Remove BH dependency

[jcheng5]

Replaced boost/optional with https://github.com/akrzemi1/Optional.

parse_http_date_string() used boost/date_time; replaced this with sscanf() and timegm(). But timegm() isn't available on mingw, and the first workaround I tried caused the DLL not to load during CI. The final workaround reimplements timegm() using Win32 APIs.

See also r-lib/later#147

[jcheng5]

get_time does seem much, much better! Thanks for the tip.

For testing, I tried replacing the locale ("C") with a locale that is not installed, and that caused an exception; the try/catch(...) catches that exception. I also tried replacing the locale with a valid locale in a different language, and that caused the parsing to fail gracefully (which confirms that get_time does respect the imbued locale).

[wch]

It's nice that you can remove all of the manual conversion stuff up above by setting the locale for the stream object.

Just curious: do you know what it does with the day of week (like "Tue")? Does that get thrown away, or does it check for a mismatch?

[wch]

To answer my own question: the day of week gets put in the tm object:

cpp11::cpp_source(
  code = '
#include <cpp11.hpp>
#include <sstream>
#include <iomanip>

using namespace cpp11;

[[cpp11::register]]
cpp11::integers datetest(std::string date) {
  std::tm t = {};

  std::istringstream date_ss(date);
  date_ss >> std::get_time(&t, "%a, %d %b %Y %H:%M:%S GMT");
  return writable::integers({
    t.tm_isdst,
    t.tm_yday,
    t.tm_wday,
    t.tm_year,
    t.tm_mon,
    t.tm_mday,
    t.tm_hour,
    t.tm_min,
    t.tm_sec
  });
}
')

datetest("Tue, 30 Mar 2021 15:14:00 GMT")
#> [1]   0   0   2 121   2  30  15  14   0
datetest("Wed, 30 Mar 2021 15:14:00 GMT")
#> [1]   0   0   3 121   2  30  15  14   0

But I think it gets ignored for date-time comparisons later on.

[wch]

As of gcc 4.9.3 (which I believe is the version included with rtools 3.5), get_time is not implemented. This is item 27.7 in the table here: https://gcc.gnu.org/onlinedocs/gcc-4.9.3/libstdc++/manual/manual/status.html

[wch]

One piece of code that should be changed, but otherwise looks good.