Justification for doX Processing Target Strings

One of the rules of coding that I try to abide by, both at work and at home, is always to have a reason for doing something some way. Whether it be choosing to use if/elsif vs. switch statements or whether to make something a member of some class, I try to have a reason for every choice that I encounter. Sometimes I get distracted or something, and make what I later feel to be a bad choice. When correcting it, again, I require myself to have a justification. Here is one such time.

In the sequence of events involved with commands it actually seems to handwave about the final step. Ultimately a method is invoked on the character object that is performing a command that actually has the implementation of the action that takes place. For instance, if you type, "say hi", at some point character.doSay("hi") is called. There's one such so-called doX method for each applicable command. Certain commands take targets. For instance, when you type, "get bag", something needs to interpret "bag" as an identifier in the game world and associate it with a real Dealie instance, which is then manipulated by doGet. In this example, "bag" is what I refer to as a "target string."

Originally, the GetCommand class would do a minimal amount of processing on the arguments to the command, then pass on basically the raw target string to doGet, which would lookup the target string in the current room, find the dealie, and mess with it.

At some point, I got the crazy idea that the doX methods Really Should take the final objects themselves. For example, whereas the signature for doGet was something like doGet(targetString) before, it then became doGet(targetDealie). The reason I came up with was that...er..."it just seems right," or some nonsense like that. OK, so I don't think I had a really solid reason for it.

So I did a bunch of work to push out most of the target string to target object code into the Command classes. In a way, this wasn't so bad, because this code was kind of old and it got a well-deserved polishing. Then I came to my senses when I realized a few reasons why it shouldn't be that way.

Firstly, it violated one of Scott Meyers' rules about good design: make interfaces easy to use correctly and hard to use incorrectly. Before, a caller passed in simply a string, and it was thoroughly validated within the doX method. Afterwards, the caller could pass in anything they really wanted, and its validity needed to be thoroughly validated within. A lot of the checks that are done in the process of resolving the target string are kind of implicit, and making them explicit was not only tedious, but also prone to becoming outdated with the arrival of new features, and probably duplicate code for something the caller does to perform the mapping himself.

For example, in the "get" example above, the target string is looked for only on the ground of the room the character is standing in. After my silly change, the caller can pass in an object that is already in the character's inventory, which is a clearly invalid case. The root cause of this class of problems is that the real logic of the functionality is now spread between two different functions--one that does the target string lookup, and one that does the processing on the target; but these pieces of logic are closely tied together, and so should really live close together.

Secondly, the previous way represented the behavior of a character better. A command like, "get 3.bag" means something like, "get the third thing identified as a bag nearby [in this room]." Then code within the Mob class is responsible for finding the real object--i.e. looking for it with his eyes--and picking it up. The newer way detached these steps from the Mob class and put them in the Command classes instead, which have no similar analog in the MUD world.

Finally, the newer way violates the Law of Demeter worse than the older way did. I have a lot of places in my code where I disobey the Law of Demeter, but reducing those occurrences is widely considered a good thing to do. Basically, the Law of Demeter says that objects shouldn't really try to access objects that aren't direct "neighbors" in some sense. Specifically, the code in the Command classes would do things like query the character for what room they're in and call functions on that to look up dealies or mobs in that room. This kind of interaction between the Command classes and things that really don't concern them (rooms, dealies, etc.) leads to tight coupling, which eventually becomes hard to break.

But I wasn't able to remove this entirely. There is a reason to support these kinds of calls receiving target objects directly. Here's an example. A certain nipic is programmed to whisper some words to anyone who whispers something to them. More specifically, when this nipic receives any WhisperEvento, he needs to whisper some string to evento.source, which is generally an instance of Mob. With the old way, the rough code to do this would look like so:

def receiveEvento(evento)
    if evento.source != manager().mob
        if evento.kind_of?(WhisperEvento)
            manager().mob().doWhisper(mapTargetToTargetString(evento.source), "Don't whisper at me!")
end # function receive evento

As you can see here, in order to whisper to someone whom the nipic already has unambiugously identified, the code is performing a reverse mapping, from unambiguous source object to more ambiguous target string. If you imagine that the source's short description is "a towering man wearing glasses," that target string might look like "4."man big giant glasses"", which includes all of its keywords in an attempt to make it as precise as possible.

Compare that with the actual final solution that I implemented:

manager().mob().doWhisper_Direct(evento.source, "Don't whisper at me!")

The only problem is the inability to restrict who can call the doX_Direct methods. They're not just a performance optimization, but also help with correctness in certain situations, where validity checking has already taken place.


Very Controlling

Snooping is a feature that most MUDs have, including mine. I haven't talked much about it yet because it felt fairly fragile to me for a while. This morning I fixed up several problems with it, and now it feels somewhat more solid. It's time to talk about snooping and controlling.

In case you don't know, snooping is a mechanism by which one character can see everything that another character is doing or play as a Nipic as though it were alive. Due to the sensitive nature of spying on characters, it's a command typically reserved for very few immortals on the MUD, and even then is employed judiciously. Snooping and controlling Nipics, however, is a privilege given to lower level immortals, so that they can take control of Nipics strategically in roleplay situations. You can imagine a quest where a Nipic controlled by a real player is interacting with characters. Having been there personally in the past, it can be pretty cool.

Basic support for snooping (and controlling--I'll just refer to it as snooping henceforth) was one of the earlier things I implemented, because I had a gut feeling that it would prompt architectural changes to the guts of the MUD. My gut was right; several layered pieces had to be inserted to make it all work.

Let's consider the case of another character, since it's somewhat simpler. When you snoop a character, you want just one thing: that all the text that is shown to that target character is also shown to you. Each "living" entity in the game is attached to a manager that handles inserting it into the world, removing it, and keeping track of what mode of interaction with the world it's currently in (the InteractionMode).

When a character sees some text, the writeString method of ConnectionManager is called. This in turn sends the text over the network socket, which then shows in the client's telnet session. The simple thing to do is to hook or shim this call so that it also sends the text to the snooper's manager (actually, to another interaction mode, but we'll get to that shortly). This is basically what I did. MobManager keeps track of a list of "output filters." Whenever writeString is called, it invokes a method on each OutputFilter that is subscribed to receive text from this manager. As long as you're connected to the MUD, you have one output filter that sends text back across the network to you. When you get snooped, a second output filter is added, that also sends the text to someone else. As a relatively minor detail, this is done for both input and output, so the snooper actually sees the exact commands that someone else types in.

I hinted that something is different about snooping Nipics. The obvious thing is that you not only get to see what they're seeing, but you also get to control them. Let's dig into this a little more. In a regular situation, when you type a command, it gets translated into an Invocation of some Command object and is ultimately excuted by calling the callCommand method of Command. Here's the code behind one such command:

def callCommand(invocation, intermode, args)
    if args.empty?()
        intermode.mob().seeText('What do you want to say?')
end # function call command

The intermode parameter here is always the interaction mode that produced the command, so even if you're snooping a Nipic, it would be your (the human's) interaction mode, not the Nipic's. But the mob() call... hmm... If we redirected that call to point to the target Nipic instead of ourselves as it normally does, then commands we type will affect the target Nipic instead. Turns out this is exactly what I did, and it works just as it's expected.

So I just mentioned overriding the mob method of the interaction mode. Since when you start snooping someone, you can't do anything yourself until you're done, this is a prime candidate for having an interaction mode. Indeed, when you snoop someone you enter the ControlInteractionMode, which shims the mob call and inserts the input and output filters into the target mob. When you type exit you leave the interaction mode and remove the filters.

My motto is, "there's always gotchas," and this is no exception. With the introduction of eventos, Nipics don't actually print out text at all for any eventos. Why would they need to? They don't care what the display text for an evento is; the relevant data for the evento is encoded in the object, and they fetch and act on that directly. This poses a problem for snooping: if the Nipic doesn't display text, then that text won't get forwarded to the snooper.

I fixed this much in the same way as the filters. Since the manager already receives and processes eventos--in this case, the NipicManager--it can now keep track of a list of evento listeners that should receive notifications of events. For characters, this listener just sends the text to the network. For Nipics, this listener currently does very little, but is the connection point for mobprogs. When a snooper enters the ControlInteractionMode, when it sets up the input filters, it also adds the same evento filter that characters get into the target Nipic's manager. The only thing this does is cause events to display text to the Nipic, like characters see. Then--cleverly, I feel--the output filter correctly forwards the text to the snooper.

What's left to do? Well, if you snoop a Nipic and use the "ooc" command, which sends an out-of-character message to all characters on the MUD, the MUD currently crashes, because the doOOC method is only implemented on Character, not Nipic. This makes sense, because Nipics have no out-of-character presence. Something in the ControlInteractionMode will probably need to ensure that you can't issue commands to Nipics that aren't valid.