CEP27: toolkit Snippet

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+CEP 27 - Toolkit Capabilities Injection into an Archetype
+*********************************************************
+
+:CEP: 27
+:Title: Agent Toolkit Capabilities
+:Last-Modified: 2019-10-28
+:Author: Baptiste Mouginot <[email protected]>
+:Status: Active
+:Type: Standards Track
+:Created: Baptiste Mouginot
+
+
+Abstract
+========
+
+The |Cyclus| toolkit is designed to easily implement specific capabilities in
+newly developed archetypes, such as a trading policy, commodity producers, etc.
+To add characteristics to archetypes such as `Position` or `Metadata`, the
+actual implementation method is very verbose. It relies on adding the new
+specification in the arcehtype header, assignng it and use it in the cpp
+file. The developers have to manually ensure the consistency in the variable naming and
+implementation across multiple archetypes/files.
+This CEP explains introduces the concept of snippets to simplify and maintain consistency
+in the implementation and the usage, across multiple archetypes.
+
+
+Toolkit Implementation
+======================
+
+Each |Cyclus| toolkit component will contain 3 different files:
+- 2 for the definition of the feature C++ class (``cpp`` and header) that allows
+  the use of the capabilities, and optionally to register its values in the
+  output database,
+- a snippet definition file used to simplify the implementation and ensure
+  consistency accross its integration in the different archetypes.
+
+The snippet definition file will be included in the ``private`` section of the
+archetype header as: ``#include toolkit/my_snippet.cycpp.h``. (The use of the
+``cycpp.h`` has been chosen to allow syntax highlighting and inform developers
+that this is not a standard C++ header.)
+
+The snippet file, will contain the declaration of all the variables required
+to use the capabilities class:
+
+- the definition of the capability class as a member variable.
+
+- initialisation of the added variables.
+
+- (optional) if the capability requires/allows variable input from users,
+  standard |Cyclus| member variable declaration with variable ``#pragma`` is
+  required. In addition, to the standard variable declaration and the
+  ``#pragma`` the method also require a ``std::vector<int>
+  cycpp_shape_myvariable`` to be declared for each of the decorated variable
+  that are in the `toolkit/my_snippet.cycpp.h` file. (``cycpp preprocessor`` is
+  not able at the time to add them automatically for included files.)
+
+
+The main purpose of this include method would be to ensure consistency across
+archetypes using the same toolkit capability requiring user input, avoiding 1
+set of input syntax per archetypes for the same capability.
+
+If the toolkit features needs the capabilities to write in the output database a
+``RecordSnippet(Agent* agent)`` method will be implemented in the toolkit class to avoid
+multiplication of implementation in the each archetype using the feature.
+
+
+Archetypes Integration
+======================
+
+When the capability is integrated in an Archetype the following implementations
+have to be done:
+
+1. Include the snippet in the class header core: 
+   ``#include toolkit/my_snippet.cycpp,h``.
+
+2. (optional) In the ``Archetype::EnterNotify()``, initialise the toolkit class member
+   variables with variables.
+
+3. (optional) If required, call the ``RecordSnippet()`` method when necessary during the
+   Archetype operation.
+
+
+Class member vs Inheritance
+===========================
+
+With inheritance of the capability class, one will need to also modify the
+archetype class declaration in addition to simply including the snippet at the
+right place.
+This may also lead to confusion, as one can believe that the user input value
+for variable are passed in the constructor of the class and might lead the
+develop to skip the assignation of the value in the inherited class in the
+``EnterNotify``...
+
+Otherwise behavior would be very similar.
+
+Example:
+========
+
+
+Without Inheritance:
+--------------------
+``toolkit/my_snippet.cycpp.h``:
+.. highlight:: c
+    cyclus::toolkit::Position coordinates(0,0);
+
+    #pragma cyclus var { \
+        "default": 0.0, \
+        "uilabel": "Geographical latitude in degrees as a double", \
+        "doc": "Latitude of the agent's geographical position. The value should " \
+           "be expressed in degrees as a double." }
+    double latitude = 0;
+    // required for compilation but not added by the cycpp preprocessor...
+    std::vector<int> cycpp_shape_latitude;
+
+    #pragma cyclus var { \
+           "default": 0.0, \
+           "uilabel": "Geographical longitude in degrees as a double", \
+           "doc": "Longitude of the agent's geographical position. The value should" \
+           "be expressed in degrees as a double." }
+    double longitude = 0;
+    // required for compilation but not added by the cycpp preprocessor...
+    std::vector<int> cycpp_shape_longitude;
+
+``my_archetype_example.h``:
+.. highlight:: c
+    #include 'toolkit/Position.h'
+
+     class fun_archetype : public cyclus::facility{
+        public:
+        [...]
+        private:
+        [...]
+        #include "toolkit/my_snippet.cycpp.h"
+    }
+
+``my_archetype_example.cpp``:
+.. highlight:: c
+    void fun_archetype::EnterNotify() {
+        coordinates.set_position(latitude, longitude);
+        coordinates.RecordPosition(this);
+        [...]
+        }
+
+With Inheritance:
+-----------------
+``toolkit/my_snippet.cycpp.h``:
+.. highlight:: c
+    #pragma cyclus var { \
+        "default": 0.0, \
+        "uilabel": "Geographical latitude in degrees as a double", \
+        "doc": "Latitude of the agent's geographical position. The value should " \
+           "be expressed in degrees as a double." }
+    double latitude = 0;
+    // required for compilation but not added by the cycpp preprocessor...
+    std::vector<int> cycpp_shape_latitude;
+
+    #pragma cyclus var { \
+           "default": 0.0, \
+           "uilabel": "Geographical longitude in degrees as a double", \
+           "doc": "Longitude of the agent's geographical position. The value should" \
+           "be expressed in degrees as a double." }
+    double longitude = 0;
+    // required for compilation but not added by the cycpp preprocessor...
+    std::vector<int> cycpp_shape_longitude;
+
+``my_archetype_example.h``:
+.. highlight:: c
+    #include 'toolkit/Position.h'
+
+    class fun_archetype : public cyclus::facility, public Position {
+        public:
+        [...]
+        private:
+        [...]
+        #include "toolkit/my_snippet.cycpp.h"
+    }
+
+``my_archetype_example.cpp``:
+.. highlight:: c
+    void fun_archetype::EnterNotify() {
+        this.set_position(latitude, longitude);
+        this.RecordPosition(this);
+        [...]
+        }