Don Cross dd0245cbf8 C: Astronomy_GetCurrentTime supports microsecond resolution. ...

This is a follow-up to work provided by:
Eric Wheeler, KJ7LNW <astronomy-git@z.ewheeler.org>

Before now, the C function Astronomy_GetCurrentTime returned
the current time from the system clock, but only with whole
second resolution. Now it supports microsecond resolution on
Linux/Unix, Mac OS, and Windows.

For unsupported platforms, a compiler error will occur
to indicate that microsecond resolution is not available.
However, it is possible to define one of the following two
preprocessor symbols to work around the compiler error:

1. ASTRONOMY_ENGINE_NO_CURRENT_TIME
   Excludes the function Astronomy_CurrentTime from the build.
   If your project does not need to obtain the current time,
   or your hardware platform does not provide current date
   and time in the first place, this is likely the better option.

2. ASTRONOMY_ENGINE_WHOLE_SECOND
   If your project does need to use the current date and time
   for astronomy calculations, and it can tolerate whole
   second resolution, this option provides a version of
   Astronomy_CurrentTime that uses a call to `time(NULL)`.

Notes:

- Added unit test to confirm at least millisecond resolution.
  Because these tests have to run on GitHub Actions cloud platform,
  and those systems can be heavily CPU-loaded, I want to be tolerant
  of resolution and avoid false failures.

- Added detection of Mac platform.

- Added preprocessor options documented above.

- On Windows, use ULARGE_INTEGER and eliminated one integer division.

- Added comments and developer documentation.

- Converted tabs to spaces in astronomy.c, for consistent code format.

2023-08-28 12:24:20 -04:00

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