From fa09dc8c7155e0618c647723d700b02f0891453b Mon Sep 17 00:00:00 2001
From: Carsten Schmiemann <carsten.schmiemann@pfalz-mail.de>
Date: Tue, 1 Nov 2022 14:21:27 +0100
Subject: [PATCH] Original project changes: fix negative integer DBC numbers
 having wrong values

---
 OVMS.V3/components/dbc/src/dbc.cpp |  6 ++++--
 OVMS.V3/main/ovms_utils.h          | 14 ++++++++++++++
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/OVMS.V3/components/dbc/src/dbc.cpp b/OVMS.V3/components/dbc/src/dbc.cpp
index a0a8644..503e041 100644
--- a/OVMS.V3/components/dbc/src/dbc.cpp
+++ b/OVMS.V3/components/dbc/src/dbc.cpp
@@ -830,8 +830,10 @@ dbcNumber dbcSignal::Decode(CAN_frame_t* msg)
 
   if (m_value_type == DBC_VALUETYPE_UNSIGNED)
     result.Cast((uint32_t)val, DBC_NUMBER_INTEGER_UNSIGNED);
-  else
-    result.Cast((uint32_t)val, DBC_NUMBER_INTEGER_SIGNED);
+  else {
+    int32_t signed_val = sign_extend<uint32_t, int32_t>((uint32_t)val, m_signal_size-1);
+    result.Cast(static_cast<uint32_t>(signed_val), DBC_NUMBER_INTEGER_SIGNED);
+  }
 
   // Apply factor and offset
   if (!(m_factor == 1))
diff --git a/OVMS.V3/main/ovms_utils.h b/OVMS.V3/main/ovms_utils.h
index 55be399..cae50ed 100644
--- a/OVMS.V3/main/ovms_utils.h
+++ b/OVMS.V3/main/ovms_utils.h
@@ -338,4 +338,18 @@ double float2double(float f);
 std::string idtag(const char* tag, void* instance);
 #define IDTAG idtag(TAG,this)
 
+/**
+ * sign_extend: Sign extend an unsigned to a signed integer of the same size.
+ */
+template <typename UINT, typename INT>
+INT sign_extend(UINT uvalue, uint8_t signbit)
+  {
+  typedef typename std::make_unsigned<INT>::type uint_t;
+  uint_t newuvalue = uvalue;
+  if (newuvalue & (UINT(1U) << signbit)) {
+    newuvalue |= ~((uint_t(1U) << signbit) - 1);
+  }
+  return reinterpret_cast<INT &>(newuvalue);
+  }
+
 #endif // __OVMS_UTILS_H__