.. _developers-checksum:

Checksum regression tests
=========================

WarpX has checksum regression tests: as part of CI testing, when running a given test, the checksum module computes one aggregated number per field (``Ex_checksum = np.sum(np.abs(Ex))``) and compares it to a reference (benchmark). This should be sensitive enough to make the test fail if your PR causes a significant difference, print meaningful error messages, and give you a chance to fix a bug or reset the benchmark if needed.

The checksum module is located in ``Regression/Checksum/``, and the benchmarks are stored as human-readable `JSON <https://www.json.org/json-en.html>`__ files in ``Regression/Checksum/benchmarks_json/``, with one file per benchmark (for instance, test ``Langmuir_2d`` has a corresponding benchmark ``Regression/Checksum/benchmarks_json/Langmuir_2d.json``).

For more details on the implementation, the Python files in ``Regression/Checksum/`` should be well documented.

From a user point of view, you should only need to use ``checksumAPI.py``. It contains Python functions that can be imported and used from an analysis Python script. It can also be executed directly as a Python script. Here are recipies for the main tasks related to checksum regression tests in WarpX CI.

Include a checksum regression test in an analysis Python script
---------------------------------------------------------------

This relies on function ``evaluate_checksum``:

.. doxygenfunction:: evaluate_checksum

For an example, see

.. literalinclude:: ../../../Examples/analysis_default_regression.py
   :language: python

This can also be included in an existing analysis script. Note that the plotfile must be ``<test name>_plt?????``, as is generated by the CI framework.

Evaluate a checksum regression test from a bash terminal
--------------------------------------------------------

You can execute ``checksumAPI.py`` as a Python script for that, and pass the plotfile that you want to evaluate, as well as the test name (so the script knows which benchmark to compare it to).

.. code-block:: bash

   ./checksumAPI.py --evaluate --plotfile <path/to/plotfile> --test-name <test name>

See additional options

* ``--skip-fields`` if you don't want the fields to be compared (in that case, the benchmark must not have fields)
* ``--skip-particles`` same thing for particles
* ``--rtol`` relative tolerance for the comparison
* ``--atol`` absolute tolerance for the comparison (a sum of both is used by ``numpy.isclose()``)

Reset a benchmark from a plotfile you know is correct
-----------------------------------------------------

This is using ``checksumAPI.py`` as a Python script.

.. code-block:: bash

   ./checksumAPI.py --reset-benchmark --plotfile <path/to/plotfile> --test-name <test name>

See additional options

* ``--skip-fields`` if you don't want the benchmark to have fields
* ``--skip-particles`` same thing for particles

Since this will automatically change the JSON file stored on the repo, make a separate commit just for this file, and if possible commit it under the ``Tools`` name:

.. code-block:: bash

   git add <test name>.json
   git commit -m "reset benchmark for <test name> because ..." --author="Tools <warpx@lbl.gov>"