gcount, tellg, seekg functions

[ferdymercury]

tellg(), gcount() and seekg() functions might be helpful for estimating file size.

See https://stackoverflow.com/questions/2409504/using-c-filestreams-fstream-how-can-you-determine-the-size-of-a-file

[sandsmark]

I think tellg works in general now, I needed it for debugging some file format issues so I tried to fix it:

81db815#diff-f957a3c12ea388ffadfa08b526d7ae5c52ce99fa70fce25fe583fbbadb27a624 (which is ironically the one that broke the test).

[ferdymercury]

instead of seekg to ios::end (or ios::ate) and tellg, an alternative way of getting the filesize would be:
https://en.cppreference.com/w/cpp/filesystem/file_size

For the 'uncompressed' file size, it could be:
std::distance(std::istreambuf_iterator<char>(file), std::istreambuf_iterator<char>());