Add a FIXME comment describing the current issues with the query sum implementations.

DCDBLib/src/c_api.cpp

@@ -40,6 +40,27 @@
 
 using namespace DCDB;
 
+/* FIXME:
+ * All these functions suffer from a minor logical error in handling
+ * week-stamp transitions. When calling query sum in a time range
+ * that contains a week stamp transition, this will result in multiple
+ * calls to SensorDataStoreImpl::querySum() with their results being
+ * summed up. When transitioning from week stamp w to w+1 in the
+ * evaluation, this results in the algorithm ignoring the interval
+ * between the last reading within w and the first reading within w+1.
+ *
+ * To fix this, one would have to replicate the implementation of
+ * integrals from SensorDataStoreImpl::querySum for the last reading
+ * in w and the first reading in w+1.
+ *
+ * An alternative fix would be to shift the whole querySum logic
+ * including the expansion of the public sensor name into patterns
+ * and subsequent querying of multiple SensorIDs for forming a
+ * continuous (but week stamp separated) time series into a function
+ * that is part of the DCDBLib C++ API (i.e. rework the current
+ * SensorDataStoreImpl::querySum() function.
+ */
+
 DCDB_C_RESULT dcdbQuerySum(
     int64_t* result,
     const char* hostname,