Decode S300 tail payload RPM

lib/core/protocol/s300_parser.dart

@@ -39,11 +39,14 @@ SensorState parseS300(Uint8List frame, {bool validateChecksum = true}) {
     throw ArgumentError('S300 frame NEG8 checksum failed');
   }
 
-  // RPM: bytes 2..3 uint16 BE, used directly.
-  // The S300 live frame starts its sensor payload at byte 2; byte 0..1 are
-  // the protocol header. Reading from byte 3 shifts all high-rate values.
-  final int rpmRaw = _readUint16BE(frame, 2);
-  final double rpm = (rpmRaw - 256).clamp(0, 12000).toDouble();
+  // Some S300 Bluetooth frames carry live RPM in the tail payload at 82..83,
+  // 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 : rpmRaw - 256).clamp(0, 12000).toDouble();
 
   // VSS: bytes 5..6 little-endian pulse period.
   final int vssRaw = _readUint16LE(frame, 5);
@@ -180,3 +183,9 @@ int _readUint16BE(Uint8List frame, int offset) {
 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;
+}

test/s300_parser_test.dart

@@ -189,5 +189,37 @@ void main() {
 
       expect(isS300PlaceholderFrame(frame), isTrue);
     });
+
+    test('uses tail payload RPM for 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[82] = 0x32;
+      frame[83] = 0x0C;
+      frame[127] = calculateNeg8(frame.sublist(0, 127));
+
+      final state = parseS300(frame);
+      expect(state.rpm, equals(3122.0));
+    });
   });
 }