Intermediate LPC
Descartes of Borg
Novermber 1993

                         Chapter 2: The LPMud Driver

2.1 Review of Basic Driver/Mudlib Interaction
In the LPC Basics textbook, you learned a lot about the way the mudlib
works, specifically in relation to objects you code in order to build your
realm.  Not much was discussed about the interaction between the
mudlib and the driver.  You should know, however, that the driver
does the following:
1) When an object is first loaded into memory, the driver will call
create() in native muds and reset() in compat muds.  A creator
uses create() or reset() to give initial values to the object.
2) At an interval setup by the game administrator, the driver calls the
function reset().  This allows the object to regenerate monsters and
such.  Notice that in a compat mud, the same function is used to set up
initial values as is used to reset the room.
3) Any time a living object comes in contact with an object of any sort,
the driver calls init() in the newly encountered object.  This allows
newly encountered objects to give living objects commands to execute
through the add_action() efun, as well as perform other actions which
should happen whenever a living thing encounters a given object.
4) The driver defines a set of functions known as efuns which are
available to all objects in the game.  Examples of commonly used efuns
are: this_player(), this_object(), write(), say(), etc.

2.2 The Driver Cycle
The driver is a C program which runs the game.  Its basic functions are
to accept connections from the outside world so people can login,
interpret the LPC code which defines LPC objects and how they
function in the game, and accept user input and call the appropriate LPC
functions which match the event.  In its most simplest essence, it is an
unending loop.

Once the game has booted up and is properly functioning (the boot up
process will be discussed in a future, advanced LPC textbook), the
driver enters a loop which does not terminate until the shutdown() efun
is legally called or a bug causes the driver program to crash.  First off,
the driver handles any new incoming connections and passes control of
the connection to a login object.  After that, the driver puts together a
table of commands which have been entered by users since the last cycle
of the driver.  After the command table is assembled, all messages
scheduled to be sent to the connection from the last driver cycle are sent
out to the user.  At this point, the driver goes through the table of
commands to be executed and executes each set of commands each
object has stored there.  The driver ends its cycle by calling the function
heart_beat() in every object with a heart_beat() set and finally
performing all pending call outs.  This chapter will not deal with the
handling of connections, but instead will focus on how the driver
handles user commands and heartbeats and call outs.

2.3 User Commands
As noted in section 1.2, the driver stores a list of commands for each
user to be executed each cycle.  The commands list has the name of the
living object performing the command, the object which gave the living
object that command, and the function which is to be executed in order
to perform the command.  The driver refers to the object which typed in
the command as the command giver.  It is the command giver which
gets returned as this_player() in most cases.

The driver starts at the top of the list of living objects with pending
commands, and successively performs each command it typed by calling
the function associated with the command and passing any arguments
the command giver gave as arguments to the function.  As the driver
starts with the commands issued by a new living object, the command
giver variable is changed to be equal to the new living object, so that
during the sequence of functions initiated by that command, the efun
this_player() returns the object which issued the command.

Let's look at the command buffer for an example player.  Since the
execution of his last command, Bozo has typed "north" and "tell
descartes when is the next reboot".  The command "north" is associated
with the function "Do_Move()" in the room Bozo is in (the command
"north" is automatically setup by the set_exits() efun in that room).  The
command "tell" is not specifically listed as a command for the player,
however, in the player object there is a function called "cmd_hook()"
which is associated with the command "", which matches any possible
user input.

Once the driver gets down to Bozo, the command giver variable is set to
the object which is Bozo.  Then, seeing Bozo typed "north" and the
function "north" is associated with, the driver calls Bozo's_Room-
>Do_Move(0).  An argument of 0 is passed to the function since Bozo
only typed the command "north" with no arguments.  The room
naturally calls some functions it needs, all the while such that the efun
this_player() returns the object which is Bozo.  Eventually, the room
object will call move_player() in Bozo, which in turn calls the
move_object() efun.  This efun is responsible for changing an object's
environment.

When the environment of an object changes, the commands available to
it from objects in its previous environment as well as from its previous
environment are removed from the object.  Once that is done, the driver
calls the efun init() in the new environment as well as in each object in
the new environment.  During each of these calls to init(), the object
Bozo is still the command giver.  Thus all add_action() efuns from this
move will apply to Bozo.  Once all those calls are done, control passes
back from the move_object() efun to the move_player() lfun in Bozo. 
move_player() returns control back to Do_Move() in the old room,
which returns 1 to signify to the driver that the command action was
successful.  If the Do_Move() function had returned 0 for some reason,
the driver would have written "What?" (or whatever your driver's
default bad command message is) to Bozo.

Once the first command returns 1, the driver proceeds on to Bozo's
second command, following much the same structure.  Note that with
"tell descartes when is the next reboot", the driver passes "descartes
when is the next reboot" to the function associated with tell.  That
function in turn has to decide what to do with that argument.  After that
command returns either 1 or 0, the driver then proceeds on to the next
living object with commands pending, and so on until all living objects
with pending commands have had their commands performed.

2.4 The Efuns set_heart_beat() and call_out()
Once all commands are performed for objects with commands pending,
the driver then proceeds to call the heart_beat() function in all objects
listed with the driver as having heartbeats.  Whenever an object calls the
efun set_heart_beat() with a non-zero argument (depending on your
driver, what non-zero number may be important, but in most cases you
call it with the int 1).  The efun set_heart_beat() adds the object which
calls set_heart_beat() to the list of objects with heartbeats.  If you call it
with an argument of 0, then it removes the object from the list of objects
with heartbeats.

The most common use for heartbeats in the mudlib is to heal players and
monsters and perform combat.  Once the driver has finished dealing with
the command list, it goes through the heartbeat list calling heart_beat() in
each object in the list.  So for a player, for example, the driver will call
heart_beat() in the player which will:
1) age the player
2) heal the player according to a heal rate
3) check to see if there are any hunted, hunting, or attacking objects
around
4) perform an attack if step 3 returns true.
5) any other things which need to happen automatically roughly every
second

Note that the more objects which have heartbeats, the more processing
which has to happen every cycle the mud is up.  Objects with heartbeats
are thus known as the major hog of CPU time on muds.  

The call_out() efun is used to perform timed function calls which do not
need to happen as often as heartbeats, or which just happen once.  Call
outs let you specify the function in an object you want called.  The
general formula for call outs is:
call_out(func, time, args);
The third argument specifying arguments is optional.  The first argument
is a string representing the name of the function to be called.  The second
argument is how many seconds should pass before the function gets
called.

Practically speaking, when an object calls call_out(), it is added to a list
of objects with pending call outs with the amount of time of the call out
and the name of the function to be called.  Each cycle of the driver, the
time is counted down until it becomes time for the function to be called. 
When the time comes, the driver removes the object from the list of
objects with pending call outs and performs the call to the call out
function, passing any special args originally specified by the call out
function.

If you want a to remove a pending call before it occurs, you need to use
the remove_call_out() efun, passing the name of the function being
called out.  The driver will remove the next pending call out to that
function.  This means you may have some ambiguity if more than one
call out is pending for the same function.

In order to make a call out cyclical, you must reissue the call_out() efun
in the function you called out, since the driver automatically removes the
function from the call out table when a call out is performed.  Example:

void foo() { call_out("hello", 10); }

void hello() { call_out("hello", 10); }

will set up hello() to be called every 10 seconds after foo() is first called. 
There are several things to be careful about here.  First, you must watch
to make sure you do not structure your call outs to be recursive in any
unintended fashion.  Second, compare what a set_heart_beat() does
when compared directly to what call_out() does.

set_heart_beat():
a) Adds this_object() to a table listing objects with heartbeats.
b) The function heart_beat() in this_object() gets called every single
driver cycle.

call_out():
a) Adds this_object(), the name of a function in this_object(), a time
delay, and a set of arguments to a table listing functions with pending
call outs.  
b) The function named is called only once, and that call comes after the
specified delay.

As you can see, there is a much greater memory overhead associated
with call outs for part (a), yet that there is a much greater CPU overhead
associated with heartbeats as shown in part (b), assuming that the delay
for the call out is greater than a single driver cycle. 

Clearly, you do not want to be issuing 1 second call outs, for then you
get the worst of both worlds.  Similarly, you do not want to be having
heart beats in objects that can perform the same functions with call outs
of a greater duration than 1 second.  I personally have heard much talk
about at what point you should use a call out over a heartbeat.  What I
have mostly heard is that for single calls or for cycles of a duration
greater than 10 seconds, it is best to use a call out.  For repetitive calls of
durations less than 10 seconds, you are better off using heartbeats.  I do
not know if this is true, but I do not think following this can do any
harm.

2.5 Summary
Basic to a more in depth understanding of LPC is and understanding of
the way in which the driver interacts with the mudlib.  You should now
understand the order in which the driver performs functions, as well as a
more detailed knowledge of the efuns this_player(), add_action(), and
move_object() and the lfun init().  In addition to this building upon
knowledge you got from the LPC Basics textbook, this chapter has
introduced call outs and heartbeats and the manner in which the driver
handles them.  You should now have a basic understanding of call outs
and heartbeats such that you can experiment with them in your realm
code.

Copyright (c) George Reese 1993