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Concepts

The goal of most constraints is to produce a Windows object through operations on profiles and activity instances. This is a conceptual guide to what they represent.

Profiles​

Profiles represent functions over time to a specific type, and usually come from mission model resources. For example, a "mission phase" resource might be simulated by the mission model and exposed to the constraints with intervals of time labelled as cruise, edl, and surface; a "battery charge" resource could be represented as a real number between 0 and 1 that varies with activities; etc. These resources can be referenced by calling Discrete.Resource("mission phase") or Real.Resource("battery charge") in the constraint, allowing you to transform them and do comparisons.

Real profiles are for integers and floating point numbers, and provide methods for some basic math operations like addition and derivatives. Discrete profiles are for everything else, like strings or objects.

Profiles can have gaps, or intervals where the value is unknown. This comes up most often when dealing with external datasets. In most cases it is best to apply a default value to a profile's gaps ASAP using the profile.assignGaps(defaultValue) method.

Windows​

Windows are like a boolean profile, augmented with some extra functionality. For many constraints, the final result is a Windows object, which tells Aerie what times are violations of the constraint. true means the state is nominal and false means the state is a violation. This means that you should describe the conditions you want to happen, not the conditions you don’t want to happen.

There are a few ways to calculate a Windows object from profiles, and many operations that can be done on them; including the traditional boolean and, or, and not. These are all in the API documentation. Like all profiles, Windows can have gaps too. However, some operations (such as converting to Spans or splitting segments) are not possible on gaps, so you'll be required to apply a default value using windows.assignGaps(boolean).

You can directly return your Windows object from the constraint function, and it will be automatically converted in to a Constraint object.

Spans​

Spans are designed to work around a limitation of Windows: if two true segments of a Windows object are transformed so that they touch, they are combined or "coalesced" into a single segment, and the knowledge that they were originally separate is lost. In situations where this is not acceptable (such as when working with activities that can overlap), Spans can be used. Spans are not a type of profile at all; instead they’re just a collection of intervals on the timeline. They share some operations available for Windows, but not all are valid (such as not).

Activity Instances​

Activity instances are accessible through two functions: Constraint.ForEachActivity and Spans.ForEachActivity. These functions provide you with a reference to each activity of a given type, and allow you to access the activity’s location in time as a Windows or Spans object, and exposes its parameters as profiles.

Other Constraint Types​

Not all constraints are based solely off of a Windows object. The main exceptions are constraints that deal with individual activity instances, because these will use the Constraint.ForEachActivity(…) function instead of returning a basic Windows. This will evaluate your Windows expression once for each instance of the given activity type, and associate any violations in those expressions with the activity instance it was evaluated on, which can be helpful in the UI to figure out which activity caused the violation.

Mental Model for Evaluation​

A constraint doesn't directly query simulation data, or directly return violations. Instead, your constraint code defines an expression to be interpreted by Aerie. The exact implementation details don’t matter for constraint authors, but for this reason you cannot directly inspect a profile's values or a plan's activities. This is also why there are no plans to support querying external profiles directly from a web request or filesystem access inside the constraint code. For that, see the external dataset documentation.