From c2f92db2174aab8803e03176adda449df82ef12e Mon Sep 17 00:00:00 2001
From: HVBT Dev <dev@hvbt>
Date: Tue, 28 Jul 2026 01:35:04 +0530
Subject: [PATCH] Calibrate S300 tail RPM decode

---
 lib/core/protocol/s300_parser.dart | 5 +++--
 test/s300_parser_test.dart         | 5 +++--
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/lib/core/protocol/s300_parser.dart b/lib/core/protocol/s300_parser.dart
index 5665e13..b3593cd 100644
--- a/lib/core/protocol/s300_parser.dart
+++ b/lib/core/protocol/s300_parser.dart
@@ -45,8 +45,9 @@ SensorState parseS300(Uint8List frame, {bool validateChecksum = true}) {
   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();
+  final double rpm = (hasTailPayload ? rpmRaw - 1089 : rpmRaw - 256)
+      .clamp(0, 12000)
+      .toDouble();
 
   // VSS: bytes 5..6 little-endian pulse period.
   final int vssRaw = _readUint16LE(frame, 5);
diff --git a/test/s300_parser_test.dart b/test/s300_parser_test.dart
index 3de83f2..4ac6682 100644
--- a/test/s300_parser_test.dart
+++ b/test/s300_parser_test.dart
@@ -190,7 +190,8 @@ void main() {
       expect(isS300PlaceholderFrame(frame), isTrue);
     });
 
-    test('uses tail payload RPM for S300 Bluetooth prefix frames', () {
+    test('uses calibrated tail payload RPM for S300 Bluetooth prefix frames',
+        () {
       final frame = Uint8List(128);
       final prefix = [
         0x1B,
@@ -219,7 +220,7 @@ void main() {
       frame[127] = calculateNeg8(frame.sublist(0, 127));
 
       final state = parseS300(frame);
-      expect(state.rpm, equals(3122.0));
+      expect(state.rpm, equals(2033.0));
     });
   });
 }