# Code Structure

## Legend for class diagrams

* **Abstract classes** are shown in the yellow boxes.
* **Concrete classes** are shown in the green boxes.
* **Inheritance** is denoted by an arrow.
* **Aggregation** is denoted by the a diamond and arrow.

## Application

:::{figure-md} fig-developer-classes-pylithapp
<img src="figs/classdiagram_pylithapp.*" alt="PyLithApp and its data member objects." width="80%" />

Diagram showing the relationships among objects associated with PyLithApp.
:::

## Problem

:::{figure-md} fig-developer-classes-problem
<img src="figs/classdiagram_problem.*" alt="Python and C++ Problem objects and their data members." width="100%" />

Diagram showing the relationships among the Python and C++ `Problem` objects and their data members.
:::

## Solution

:::{figure-md} fig-developer-classes-solution
<img src="figs/classdiagram_solution.*" alt="Python and C++ Solution objects and their data members." width="100%" />

Diagram showing the relationships among the Python and C++ `Solution` objects and their data members.
:::

## Physics and Finite-Element Objects

We separate the specification of the physics from the finite-element operations.
That is, we have one set of objects that specify the physics through materials, boundary conditions, and faults; another set of objects perform the finite-element operations required to solve the equations.
{numref}`fig-developer-physics-fem` illustrates this separation.
The user specifies the parameters for the `Physics` objects, which each create the appropriate integrator and/or constraint via factory methods.

:::{figure-md} fig-developer-physics-fem
<img src="figs/classdiagram_physics_fem.*" alt="Hierarchy of physics and corresponding finite-element objects." width="70%" />

Diagram showing the relationships among objects specifying the physics and the finite-element implementations.
:::

We generalize the finite-element operations into two main classes: `Integrator` and `Constraint`.
The `Integrator` is further separated into concrete classes for performing the finite-element integrations over pieces of the domain (`IntegratorDomain`), pieces of the domain boundary (`IntegratorBoundary`), and interior interfaces (`IntegratorInterface`).
We implement several kinds of constraints, corresponding to how the values of the constrained degrees of freedom are specified.
`ConstraintSpatialDB` gets values for the constrained degrees of freedom from a spatial database; `ConstraintUserFn` gets the values for the constrained degrees of freedom from a function (this object is widely used in tests); `ConstraintSimple` is a special case of `ConstraintUserFn` with the constrained degrees of freedom set programmatically using a label (this object is used for constraining the edges of the fault).

`Problem` holds the `Physics` objects as materials, boundary conditions, and interfaces.
During initialization of `Problem`, each `Physics` object creates any necessary `Integrator` and `Constraint` objects to implement the physics.
For example, a material will create an `IntegratorDomain` object that performs integration over that material's cells.

## Materials

:::{figure-md} fig-developer-classes-material
<img src="figs/classdiagram_material.*" alt="Hierarchy of materials related objects." width="60%" />

Diagram showing the relationships among objects associated with materials.
:::

## Boundary Conditions

:::{figure-md} fig-developer-classes-bc
<img src="figs/classdiagram_bc.*" alt="Hierarchy of boundary condition related objects." width="60%" />

Diagram showing the relationships among objects associated with boundary conditions.
:::

## Interior Interfaces (Faults)

:::{figure-md} fig-developer-classes-fault
<img src="figs/classdiagram_fault.*" alt="Hierarchy of fault related objects." width="60%" />

Diagram showing the relationships among objects associated with faults.
:::

## Mesh Importing

:::{figure-md} fig-developer-classes-mesher
<img src="figs/classdiagram_mesher.*" alt="Hierarchy of mesh generation and importing related objects." width="60%" />

Diagram showing the relationships among objects associated with mesh generation and importing.
:::

## Output

:::{figure-md} fig-developer-classes-output
<img src="figs/classdiagram_output.*" alt="Hierarchy of output related objects." width="100%" />

Diagram showing the relationships among objects associated with output.
:::