3. Solutions
=============

Representing a solution to a specific problem is very important in an optimisation process. In this example, we will always use the **knapsack problem** as a basis.

In a first step, the management of the solutions by the macop package will be presented. Then a specific implementation for the current problem will be detailed.

3.1. Generic Solution
~~~~~~~~~~~~~~~~~~~~~~~~~

Inside ``macop.solutions.base`` module of `Macop`, the ``Solution`` class is available. It's an abstract solution class structure which:

- stores the solution data representation into its ``data`` attribute
- get ``size`` (shape) of specific data representation
- stores the ``score`` of the solution once a solution is evaluated

Some specific methods are available:

.. code-block:: python

    class Solution():

        def __init__(self, data, size):
            """
            Abstract solution class constructor
            """
            ...

        def isValid(self, validator):
            """
            Use of custom function which checks if a solution is valid or not
            """
            ...

        def evaluate(self, evaluator):
            """
            Evaluate solution using specific `evaluator`
            """
            ...

        def fitness(self):
            """
            Returns fitness score
            """
            ...

        @staticmethod
        def random(size, validator=None):
            """
            Initialize solution using random data with validator or not
            """
            ...

        def clone(self):
            """
            Clone the current solution and its data, but without keeping evaluated `_score`
            """
            ...


From these basic methods, it is possible to manage a representation of a solution to our problem. 

Allowing to initialize it randomly or not (using constructor or ``random`` method), to evaluate it (``evaluate`` method) and to check some constraints of validation of the solution (``isValid`` method).

.. note::
    Only one of these methods needs specification if we create our own type of solution. This is the ``random`` method, which depends on the need of the problem.

We will now see how to define a type of solution specific to our problem.

3.2. Solution representation for knapsack
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We will now use the abstract ``Solution`` type available in the ``macop.solutions.base`` module in order to define our own solution.
First of all, let's look at the representation of our knapsack problem. **How to represent the solution?**

3.2.1. Knapsack solution
************************

A valid solution can be shown below where the sum of the object weights is 15 and the sum of the selected objects values is 8 (its fitness):

.. image:: ../_static/documentation/project_knapsack_solution.png
   :width:  800 px
   :align: center

Its representation can be translate as a **binary array** with value:

.. code-block::

    [1, 1, 0, 0, 1]

where selected objects have **1** as value otherwise **0**.

3.2.2. Binary Solution
**********************

We will now define our own type of solution by inheriting from ``macop.solutions.base.Solution``, which we will call ``BinarySolution``.

First we will define our new class as inheriting functionality from ``Solution`` (such as child class). 
We will also have to implement the ``random`` method to create a new random solution.

.. code-block:: python

    """
    modules imports
    """
    from macop.solutions.base import Solution
    import numpy as np

    class BinarySolution(Solution):
        
        @staticmethod
        def random(size, validator=None):

            # create binary array of specific size using numpy random module
            data = np.random.randint(2, size=size)
            # initialize new solution using constructor
            solution = BinarySolution(data, size)

            # check if validator is set
            if not validator:
                return solution

            # try to generate solution until solution validity (if validator is provided)
            while not validator(solution):
                data = np.random.randint(2, size=size)
                solution = BinarySolution(data, size)

            return solution

.. note::
    The current developed ``BinarySolution`` is available into ``macop.solutions.discrete.BinarySolution`` in **Macop**.

Using this new Solution representation, we can now generate solution randomly:

.. code-block:: python

    solution = BinarySolution.random(5)

In the next part, we will see how to verify that a solution meets certain modeling constraints of the problem.