int64 CopyStream(Stream& dest, Stream& src, int64 count, Gate<int64, int64> progress, int chunk_size)
{
	int block = (int)min<int64>(count, chunk_size);
	Buffer<byte> temp(block);
	int loaded;
	int64 done = 0;
	int64 total = count;
	while(count > 0 && (loaded = src.Get(~temp, (int)min<int64>(count, block))) > 0) {
		dest.Put(~temp, loaded);
		int64 pos = dest.GetPos();
		if(pos > done) { // at least one byte is written to dest.
			count -= pos;
			done  =  pos;
			if(progress(done, total))
				return -1;
		}
		if(dest.IsError())
			break;
	}
	return done;
}