add developer documentation and design principles

Docs/source/developer/design_principles.rst

@@ -0,0 +1,134 @@
+Design Principles
+=================
+
+This document describes fundamental design choices of PICMI.
+
+Main Structure
+--------------
+
+PICMI defines a Python interface to specify a PIC input parameter set.
+It is not a pure schema, i. e. can do computation.
+
+In practical terms, PICMI defines a set of classes which implementations
+must inherit from with well-defined names. These classes’ main purpose
+is to provide a common user experience.
+
+Semantics
+---------
+
+PICMI is (aims to be) well-defined enough to compare codes.
+
+Scope
+-----
+
+PICMI aims to define all parameters for most PIC scenarios. Obscure
+parameters may be implemented by codes when needed.
+
+   Rule of thumb: If multiple codes implement a feature, put it into
+   PICMI.
+
+Grouping Principles
+-------------------
+
+Parameters defining physics are separated from parameters defining
+numerics (i. e. how these physics are represented in the simulation).
+
+Parameters should be optional if possible.
+
+Parameters are grouped conveniently for the user: Classes may contain more
+than the *minimal required set of parameters*, or in other terms:
+Forming sub-groups of parameters is only done when **convenient**, not
+mereley because sub-grouping would be possible.
+
+One PICMI object *should* represent one physical thing.
+
+Computations inside PICMI
+-------------------------
+
+PICMI defines a parameter structure. Convenient computations are
+performed automatically.
+
+   "Computations" here are filling in different representations of the
+   same input data.
+
+Redundancies are allowed, though should be limited to different representations of a physical concept (e.g. laser a0/e0).
+Inconsistencies must be caught by checks when invoked explicitly.
+
+Python Features
+---------------
+
+PICMI is implemented using Python. This section addresses some practical
+approaches to the behavior and treatment of PICMI objects.
+
+PICMI Object Lifecycle
+~~~~~~~~~~~~~~~~~~~~~~
+
+0. Parameters are collected/loaded/created by the user.
+1. PICMI object is constructed, **all** parameters are passed.
+   Automated computations are invoked.
+   Their results and the original parameters are stored to be retrievable by the code.
+   Note that the variables are exposed to the implementing code, not necessarily to the user.
+2. The implementing code extracts these parameters.
+
+PICMI objects mainly hold data and are not designed to be modified
+after they have been initialized with their content.
+
+Note that even if not recommended direct access to member variables
+of objects is still possible.
+Accessing the member variables directly (both reading and writing) results in **undefined behavior**.
+
+Implementing codes may invoke checks on PICMI objects to ensure their integrity.
+
+Types
+~~~~~
+
+For simplicity **strong typing** is used, i.e. variables are checked
+against a finite list of type specifications. However, this finite list
+of type specifications is kept **permissive**, including (1) general
+types (e.g. iterable instead of "list"), and (2) common library types
+(e.g. numpy types).
+
+Identifying Objects
+~~~~~~~~~~~~~~~~~~~
+
+Two PICMI objects are considered to be equal (and hence should yield the
+same simulation initialization) if they fullfill the Python equals
+operator (``__eq__()``, invoked by ``==``).
+
+PICMI reserves the option to redefine ``__eq__()`` to follow either of
+these approaches:
+
+- Two PICMI objects are considered equal if all their attributes are equal (dataclass approach).
+- Two PICMI objects are considered equal if they point to the same object,
+  i.e. the same memory region (python ``is`` operator, python vanilla for custom objects).
+
+Friendly towards Implementations
+--------------------------------
+
+It should be *easy* to implement PICMI.
+
+Extensible
+~~~~~~~~~~
+
+PICMI passes through all arguments (including codespecific variables)
+which can be handled by implementations.
+
+It is not a library, i. e. it does not provide functionality to be used
+by the implementation.
+
+Safety
+~~~~~~
+
+PICMI aims to provide a well defined interface to implementing codes.
+PICMI guarantees the integrity of its structure by providing ``check()``
+methods, which can be called explicitly.
+
+PICMI is **predictable** in the sense that any variable provided by the
+user is passed through to the implementation using **the same name**.
+
+Implementations are not Bound by PICMI
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Implementations may extend PICMI at arbitrary points. PICMI provides
+simple interfaces for that. Implementations may only implement PICMI
+partially.

Docs/source/developer/developer.rst

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+Developer Documentation
+=======================
+
+This section is aimed at the developer of PICMI and guide them through general design and internal functionality of PICMI itself, i.e. the python module ``picmistandard``.
+It does **not** explain how to implement PICMI in an existing PIC simulation.
+
+.. toctree::
+   :maxdepth: 1
+
+   design_principles

Docs/source/index.rst

@@ -31,3 +31,4 @@ For more details on the standard and how to use it, see the links below.
 
    how_to_use/how_to_use.rst
    standard/standard.rst
+   developer/developer.rst