Fix #2 Preserve unsigned 64-bit decoded values

dbc_decoder.cpp

@@ -55,10 +55,12 @@ DecodedFrameValue DbcDecoder::Decode (const DecodeDatabase &database,
     if (signal.valueType == ValueType::Signed)
       decoded.rawValue = SignExtend (unsignedValue, signal.length);
     else
-      decoded.rawValue = static_cast<std::int64_t> (unsignedValue);
+      decoded.rawValue = unsignedValue;
 
     decoded.physicalValue =
-      (static_cast<double> (decoded.rawValue) * signal.factor) + signal.offset;
+      std::visit ([&signal] (auto rawValue) {
+        return (static_cast<double> (rawValue) * signal.factor) + signal.offset;
+      }, decoded.rawValue);
     decoded.valid = true;
 
     result.signals.push_back (decoded);
@@ -137,16 +139,17 @@ bool DbcDecoder::ExtractMotorola (const std::vector<std::uint8_t> &data,
 }
 
 std::int64_t DbcDecoder::SignExtend (std::uint64_t value, std::uint32_t bitLength) {
-  if ((bitLength == 0U) || (bitLength >= 64U))
-    return static_cast<std::int64_t> (value);
-
-  const std::uint64_t signMask = (static_cast<std::uint64_t> (1U) << (bitLength - 1U));
-  const std::uint64_t valueMask = (static_cast<std::uint64_t> (1U) << bitLength) - 1U;
+  if ((bitLength == 0U) || (bitLength > 64U))
+    return 0;
 
+  const std::uint64_t signMask = std::uint64_t (1U) << (bitLength - 1U);
+  const std::uint64_t valueMask = (bitLength == 64U)
+    ? ~std::uint64_t (0U) : (std::uint64_t (1U) << bitLength) - 1U;
   value &= valueMask;
 
   if ((value & signMask) == 0U)
     return static_cast<std::int64_t> (value);
 
-  return static_cast<std::int64_t> (value | (~valueMask));
+  // The complemented magnitude fits int64_t, including for INT64_MIN.
+  return -1 - static_cast<std::int64_t> ((~value) & valueMask);
 }