM frames

lib/core/protocol/s300_parser.dart

@@ -28,6 +28,10 @@ bool isS300PlaceholderFrame(Uint8List frame) {
       frame[16] == 0xC4;
 }
 
+bool hasS300TailRpmPayload(Uint8List frame) {
+  return _s300TailRpmRaw(frame) != null;
+}
+
 /// Parses a 128-byte S300 ECU response frame into a [SensorState].
 /// Throws [ArgumentError] if frame length is wrong, or if [validateChecksum]
 /// is true and the NEG8 checksum fails.
@@ -39,13 +43,12 @@ SensorState parseS300(Uint8List frame, {bool validateChecksum = true}) {
     throw ArgumentError('S300 frame NEG8 checksum failed');
   }
 
-  // Some S300 Bluetooth frames carry live RPM in the tail payload at 82..83,
+  // Some S300 Bluetooth frames carry live RPM in the tail payload,
   // while the header bytes stay fixed at 1b 00 14 00 and decode to a bogus
   // 4864 rpm. Prefer the tail value when it looks like real engine speed.
-  final bool hasTailPayload = _hasS300TailPayload(frame);
-  final int rpmRaw =
-      hasTailPayload ? _readUint16LE(frame, 82) : _readUint16BE(frame, 2);
-  final double rpm = (hasTailPayload ? rpmRaw - 1089 : rpmRaw - 256)
+  final int? tailRpmRaw = _s300TailRpmRaw(frame);
+  final int rpmRaw = tailRpmRaw ?? _readUint16BE(frame, 2);
+  final double rpm = (tailRpmRaw != null ? rpmRaw - 1089 : rpmRaw - 256)
       .clamp(0, 12000)
       .toDouble();
 
@@ -185,8 +188,11 @@ int _readUint16LE(Uint8List frame, int offset) {
   return frame[offset] | (frame[offset + 1] << 8);
 }
 
-bool _hasS300TailPayload(Uint8List frame) {
-  if (!isS300PlaceholderFrame(frame)) return false;
-  final rpm = _readUint16LE(frame, 82);
-  return rpm > 300 && rpm < 12000;
+int? _s300TailRpmRaw(Uint8List frame) {
+  if (!isS300PlaceholderFrame(frame)) return null;
+  for (final offset in const [82, 86]) {
+    final rpm = _readUint16LE(frame, offset);
+    if (rpm > 300 && rpm < 12000) return rpm;
+  }
+  return null;
 }

lib/core/providers/sensor_provider.dart

@@ -15,6 +15,9 @@ final sensorStateProvider = StreamProvider<SensorState>((ref) async* {
 
   await for (final frame in btNotifier.frameStream) {
     if (ecuType == EcuType.s300) {
+      if (isS300PlaceholderFrame(frame) && !hasS300TailRpmPayload(frame)) {
+        continue;
+      }
       yield parseS300(frame, validateChecksum: false);
     } else {
       yield parseKPro(frame);

test/s300_parser_test.dart

@@ -190,8 +190,7 @@ void main() {
       expect(isS300PlaceholderFrame(frame), isTrue);
     });
 
-    test('uses calibrated tail payload RPM for S300 Bluetooth prefix frames',
-        () {
+    test('uses calibrated tail payload RPM at offset 82', () {
       final frame = Uint8List(128);
       final prefix = [
         0x1B,
@@ -222,5 +221,68 @@ void main() {
       final state = parseS300(frame);
       expect(state.rpm, equals(2033.0));
     });
+
+    test('uses calibrated tail payload RPM at offset 86', () {
+      final frame = Uint8List(128);
+      final prefix = [
+        0x1B,
+        0x00,
+        0x14,
+        0x00,
+        0x00,
+        0xFF,
+        0xFF,
+        0xF2,
+        0x03,
+        0x18,
+        0x00,
+        0x00,
+        0x10,
+        0x00,
+        0x00,
+        0x00,
+        0xC4,
+      ];
+      for (var i = 0; i < prefix.length; i++) {
+        frame[i] = prefix[i];
+      }
+      frame[86] = 0x31;
+      frame[87] = 0x0C;
+      frame[127] = calculateNeg8(frame.sublist(0, 127));
+
+      expect(hasS300TailRpmPayload(frame), isTrue);
+      final state = parseS300(frame);
+      expect(state.rpm, equals(2032.0));
+    });
+
+    test('detects empty S300 Bluetooth prefix frames', () {
+      final frame = Uint8List(128);
+      final prefix = [
+        0x1B,
+        0x00,
+        0x14,
+        0x00,
+        0x00,
+        0xFF,
+        0xFF,
+        0xF2,
+        0x03,
+        0x18,
+        0x00,
+        0x00,
+        0x10,
+        0x00,
+        0x00,
+        0x00,
+        0xC4,
+      ];
+      for (var i = 0; i < prefix.length; i++) {
+        frame[i] = prefix[i];
+      }
+      frame[127] = calculateNeg8(frame.sublist(0, 127));
+
+      expect(isS300PlaceholderFrame(frame), isTrue);
+      expect(hasS300TailRpmPayload(frame), isFalse);
+    });
   });
 }