# Source code for gemseo.problems.analytical.rastrigin

# Copyright 2021 IRT Saint Exupéry, https://www.irt-saintexupery.com
#
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# modify it under the terms of the GNU Lesser General Public
#
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# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
# Lesser General Public License for more details.
#
# You should have received a copy of the GNU Lesser General Public License
# along with this program; if not, write to the Free Software Foundation,
# Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
# Contributors:
#    INITIAL AUTHORS - API and implementation and/or documentation
#        :author: Damien Guenot
#        :author: Francois Gallard
#    OTHER AUTHORS   - MACROSCOPIC CHANGES
"""
The Rastrigin analytic problem
******************************
"""
from __future__ import annotations

import logging
from cmath import cos
from cmath import pi
from cmath import sin

from numpy import array
from numpy import full
from numpy import zeros

from gemseo.algos.design_space import DesignSpace
from gemseo.algos.opt_problem import OptimizationProblem
from gemseo.core.mdofunctions.mdo_function import MDOFunction

LOGGER = logging.getLogger(__name__)

[docs]class Rastrigin(OptimizationProblem):

r"""**Rastrigin** :class:.OptimizationProblem
uses the Rastrigin objective function
with the :class:.DesignSpace :math:[-0.1,0.1]^2

From http://en.wikipedia.org/wiki/Rastrigin_function:

the Rastrigin function is a non-convex function used as a
performance test problem for optimization algorithms.
It is a typical example of non-linear multimodal function.
It was first proposed by [Rastrigin] as a 2-dimensional
function and has been generalized by [MuhlenbeinEtAl].
Finding the minimum of this function is a fairly difficult
problem due to its large search space and its large
number of local minima.
It has a global minimum at :math:x=0 where :math:f(x)=0.
It can be extended to :math:n>2 dimensions:

.. math::

f(x) = 10n + \sum_{i=1}^n [x_i^2 - 10\cos(2\pi x_i)]

[Rastrigin] Rastrigin, L. A. "Systems of extremal control."
Mir, Moscow (1974).

[MuhlenbeinEtAl] H. Mühlenbein, D. Schomisch and J. Born.
"The Parallel Genetic Algorithm as Function Optimizer ".
Parallel Computing, 17, pages 619–632, 1991.
"""

def __init__(self):
design_space = DesignSpace()
design_space.set_current_value(full(2, 0.01))
super().__init__(design_space)
self.objective = MDOFunction(
self.rastrigin,
name="Rastrigin",
f_type="obj",
jac=self.rastrigin_jac,
expr="20 + sum(x[i]**2 - 10*cos(2pi*x[i]))",
args=["x"],
)

[docs]    @staticmethod
def rastrigin(x_dv):
"""Evaluate the 2nd order Rastrigin function.

Args:
x_dv: The design variables.

Returns:
The Rastrigin function output.
"""
a_c = 10.0
return (
a_c * 2.0
+ (x_dv[0] ** 2 - a_c * cos(2 * pi * x_dv[0]))
+ (x_dv[1] ** 2 - a_c * cos(2 * pi * x_dv[1]))
)

[docs]    @staticmethod
def get_solution():
"""Return theoretical optimal value of Rastrigin function.

Returns:
The design variable and objective function at optimum.
"""
return zeros(2), 0.0

[docs]    @staticmethod
def rastrigin_jac(x_dv):
"""Compute the analytical gradient of 2nd order Rastrigin function.

Args:
x_dv: The design variable vector.

Returns:
The analytical gradient vector of Rastrigin function.
"""
a_c = 10.0
return array(
[
2 * x_dv[0] + 2 * pi * a_c * sin(2 * pi * x_dv[0]),
2 * x_dv[1] + 2 * pi * a_c * sin(2 * pi * x_dv[1]),
]
).real