Allow retrieving the decompressed size

[feliwir]

It would be great to have a replacement function to calculate the uncompressed size. Usually people do something like that:

  bxz::ifstream is(filename, std::ios::binary);
  is.seekg (0, std::ios::end);
  length = UInt(is.tellg());
  is.seekg (0, std::ios::beg);

However with bxzstr seeking from end is not supported. However the decompressed size can be retrieved with ZSTD_getFrameContentSize for example (when using ZStd).
Are there any plans to add such a function

[tmaklin]

For zlib, libbz2, and liblzma calculating the uncompressed size isn't typically (as far as I know) possible without decompressing the file hence the decision not to support reading from end. I can have a look at the zstd function and see if it would be feasible to add for zstd compressed files, thanks for pointing it out!