Data Manipulation Basics
[Context: HL Kit … Basic Concepts and Terminology]
Overview
In order to handle the complexities of every game system, the HL engine is a very sophisticated tool. However, we've invested significant time and effort to keep things as simple and streamlined as possible. The net result is that the Kit makes extensive use of a few powerful mechanisms that give you complete control over both how data is manipulated and when it is manipulated.
Given the volume of information involved in many game systems, data manipulation entails the proper sequencing of tasks. The first key mechanism is the evaluation cycle that governs when the data is manipulated. The second key mechanism is a fast and flexible classification system, called tag expressions, to identify which objects are to be manipulated and which manipulations should be applied to them. The final key mechanism is the scripting language that allows the actual manipulation of the data.
Topics
Each of the above mechanisms is introduced in the sections below. An in-depth discussion of these mechanisms is provided in separate sections of the documentation.
Scripts
HL makes extensive use of scripts to allow the data file author substantial freedom and flexibility. In fact, scripting is such a fundamental and diverse topic that huge sections of the documentation are dedicated to various facets of writing scripts. This section merely provides a brief overview.
The scripting language syntax within the Kit is relatively simple. You can declare variables, assign values, perform simple conditional tests, and utilize a number of built-in intrinsic functions for various purposes. There is also a syntax that allows access to all of the objects within a given actors, such as the various picks and containers, plus the field values and tags that may be assigned to them. The language syntax itself is somewhat similar to the age-old Basic language. Using scripts, an author can pretty much do whatever is necessary to properly model the requirements of a given game system.
To make the writing of scripts easier, the Kit supports re-usable procedures. A procedure is nothing more than a mini-script that can be called from multiple places. The data files for each game system make extensive use of procedures so that many scripts can be reduced to simply calling one or two procedures to do all the work.
The starting data files provided by the Kit include numerous scripts and procedures that you can use, adapt, and extend according to the needs of the game system you're implementing.
Thing-Based Scripts
The most common type of script that you will find yourself writing the "eval script". Of all the different types of scripts, most are triggered in response to a specific action. However, an eval script is scheduled to occur at a specific phase and priority during the evaluation cycle, hence the name.
Every eval script is associated with a thing. This means that every pick derived from a given thing inherits all of the eval scripts for that thing.
Since an eval script is performed during evaluation processing, a separate task is always created for each eval script, which is then scheduled within the overall task list. If a thing is added to a container multiple times, each eval script must be processed separately for every pick. Consequently, separate eval script tasks are created and scheduled for every pick.
Eval scripts are the most commonly used script due to the ability to schedule them. So many facets of a complex game system are inter-dependent. Dependent calculations must be performed in a carefully ordered sequence to ensure that all of the game mechanics are accurately implemented within the data files. Eval scripts provide the means for this scheduling.
Thing-Based Rules
Sometimes, you don't need to apply a modification to anything and instead need to verify whether a required condition is satisfied. For example, the "Knowledge" skill might require that the user specify the domain to which the skill applies. This entails checking that the user has entered a suitable value.
To handle situations like this, the Kit provides "eval rules" as a companion to eval scripts, and you will likely find yourself writing a number of these as well. Just like eval scripts, eval rules are scheduled as tasks and performed with specific timing during the evaluation process. Unlike eval scripts, the purpose of an eval rule is to verify a condition, so an eval rule must always determine whether its condition is satisfied or not. If the rule is not satisfied, the corresponding pick is flagged as being invalid and an appropriate message is displayed to the user within the validation report.
Component-Based Scripts and Rules
There are many situations where you'll want the same script or rule to be applied to all things that derive from a particular component. For example, every piece of gear the character is wielding should be verified to not be stored in a backpack or some other container. The Kit makes it easy to handle situations like this by allowing you to actually define the script or rule directly on the component.
Whenever a script or rule is defined on a component, that script/rule is treated as if it were individually assigned to every thing that derives from the component. This means that a new task is automatically created and scheduled for the script/rule on every pick that is added to the character.
It is reasonably common to have both component scripts/rules and individual script/rules associated with a particular thing.
