Windows support

[HertzDevil]

The following implementation will work on Windows: (for simplicity it uses LibC declarations in the standard library rather than copying the used definitions into a separate, private lib)

# consoleapi2.h
lib LibC
  struct COORD
    x : Short
    y : Short
  end

  struct SMALL_RECT
    left : Short
    up : Short
    right : Short
    bottom : Short
  end

  struct CONSOLE_SCREEN_BUFFER_INFO
    dwSize : COORD
    dwCursorPosition : COORD
    wAttributes : WORD
    srWindow : SMALL_RECT
    dwMaximumWindowSize : COORD
  end

  fun GetConsoleScreenBufferInfo(hConsoleOutput : HANDLE, lpConsoleScreenBufferInfo : CONSOLE_SCREEN_BUFFER_INFO*) : BOOL
end

module Term
  module Screen
    def size
      {% if flag?(:win32) %}
        check_size(size_from_win_api) ||
          check_size(size_from_default) ||
          size_from_default
      {% else %}
        # ...
      {% end %}
    end

    def size_from_win_api
      handle = LibC._get_osfhandle(STDOUT.fd)
      return nil if handle == -1

      ret = LibC.GetConsoleScreenBufferInfo(LibC::HANDLE.new(handle), out buffer_info)
      return nil if ret == 0

      lines = buffer_info.srWindow.right - buffer_info.srWindow.left + 1
      cols = buffer_info.srWindow.bottom - buffer_info.srWindow.up + 1
      {lines.to_i, cols.to_i}
    end
  end
end

In case you are wondering, this is how ioctl from Cygwin's runtime does it as well, so flag?(:win32) will suffice, not just flag?(:msvc):

• https://github.com/msys2/msys2-runtime/blob/991a73778838b67d32a53bb1c2fb2edda750953b/winsup/cygwin/fhandler_console.cc#L1625-L1640
• https://github.com/msys2/msys2-runtime/blob/b54860d002ad85de2949bec05ca579aa62f0ef7c/winsup/cygwin/fhandler_console.cc#L880-L904

For this to truly compile, all the POSIX-specific stuffs must be guarded away on Windows as well, and vice versa. The simplest solution is to guard the individual bodies of the size_from_* methods, and then let #size continue to try all alternatives.

[watzon]

I don't have a ton of time to try this myself at the moment, but I'd be more than happy to merge a PR if you can get one up!