/* * This subroutine adds the data at next_in/avail_in to the output history * without performing any output. The output buffer must be "caught up"; * i.e. no pending output (hence s->read equals s->write), and the state must * be BLOCKS (i.e. we should be willing to see the start of a series of * BLOCKS). On exit, the output will also be caught up, and the checksum * will have been updated if need be. */