This refactors Alert in both c++ and java to fix the issues with the current c++ implementation and improve performance.
Currently, constructing an Alert adds it to a list of Alerts with the same group and type. Activating an alert sets a flag on the alert. When the SendableAlerts is polled (GetStrings), the entire list is iterated over, filtered, and the filtered list is sorted by timestamp. This leads to a worst case O(m + nlog(n)) time complexity for GetStrings, where m and n are the count of total constructed alerts active alerts respectively. It also allocates intermediate data structures to hold the active alert strings for sorting.
This changes the implementation to improve the performance of GetStrings, by shifting the sorting overhead to Alert.Set
Constructing the Alert only initializes the alert's initial state, and initializes the SendableAlerts for the group if it is not already initialized.
Activating or deactivating an alert sets an internal flag for state tracking, and inserts or removes a structure containing the timestamp and text into a self-sorting data structure (std::set, TreeSet) containing other active alerts with the same group and type. (worst case O(log(n))
Now, SendableAlerts.GetStrings only has to iterate over the structure and copy the strings to the returned array. (amortized O(n))
This also fixes the c++ implementation by removing the need for SendableAlerts to directly access the Alert.
This also adds a helper method to SendableRegistry to force initialization of the instance to prevent static initialization ordering issues.
PMD requires that variables only initialized in the constructor be
final. The compiler errors if those final variables aren't guaranteed to
be initialized, so extra else branches were added to ensure that.
PMD also requires that classes with only private constructors be final.
The equivalent C++ classes were finalized as well, except for
TimeInterpolatableBuffer because it doesn't expose factory functions.
The non-NT portion has been moved to wpiutil.
The NT portion has been moved to ntcore (as NTSendable).
SendableBuilder similarly split and moved.
SendableRegistry moved to wpiutil.
In C++, SendableHelper also moved to wpiutil.
This enables use of Sendable from wpimath and also enables
moving several classes from wpilib to wpimath.
