@martinjrobins/diffeq-js

Diffeq-js

This library provides a set of classes for solving differential-algebraic equations (DAEs) in JavaScript. It is a thin wrapper around a WebAssembly (WASM) runtime that is compiled from a domain specific language (DSL) for specifying DAE systems. Compilation of the DSL to WASM is performed on a remote server, so an active internet connection is required to use the library. Once compiled, the WASM module is returned to the client and can be used to solve the DAE system as many times as required.

For an online interactive demo of the library, see the diffeq-web website.

For an example of how to use the library directly, the following code solves a classic DAE testcase, the Robertson (1966) problem, which models the kinetics of an autocatalytic reaction, given by the following set of equations:

$$ \begin{align} \frac{dx}{dt} &= -0.04x + 10^4 y z \ \frac{dy}{dt} &= 0.04x - 10^4 y z - 3 \cdot 10^7 y^2 \ 0 &= x + y + z - 1 \end{align} $$

The javascript code to solve this problem is as follows:

import { compileModel, Options, Solver, Vector } from 'diffeq-js';

const code = `
in = [k1, k2, k3]
k1 { 0.04 }
k2 { 10000 }
k3 { 30000000 }
u_i {
  x = 1,
  y = 0,
  z = 0,
}
dudt_i {
  dxdt = 1,
  dydt = 0,
  dzdt = 0,
}
F_i {
  dxdt,
  dydt,
  0,
}
G_i {
  -k1 * x + k2 * y * z,
  k1 * x - k2 * y * z - k3 * y * y,
  1 - x - y - z,
}
out_i {
  x,
  y,
  z,
}`;

const model = compileModel(code).then((model) => {
  const options = new Options();

  // create solver with default options
  const solver = new Solver(options);

  // solve the model at k1 = 0.04, k2 = 1e4, k3 = 3e7
  const inputs = new Vector([0.04, 1e4, 3e7]);

  // create a vector to store the output
  const outputs = new Vector([]);

  // solve the model from t = 0 to t = 1e5
  const times = new Vector([0, 1e5]);

  // solve the model, afterwards times will contain the times at which the
  // solution was computed, and outputs will contain the solution itself
  // in a vector of length 3 * times.length, where the first 3 elements
  // are the solution at times[0], the next 3 elements are the solution at
  // times[1], etc.
  solver.solve(times, inputs, outputs);

  // The contents of times and outputs are stored in WASM linear memory.
  // To access the contents of the vectors, use the getFloat64Array method
  // which returns a Float64Array view of the vector's contents
  console.log('times', times.getFloat64Array());
  console.log('outputs', outputs.getFloat64Array());
});

Installation

This library is available on npm, and can be installed with the following command:

npm install @martinjrobins/diffeq-js

DiffSL Domain Specific Language (DSL)

Please see the language documentation

Compiling the DSL

The DSL is compiled to a WASM module using the compileModel function:

import { compileModel } from 'diffeq-js';

const code = `...`;
compileModel(code).then(() => {
  // create Solver, Vector etc here
});

The compileModel function requires an active internet connection as it sends the model code to a remote server for compilation to WASM. All the classes in the library are wrappers around corresponding classes in the WASM module, and so the compileModel function must be called successfully before any of the other classes can be used.

Options

The compileModel function takes an Options object as its argument. The contructor for the Options class takes the following arguments:

• print_stats - statistics about each solve are printed to the console after each successful call to solve. Default: false
• fixed_times - if false (the default), the solver will consider the first element of times to be the starting time point, and the second element to be the final time point, between these two times the solver will choose the time points to output (these are returned in the times vector). If true, the solver will only return solutions at the times specified in the input times vector. Default: false

Compilation errors

If the model code contains errors, the compileModel function will throw an error which is a string containing the error message. For example, if we try to compile the following code:

import { compileModel } from 'diffeq-js';

const code = `
k1 { 10 * k2 }
`;

compileModel(code).then(() => {
  // create Solver, Vector etc here
}).catch((error) => {
  console.log(error);
});

then the following error is thrown:

Line 1, Column 11: Error: cannot find variable k2
Line 1, Column 1: Error: tensor k1 has no elements
Line 1, Column 1: Error: missing 'u' array
Line 1, Column 1: Error: missing 'dudt' array
Line 1, Column 1: Error: missing 'F' array
Line 1, Column 1: Error: missing 'G' array
Line 1, Column 1: Error: missing 'out' array

Using diffeq from other languages

This library is only a thin wrapper around the WASM module and the remote server that provides the compilation service, and so it is possible to compile the diffeq DSL and use the provided WASM module directly from other languages.

The diffeq-js library uses the WASI browser shim to provide a WASI environment for the WASM module, and so it is possible to use the WASM module from other languages that support WASI directly, such as Rust or C, or from other WASM runtimes like the Wasmer SDK, which supports a wide range of languages including C, C++, C#, Go, Java, PHP, Python, Ruby, and Rust.

If you are interested in using the diffeq DSL from other languages, you could do one of the following:

• Contact the author directly, Martin Robinson, via email, to discuss your requirements and see if a collaboration is possible
• Consult the source code of the diffeq-js library, which is available on GitHub, and port the code to your language of choice

Contact

If you have any questions or comments, please file an issue on the GitHub repository or contact the author via email