[Orekit Developers] TimeStampedCache Improvements

[Evan Ward <evan.ward@nrl.navy.mil>]

Hi,

I've noticed that the current implementation of TimeStampedCache (TSC)
can still be a performance issue when it is highly contended. I've also
found the getNeighbors(date) method to be very valuable, even when I'm
not expecting multiple threads. So I propose refactoring the TSC to an
interface so different implementations can be used, depending on the
context. The interface would have 4 methods:

  List<T> getNeighbors(final AbsoluteDate central) throws
IllegalArgumentException; (Or TSCException)
  int getNeighborsSize();
  T getEarliest() throws IllegalStateException;
  T getLatest() throws IllegalStateException;

Besides the existing implementation I have an immutable implementation
that is lock free. The immutable version can improve performance where
all the data stored in memory. (~2x faster sequential, ~40x faster
concurrent) I think AbstractEOPHistory, Ephemeris, and UTCScale could
all use the immutable version.  User classes could also use the
lighter-weight cache for interpolating their data. (I had some
difficulty figuring out how AbstractEOPHistory is used. Correct me if
I'm wrong, I think it is only "written" in FramesFactory when it is
created and then only "read" after that.)

What do you think of this refactoring? It would also open the
possibility for a mutable cache based on a lock free ConcurrentMap.

Regards,
Evan