A question about code of the libevent book Chapter 1

[rickyrick2002]

https://libevent.org/libevent-book/01_intro.html

In the example of

Example: select()-based ROT13 server
...

in do_write, there's a line:

state->n_written = state->write_upto = state->buffer_used = 0;

why is it changed and set to 1

state->n_written = state->write_upto = state->buffer_used = 1;

in the following

Example: A low-level ROT13 server with Libevent
....

example ?
Is it a typo or intention?