Crate dexterior

dexterior provides building blocks for the discretization of partial differential equations using the mathematical framework of Discrete Exterior Calculus (DEC). These building blocks are sparse matrix operators (exterior derivative, Hodge star) and vector operands (cochains) associated with a simplicial mesh of any dimension. An effort is made to provide as many compile-time checks and type inference opportunities as possible to produce concise and correct code.

Additionally, dexterior provides some batteries-included tools for real-time visualization with wgpu in the optional visuals module. Enable the visuals feature to use them. These are less mature than the core library, currently only supporting 2D use cases.

Example

To give you a taste of the basic features available, the following code implements a simple acoustic wave simulation:

use dexterior as dex;

// type aliases for simulation variables ease readability
type Pressure = dex::Cochain<0, dex::Primal>;
type Velocity = dex::Cochain<1, dex::Primal>;

// meshes can be loaded from files generated with `gmsh`
let msh_bytes = include_bytes!("../examples/meshes/2d_square_pi_x_pi.msh");
let mesh = dex::gmsh::load_trimesh_2d(msh_bytes).expect("Failed to load mesh");

let dt = 0.1;
// generic operator expressed with its input and output type.
// type inference figures out which dimensions of `star` and `d` we need
// to make this expression match the type
let p_step: dex::Op<Velocity, Pressure> =
    -dt * mesh.star() * mesh.d() * mesh.star();
// type inference also knows which `d` we mean here
// because we multiply `p` by this operator later
let v_step = dt * mesh.d();

// integrate an initial state function into discrete cochains
let mut p: Pressure = mesh.integrate_cochain(|v| {
    f64::sin(3.0 * v[0].x) * f64::sin(2.0 * v[0].y)
});
let mut v: Velocity = mesh.new_zero_cochain();

// step the simulation forward in time
for _step in 0..10 {
    p += &p_step * &v;
    v += &v_step * &p;
}

For examples including visuals, see the examples in the repo.

Meshes

The core of a DEC discretization is the computation mesh, represented in the SimplicialMesh type. The type has one generic parameter representing the dimension of its embedding space and consequently the dimension of its highest-dimensional set of simplices.

Constructing a mesh

A mesh can be loaded from a .msh file generated with gmsh. 2D triangle meshes and 3D tetrahedral meshes are supported. See the gmsh module for details.

Alternatively, a mesh can be built manually from a list of vertices and a list of indices defining the highest-dimensional simplices.

Working with cochains and operators

Simulation variables in DEC-based simulations are expressed as cochains, which associate a value with every cell of a specific dimension in either the primal or the dual mesh. In this library, both the dimension and the association with the primal or dual mesh are expressed via generic parameters on SimplicialMesh methods. Dimension is expressed as a usize constant and the type of associated mesh as either Primal or Dual.

Say we have a two-dimensional mesh,

let mesh: SimplicialMesh<2> = some_mesh();

The following code constructs two cochains, the first associated with primal 1-simplices and the second with dual 0-simplices (i.e. vertices), on this mesh, with zeroes for all elements:

let primal_1_cochain = mesh.new_zero_cochain::<1, Primal>();
let dual_0_cochain = mesh.new_zero_cochain::<0, Dual>();

Operators

The two most important operators in DEC are the exterior derivative and Hodge star, which are constructed from a SimplicialMesh. They use generics similar to cochains to ensure compatible inputs and outputs.

Exterior derivative

The k-dimensional exterior derivative takes a k-cochain on either the primal or dual mesh and produces a k+1-cochain on the same mesh. Exterior derivative operators are constructed with SimplicialMesh::d; for example,

let primal_d_1 = mesh.d::<1, Primal>();

constructs a primal 1-dimensional exterior derivative. To apply it to a primal 1-cochain, we can use multiplication syntax:

let result: Cochain<2, Primal> = &primal_d_1 * &primal_1_cochain;

Hodge star

The Hodge star maps cochains from the primal mesh to the dual and vice versa. Hodge star operators are constructed with SimplicialMesh::star; for example,

let primal_star_1 = mesh.star::<1, Primal>();

constructs a Hodge star mapping primal 1-cochains to dual n-1-cochains, where n is the dimension of the mesh. The inverse of the k-dimensional star is the dual n-k-dimensional star, with a sign depending on the dimensions of the mesh and the star. Applying both in succession is equivalent to multiplication by (-1)^(k * (n - k)). Let us illustrate this with a 3D mesh:

let mesh: SimplicialMesh<3> = some_mesh();
let primal_1_cochain = mesh.new_zero_cochain::<1, Primal>();
assert_eq!(
    mesh.star::<2, Dual>() * mesh.star::<1, Primal>() * &primal_1_cochain,
    primal_1_cochain,
);

Operator composition

Most exterior calculus equations feature compositions of multiple operators, e.g. the 3-dimensional divergence ∇· = ٭d٭. Rather than operating on a cochain multiple times in succession with all these operators, it is desirable to first compose the operators into a single matrix. This can be done with multiplication syntax:

let divergence: ComposedOperator<_, _>
    = mesh.star::<3, Dual>()
    * mesh.d::<2, Dual>()
    * mesh.star::<1, Primal>();

The resulting type is an operator that takes the rightmost operator’s input and produces the leftmost operator’s output:

let input: Cochain<1, Primal> = mesh.new_zero_cochain();
let output: Cochain<0, Primal> = &divergence * &input;

This may seem verbose, and it is, but don’t worry – as we’ll see in the next section, most of these type annotations don’t actually need to be written.

Compile-time checks and type inference

When composing operators or applying them to cochains, the compatibility of the operands is checked at compile time. This is achieved via the Operator trait, which tracks the input and output cochain types of an operator. For example, all of the following would fail to compile:

mesh.d::<1, Primal>() * &mesh.new_zero_cochain::<2, Primal>();

mesh.star::<2, Dual>() * &mesh.new_zero_cochain::<2, Primal>();

mesh.star::<0, Dual>() * &mesh.d::<1, Primal>();

Thanks to these constraints, the compiler can infer the types of most generic expressions. The earlier divergence example could be written like this:

let divergence = mesh.star() * mesh.d() * mesh.star::<1, Primal>();

or like this, annotating the resulting ComposedOperator’s input and output types instead:

let divergence: ComposedOperator<Cochain<1, Primal>, Cochain<0, Primal>>
    = mesh.star() * mesh.d() * mesh.star();

A common pattern is to give Cochains named type aliases based on the simulation variables they represent, which enables nice definitions like this one from the membrane example (using the Op type alias for brevity):

type Pressure = Cochain<0, Primal>;
type Velocity = Cochain<1, Primal>;

struct Ops {
    p_step: Op<Velocity, Pressure>,
    v_step: Op<Pressure, Velocity>,
}

let ops = Ops {
    p_step: mesh.star() * mesh.d() * mesh.star(),
    v_step: mesh.d().into(),
};

In addition to the compatibility of operands, the existence of cochains and operators is also ensured at compile time. This means you cannot construct cochains or Hodge stars of higher dimension than your mesh or exterior derivatives whose output dimension would be higher than the mesh’s (i.e. the maximum dimension for d is n-1). For a 2D mesh, none of the following would compile:

let c = mesh.new_zero_cochain::<3, Primal>();

let star = mesh.star::<3, Primal>();

let d = mesh.d::<2, Primal>();

Visualization

Real-time visualization of the computation mesh and values of cochains is available in the dexterior-visuals crate. Currently only a few specific cases are supported; more to come later. See the examples in the repo for usage.

Modules

• cochain
  Cochains, i.e. values assigned to elements of a mesh.
• gmsh
  Utilities for loading meshes generated with gmsh.
• mesh
  The core discretization structure of DEC, the (simplicial) mesh.
• operator
  Composable operators for doing math on Cochains.

Structs

• ComposedOperator
  A composition of one or more Operators.
• Dual
  Marker type indicating a Cochain or operator corresponds to a dual mesh.
• ExteriorDerivative
  The exterior derivative, also known as the coboundary operator.
• HodgeStar
  A diagonal Hodge star operator.
• Primal
  Marker type indicating a Cochain or operator corresponds to a primal mesh.
• SimplexIter
  Iterator over a set of DIM-simplices in a mesh.
• SimplexView
  A view into a single simplex’s data.
• SimplicialMesh
  A DEC mesh where the primal cells are all simplices (points, line segments, triangles, tetrahedra etc).

Traits

• Operator
  Trait enabling operator composition checked for compatibility at compile time.

Type Aliases

• Cochain
  A vector of values corresponding to a set of k-dimensional cells on a mesh.
• Op
  A type alias for ComposedOperator to make common patterns more convenient to type.