Big Technique #4: State Machines

In a state machine, the program tracks a `current state'
It has a table that says, for each possible state and each possible input type:
- What state to be in next
- An action to perform
For example, suppose we're writing an NNTP server:

In state START:
- call &say_hello, goto MAIN
In state MAIN:
- GROUP command: call &cmd_group, goto MAIN
- QUIT command: call &cmd_quit and goto ...
- POST command: call &cmd_post, goto HEADER
- ...
In state HEADER:
- Blank line: call &article_save_header, goto BODY
- .: goto ARTICLE_FINISH
- Other: store line, goto HEADER
In state BODY:
- .: goto ARTICLE_FINISH
- Other: store line, goto BODY
In state ARTICLE_FINISH:
- call &article_check_and_post, goto MAIN