Forward seekable compressed streams

[maxnoe]

Using boost::iostreams::filtering_istream together with one of the decompression filters, e.g. boost::iostreams::gzip_decompressor results in tellg returning -1 and setting the bad bid and seekg not working.

However, at least forward seeking and telling the number read bytes would be possible and would make working with these streams much easier for certain use cases (e.g. reading custom binary files compressed with these methods).

i.e. this:

#include <boost/iostreams/filtering_stream.hpp>
#include <boost/iostreams/filter/gzip.hpp>
#include <boost/iostreams/device/file.hpp>


namespace io = boost::iostreams;
int main() {
    io::filtering_istream stream;
    stream.push(io::gzip_decompressor());
    stream.push(io::file_source("test.gzip"));
    
    std::cout << stream.tellg() << std::endl; // should print 0, prints -1
    stream.seekg(1000); // skip 1000 bytes
    std::cout << stream.tellg() << std::endl; // should print 1000, prints -1
    
    return 0;
}

See for example gzseek: https://refspecs.linuxbase.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/zlib-gzseek-1.html

and gztell: https://refspecs.linuxbase.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/zlib-gztell-1.html

[rdoeffinger]

Not sure what happens for tellg, but I am not sure if the seekg behaviour is not the best possible.
Because for cases where seeking is not strictly possible since it will always involve reading all data, it might make sense to require use of stream.ignore(). That is assuming stream.ignore() is working, I have not tested it.