Do explicit byte packing for uuids

Signed-off-by: methylDragon <methylDragon@gmail.com>

fuse_core/src/uuid.cpp

@@ -69,10 +69,14 @@ UUID generate(const std::string& namespace_string, const rclcpp::Time& stamp)
   const auto nanoseconds = stamp.nanoseconds();
   constexpr size_t buffer_size = sizeof(nanoseconds);
   std::array<unsigned char, buffer_size> buffer;
-  auto iter = buffer.begin();
-  iter = std::copy(reinterpret_cast<const unsigned char*>(&nanoseconds),
-                   reinterpret_cast<const unsigned char*>(&nanoseconds) + sizeof(nanoseconds),
-                   iter);
+
+  // Explicitly pack nanosecond bits from LSB -> MSB by masking and shifting
+  // E.g. 10 would be 0x00000000'0000000A, so the buffer would store 0x0A at buffer[0]
+  for (size_t i = 0; i < sizeof(nanoseconds); i++) {
+    auto mask = (nanoseconds & (static_cast<int64_t>(0xFF) << 8 * i));
+    buffer[i] = static_cast<unsigned char>(mask >> 8 * i);
+  }
+
   return generate(namespace_string, buffer.data(), buffer.size());
 }
 
@@ -81,13 +85,17 @@ UUID generate(const std::string& namespace_string, const rclcpp::Time& stamp, co
   const auto nanoseconds = stamp.nanoseconds();
   constexpr size_t buffer_size = sizeof(nanoseconds) + UUID::static_size();
   std::array<unsigned char, buffer_size> buffer;
-  auto iter = buffer.begin();
-  iter = std::copy(reinterpret_cast<const unsigned char*>(&nanoseconds),
-                   reinterpret_cast<const unsigned char*>(&nanoseconds) + sizeof(nanoseconds),
-                   iter);
-  iter = std::copy(id.begin(),
-                   id.end(),
-                   iter);
+
+  // Explicitly pack nanosecond bits from LSB -> MSB by masking and shifting
+  for (size_t i = 0; i < sizeof(nanoseconds); i++) {
+    auto mask = (nanoseconds & (static_cast<int64_t>(0xFF) << 8 * i));
+    buffer[i] = static_cast<unsigned char>(mask >> 8 * i);
+  }
+
+  // Then append the ID bytes
+  auto iter = &buffer[sizeof(nanoseconds)];
+  iter = std::copy(id.begin(), id.end(), iter);
+
   return generate(namespace_string, buffer.data(), buffer.size());
 }