source: branches/OKBJavaConnector/ECJClient/src/ec/README @ 9286

This README is with regard to the 'ec' package in this directory.  Various
subdirectories contain READMEs of their own for their respective packages.
If you're looking for overall documentation for ECJ, see the README file
for the parent directory of this one (that is, the 'ecj' directory).

The 'ec' package contains top-level abstract classes defining most of the
major elements of evolutionary or stochastic search processes.  You probably
don't want abstract classes: instead you may wish to examine the 'simple'
subpackage for concrete implementations of these classes which perform
basic evolutionary tasks.




DESIGN PATTERNS IN ECJ
----------------------

ECJ's basic classes sit on a set of simple interfaces defining what would now
be called 'design patterns'.  Nearly all classes in ECJ use one or more of
these interfaces, and together the interfaces are largely responsible for
ECJ's dynamic loading facilities.


ec.Setup

ec.Setup is ECJ's top-level design pattern interface.  Setup defines a single
method, setup(...), which acts as a sort of alternative constructor for
instances which implement it.  In ECJ most classes are loaded at runtime
from ECJ's ParameterDatabase.  Instances from such classes are dynamically
instantiated using their default (no-argument) constructor; the setup(...)
method then is called to give the objects a chance to set themselves up
properly from the ParameterDatabase.  This approach was largely done because
early versions of reflection in Java were inadequate.

ec.Prototype

Prototypes are Setups which are cloneable.  An class implementing Prototype
is meant to have a single instance of the class (the "prototypical instance")
loaded and setup from the Parameter Database; further instances of the class
are constructed by cloning the prototypical instance. A large number of 
ECJ objects are Prototype, including Individuals and various components of
Individuals (such as genetic programming tree nodes).

Many ECJ Prototypes support the so-called "flyweight" design pattern, whereby
large numbers of lightweight instances share a common single special object
which acts as a repository of default information common to all of them.
In Java this could be implemented using static members, but static members
are challenging to serialize (an important thing in ECJ), and so ECJ instead
uses an actual object instead.  

ec.Group

Groups are very similar to Prototypes: their primary difference is that there
is no prototypical Group: rather, new Groups can be cloned from currently
in-use Groups using the method emptyClone().  Examples of Groups include
Populations and Subpopulations.

ec.Singleton

Singletons are also Setups but are meant to be global objects: only one 
instance is ever loaded and setup during the course of a run.  Most top-level
objects (like ECJ's EvolutionState, Breeder, Evaluator, etc. objects) are
Singletons.

ec.Clique

Cliques are Setups which are small groups of instances of a single class.
Like Singletons, Cliques are global.  Once the collection of Cliques are loaded
and setup, no more are usually created.  Cliques are associated with globally
accessible tables which enable access to all members of the clique.  Cliques
include genetic programming function sets, genetic programming types, etc.



THE MAKEUP OF AN EVOLUTIONARY SYSTEM
------------------------------------

ec.Population
ec.Subpopulation
ec.Individual

ECJ's evolutionary system is centered around a single Population, which 
contains an array of Subpopulations.  Each Subpopulation contains an array of 
Individuals.  This structure is largely to support techniques such as 
coevolution which require more than one "population".  In ECJ such methods
use one or more Subpopulation class to support this.  Though you will likely
use Population and Subpopulation classes directly, almost certainly you'll
need to subclass Individual to define its representation.  A number of
predefined Individuals are available for you.

ec.Species

Individuals in ECJ support the flyweight pattern (see discussion of
ec.Prototype above) and ec.Species is their collective object.  Many Individuals
in ECJ share a common Species.  Each Subpopulation is also associated 
with the Species used by Individuals in that Subpopulation.  Note that 
multiple subpopulations, and different groups of Individuals, might 
use the same Species in theory.

The Species object contains the prototypical Individual from which other
Individuals of that Species are cloned; and also contain Breeding Pipelines
(see below) which enable Individuals to be bred from other Individuals of
that Species.

ec.Fitness

Each ECJ individual contains a Fitness.  Different kinds of individuals
can have different kinds of fitness objects (say, multiobjective fitness
objects, or single-objective GA-style fitness objects, etc.).  The Fitness
class is abstract: various Fitness subclasses perform different kinds of
Fitness concepts.




DOING EVOLUTION
---------------

ec.Evolve

This class is the entry point for ECJ (it contains the main() method), but
in fact it does relatively little: its main task is to load an EvolutionState
from the parameter database, then get it going.

ec.EvolutionState

ECJ's top-level evolutionary loop is handled by subclasses of EvolutionState.
EvolutionState also contains pointers to all the major elements of evolution,
and ultimately everything in the entire evolutionary system.  Thus the entire
process can be serialized (checkpointed) to disk simply by serializing
EvolutionState.  When ECJ creates an evolutionary process, it first constructs
an EvolutionState, then has the EvolutionState set up the rest of the process
from the parameter database.  Because EvolutionState holds everything, it is
often passed to many functions.

ec.Initializer
ec.Finisher

To create a random  population and begin evolution, EvolutionState calls upon a
subclass of Initializer. Likewise, just before quitting, EvolutionState
calls upon some subclass of Finisher to clean up.

ec.Evaluator
ec.Problem

EvolutionState calls upon Evaluator to evaluate the population.  Most
Evaluators are straightforward: go through the population and evaluate each
individual in turn.  But certain techniques require custom evaluation (such
as coevolution).  The evaluator relies on a Problem subclass to do its dirty
work: actually testing a given individual and assessing its fitness.  The
experimenter (you) is required to provide a Problem subclass for his problem.
Most of the examples in the "app" directory are largely Problem implementations.
Often the Evaluator can work in parallel, cloning several Problems, then
creating several threads, one thread per Problem, in order to assess the
population more rapidly.

ec.Breeder
ec.BreedingSource
ec.BreedingPipeline
ec.SelectionMethod

To breed a population to form a new population the EvolutionState relies on
a subclass of Breeder.  Most Breeders work by constructing a "breeding
pipeline" which selects individuals from the original population, mutates and
crosses them over, and produces new children.  The breeding pipeline typically
contains two kinds of objects: SelectionMethod objects, which pick individuals
from the old population; and BreedingPipeline objects, which receive
individuals from SelectionMethods or from other BreedingPipeline objects, and
then mutate or cross over the individuals.  Both BreedingPipeline and 
SelectionMethod objects are BreedingSources (things other BreedingPipelines
can get individuals from).  Many breeders can work in parallel,
constructing several breeding pipelines, one per thread, to speed up the task
of breeding individuals and forming the next generation.

ec.Exchanger

In between breeding and evaluation, the EvolutionState calls on a subclass
of Exchanger.  The default implementations do nothing at all, but certain
versions (like IslandExchange) take this opportunity to exchange individuals
with far-of processes on other machines.



STATISTICS
----------

ec.Statistics

ECJ relies on one or more subclasses of Statistics to write out statistics
information to various logs.  Statistics can be strung out in a chain, enabling
you to include as many different Statistics objects as you like.  Different
Statistics subclasses are available to do various tasks.  Statistics objects
are called at all sorts of times in the run, in-between nearly every major
operation, to give you a lot of hooks in which to probe the current state of
the system.