private auto _samples_addrs() const
    {
        const(AccessUnit)[] samples;

        if(_samples.keys().length > 0)
        {
            samples = _samples[_samples.keys()[0]];
        }

        return map!(x => cast(void*)x)(samples);
    }

    private static string attr(void * p)
    {
        import core.memory : GC;
        auto root = GC.addrOf(p);
        auto bi = GC.getAttr(root);

        auto map = [
            GC.BlkAttr.FINALIZE : "FINALIZE",
            GC.BlkAttr.NO_SCAN : "NO_SCAN",
            GC.BlkAttr.NO_MOVE : "NO_MOVE",
            GC.BlkAttr.APPENDABLE : "APPENDABLE",
            GC.BlkAttr.NO_INTERIOR : "NO_INTERIOR",
        ];

        string s = "";
        foreach(k, v; map)
        {
            if(bi && k)
            {
                s ~= v ~ " ";
            }
        }

        return s;
    }

    invariant()
    {
        import core.memory;
        std.stdio.stderr.writefln("-> invariant %s: as: %s samples: %s", cast(void*) this, _adapt_stream_index, _samples_addrs());

        //std.stdio.stderr.writefln("\t-> collect");
        //GC.collect();
        //std.stdio.stderr.writefln("\t<- collect");

        assert(GC.addrOf(cast(void*)this) !is null);

        foreach(as, s; _samples)
        {
            auto root = GC.addrOf(cast(void*) s.ptr);

            std.stdio.stderr.writefln("\t\tkeys ptr: %s root: %s:%s attr: %s", s.ptr, root, GC.sizeOf(root), attr(root));

            foreach(x; s)
            {
                std.stdio.stderr.writefln("\t\t[%s] keys: %s as: %s au: %s", cast(void*) Thread.getThis(), _samples.keys, _adapt_stream_index, cast(void*)x);
                assert(GC.addrOf(cast(void*)x) !is null);
                //std.file.write("/dev/null", "%s".format(x.random_access));
                //auto s1 = x.random_access;
            }

            foreach(x; s)
            {
                auto s1 = x.random_access;
            }
        }
        std.stdio.stderr.writefln("<- invariant %s", cast(void*) this);
    }

    private AccessUnit[][StreamIndex] _samples;