update the docs

docs/source/problems.rst

@@ -1,27 +1,100 @@
 Adding a problem
 ================
 
-The easiest way to add a problem is to copy an existing problem setup
-in the solver you wish to use (in its problems/ sub-directory). Three
-different files will need to be copied (created):
-
-* ``problem.py``: this is the main initialization routine. The
-  function ``init_data()`` is called at runtime by the ``Simulation``
-  object's ``initialize()`` method. Two arguments are passed in, the
-  simulation's ``CellCenterData2d`` object and the
-  ``RuntimeParameters`` object.  The job of ``init_data()`` is to fill
-  all of the variables defined in the ``CellCenterData2d`` object.
-
-* ``_problem.defaults``: this contains the runtime parameters and
-  their defaults for your problem. They should be placed in a block
-  with the heading ``[problem]`` (where ``problem`` is your problem's
-  name). Anything listed here will be available through the
-  ``RuntimeParameters`` object at runtime.
-
-* ``inputs.problem``: this is the inputs file that is used at runtime
-  to set the parameters for your problem. Any of the general
-  parameters (like the grid size, boundary conditions, etc.) as well
-  as the problem-specific parameters can be set here.  Once the
-  problem is defined, you need to add the problem name to the
-  ``__all__`` list in the ``__init__.py`` file in the ``problems/``
-  sub-directory. This lets python know about the problem.
+Problem setups are defined in a ``problems/`` subdirectory under each
+solver.  For example, the problem setups for ``compressible`` are here:
+https://github.com/python-hydro/pyro2/tree/main/pyro/compressible/problems
+
+When you install pyro via ``pip``, these problem setups become available.
+At the moment, the way to add a new problem is to directly put files
+into these directories.
+
+.. tip::
+
+   If you are working on adding problems, it is
+   recommended that you install pyro from source and do it as an
+   `editable install
+   <https://setuptools.pypa.io/en/latest/userguide/development_mode.html>`_
+   as:
+
+   .. prompt:: bash
+
+      pip install -e .
+
+Every problem needs a python module of the form *problem_name.py*.
+This will define the runtime parameters that the problem expects
+and do the initialization of the state variables.
+
+Many problems will also provide an *inputs* file that overrides
+some of the runtime parameter defaults (like the domain size and BCs)
+to be appropriate for this problem.
+
+"`problem.py`"
+--------------
+
+A python module named after the problem (we'll call it ``problem.py`` here)
+provides the following:
+
+* ``PROBLEM_PARAMS`` : this is a dictionary (it can be empty, ``{}``)
+  that defines the runtime parameters that are needed to this problem
+  setup.  For instance, for the ``compressible`` ``sod`` problem:
+
+  .. code:: python
+
+     PROBLEM_PARAMS = {"sod.direction": "x",
+                       "sod.dens_left": 1.0,
+                       "sod.dens_right": 0.125,
+                       "sod.u_left": 0.0,
+                       "sod.u_right": 0.0,
+                       "sod.p_left": 1.0,
+                       "sod.p_right": 0.1}
+
+  Each key in the dictionary should be of the form
+  *problem-name.parameter*, and the values are the default value of
+  the parameter.
+
+  Any of these runtime parameters can be overridden in an inputs file
+  or on the commandline (when running via ``pyro_sim.py``) or via the
+  ``inputs_dict`` keyword argument (when running via the
+  :func:`Pyro <pyro.pyro_sim.Pyro>` class).
+
+* ``DEFAULT_INPUTS`` : this is the name of an inputs file to be
+  read in by default when using the ``Pyro`` class interface.  It
+  can be ``None``.
+
+  This is not used when running via ``pyro_sim.py``.
+
+* ``init_data()`` : this is the main initialization routine.  It has
+  the signature:
+
+  .. code:: python
+
+     def init_data(my_data, rp)
+
+  where
+
+  * ``my_data`` is a :func:`CellCenterData2d <pyro.mesh.patch.CellCenterData2d>` or :func:`FV2d <pyro.mesh.fv.FV2d>` object.  The ``Grid`` object can be obtained from this
+    as needed.
+
+  * ``rp`` is a :func:`RuntimeParameters <pyro.util.runparams.RuntimeParameters>` object.
+    Any of the runtime parameters (including the problem-specific ones
+    defined via ``PROBLEM_PARAMS``) can be accessed via this.
+
+  .. note::
+
+     The interface for ``init_data`` is the same for all solvers.
+
+  The job of ``init_data`` is to initialize the state data that is
+  managed by the ``my_data`` object passed in.  Exactly which variables
+  are included there will depend on the solver.
+
+* ``finalize()`` : this is called at the very end of evolution.  It
+  is meant to output instructions to the user on how the can analyze the
+  data.  It takes no arguments.
+
+.. important::
+
+   Once the problem is defined, you need to add the problem name to
+   the ``__all__`` list in the ``__init__.py`` file in the
+   ``problems/`` sub-directory. This lets python know about the
+   problem.