"""
Handlers
This module provides Handler classes for the position as well as the velocity
of particles. This is necessary when boundary conditions are imposed on the PSO
algorithm. Particles that do not stay inside these boundary conditions have to
be handled by either adjusting their position after they left the bounded
search space or adjusting their velocity when it would position them outside
the search space. In particular, this approach is important if the optimium of
a function is near the boundaries.
For the following documentation let :math:`x_{i, t, d}` be the :math:`d` th
coordinate of the particle :math:`i` 's position vector at the time :math:`t`,
:math:`lb` the vector of the lower boundaries and :math:`ub` the vector of the
upper boundaries.
The algorithms in this module are adapted from [SH2010]
[SH2010] Sabine Helwig, "Particle Swarms for Constrained Optimization",
PhD thesis, Friedrich-Alexander Universität Erlangen-Nürnberg, 2010.
"""
import inspect
import logging
import numpy as np
from ..utils.reporter import Reporter
[docs]class HandlerMixin(object):
""" A HandlerMixing class
This class offers some basic functionality for the Handlers.
"""
def _merge_dicts(self, *dict_args):
"""Backward-compatible helper method to combine two dicts"""
result = {}
for dictionary in dict_args:
result.update(dictionary)
return result
def _out_of_bounds(self, position, bounds):
"""Helper method to find indices of out-of-bound positions
This method finds the indices of the particles that are out-of-bound.
"""
lb, ub = bounds
greater_than_bound = np.nonzero(position > ub)
lower_than_bound = np.nonzero(position < lb)
return (lower_than_bound, greater_than_bound)
def _get_all_strategies(self):
"""Helper method to automatically generate a dict of strategies"""
return {
k: v
for k, v in inspect.getmembers(self, predicate=inspect.isroutine)
if not k.startswith(("__", "_"))
}
[docs]class BoundaryHandler(HandlerMixin):
[docs] def __init__(self, strategy):
""" A BoundaryHandler class
This class offers a way to handle boundary conditions. It contains
methods to repair particle positions outside of the defined boundaries.
Following strategies are available for the handling:
* Nearest:
Reposition the particle to the nearest bound.
* Random:
Reposition the particle randomly in between the bounds.
* Shrink:
Shrink the velocity of the particle such that it lands on the
bounds.
* Reflective:
Mirror the particle position from outside the bounds to inside the
bounds.
* Intermediate:
Reposition the particle to the midpoint between its current
position on the bound surpassing axis and the bound itself. This
only adjusts the axes that surpass the boundaries.
The BoundaryHandler can be called as a function to use the strategy
that is passed at initialization to repair boundary issues. An example
for the usage:
.. code-block :: python
from pyswarms.backend import operators as op
from pyswarms.backend.handlers import BoundaryHandler
bh = BoundaryHandler(strategy="reflective")
ops.compute_position(swarm, bounds, handler=bh)
By passing the handler, the :func:`compute_position()` function now has
the ability to reset the particles by calling the :code:`BoundaryHandler`
inside.
Attributes
----------
strategy : str
The strategy to use. To see all available strategies,
call :code:`BoundaryHandler.strategies`
"""
self.strategy = strategy
self.strategies = self._get_all_strategies()
self.rep = Reporter(logger=logging.getLogger(__name__))
self.memory = None
[docs] def __call__(self, position, bounds, **kwargs):
"""Apply the selected strategy to the position-matrix given the bounds
Parameters
----------
position : np.ndarray
The swarm position to be handled
bounds : tuple of :code:`np.ndarray` or list
a tuple of size 2 where the first entry is the minimum bound while
the second entry is the maximum bound. Each array must be of shape
:code:`(dimensions,)`
kwargs : dict
Returns
-------
numpy.ndarray
the adjusted positions of the swarm
"""
try:
new_position = self.strategies[self.strategy](
position, bounds, **kwargs
)
except KeyError:
message = "Unrecognized strategy: {}. Choose one among: " + str(
[strat for strat in self.strategies.keys()]
)
self.rep.logger.exception(message.format(self.strategy))
raise
else:
return new_position
[docs] def nearest(self, position, bounds, **kwargs):
r"""Set position to nearest bound
This method resets particles that exceed the bounds to the nearest
available boundary. For every axis on which the coordiantes of the particle
surpasses the boundary conditions the coordinate is set to the respective
bound that it surpasses.
The following equation describes this strategy:
.. math::
x_{i, t, d} = \begin{cases}
lb_d & \quad \text{if }x_{i, t, d} < lb_d \\
ub_d & \quad \text{if }x_{i, t, d} > ub_d \\
x_{i, t, d} & \quad \text{otherwise}
\end{cases}
"""
lb, ub = bounds
bool_greater = position > ub
bool_lower = position < lb
new_pos = np.where(bool_lower, lb, position)
new_pos = np.where(bool_greater, ub, new_pos)
return new_pos
[docs] def reflective(self, position, bounds, **kwargs):
r"""Reflect the particle at the boundary
This method reflects the particles that exceed the bounds at the respective
boundary. This means that the amount that the component which is orthogonal to
the exceeds the boundary is mirrored at the boundary. The reflection is repeated
until the position of the particle is within the boundaries. The following
algorithm describes the behaviour of this strategy:
.. math::
:nowrap:
\begin{gather*}
\text{while } x_{i, t, d} \not\in \left[lb_d,\,ub_d\right] \\
\text{ do the following:}\\
\\
x_{i, t, d} = \begin{cases}
2\cdot lb_d - x_{i, t, d} & \quad \text{if } x_{i,
t, d} < lb_d \\
2\cdot ub_d - x_{i, t, d} & \quad \text{if } x_{i,
t, d} > ub_d \\
x_{i, t, d} & \quad \text{otherwise}
\end{cases}
\end{gather*}
"""
lb, ub = bounds
lower_than_bound, greater_than_bound = self._out_of_bounds(
position, bounds
)
new_pos = position
while lower_than_bound[0].size != 0 or greater_than_bound[0].size != 0:
if lower_than_bound[0].size > 0:
new_pos[lower_than_bound] = 2 * lb[lower_than_bound[1]] - \
new_pos[lower_than_bound]
if greater_than_bound[0].size > 0:
new_pos[greater_than_bound] = 2 * ub[greater_than_bound[1]] - \
new_pos[greater_than_bound]
lower_than_bound, greater_than_bound = self._out_of_bounds(
new_pos, bounds
)
return new_pos
[docs] def shrink(self, position, bounds, **kwargs):
r"""Set the particle to the boundary
This method resets particles that exceed the bounds to the intersection
of its previous velocity and the boundary. This can be imagined as shrinking
the previous velocity until the particle is back in the valid search space.
Let :math:`\sigma_{i, t, d}` be the :math:`d` th shrinking value of the
:math:`i` th particle at the time :math:`t` and :math:`v_{i, t}` the velocity
of the :math:`i` th particle at the time :math:`t`. Then the new position is
computed by the following equation:
.. math::
:nowrap:
\begin{gather*}
\mathbf{x}_{i, t} = \mathbf{x}_{i, t-1} + \sigma_{i, t} \mathbf{v}_{i, t} \\
\\
\text{with} \\
\\
\sigma_{i, t, d} = \begin{cases}
\frac{lb_d-x_{i, t-1, d}}{v_{i, t, d}} & \quad \text{if } x_{i, t, d} < lb_d \\
\frac{ub_d-x_{i, t-1, d}}{v_{i, t, d}} & \quad \text{if } x_{i, t, d} > ub_d \\
1 & \quad \text{otherwise}
\end{cases} \\
\\
\text{and} \\
\\
\sigma_{i, t} = \min_{d=1...n} \sigma_{i, t, d}
\\
\end{gather*}
"""
if self.memory is None:
new_pos = position
self.memory = new_pos
else:
lb, ub = bounds
lower_than_bound, greater_than_bound = self._out_of_bounds(
position, bounds
)
velocity = position - self.memory
# Create a coefficient matrix
sigma = np.tile(1.0, position.shape)
sigma[lower_than_bound] = (
lb[lower_than_bound[1]] - self.memory[lower_than_bound]
) / velocity[lower_than_bound]
sigma[greater_than_bound] = (
ub[greater_than_bound[1]] - self.memory[greater_than_bound]
) / velocity[greater_than_bound]
min_sigma = np.amin(sigma, axis=1)
new_pos = position
new_pos[lower_than_bound[0]] = (
self.memory[lower_than_bound[0]]
+ np.multiply(
min_sigma[lower_than_bound[0]],
velocity[lower_than_bound[0]].T,
).T
)
new_pos[greater_than_bound[0]] = (
self.memory[greater_than_bound[0]]
+ np.multiply(
min_sigma[greater_than_bound[0]],
velocity[greater_than_bound[0]].T,
).T
)
self.memory = new_pos
return new_pos
[docs] def random(self, position, bounds, **kwargs):
"""Set position to random location
This method resets particles that exeed the bounds to a random position
inside the boundary conditions.
"""
lb, ub = bounds
lower_than_bound, greater_than_bound = self._out_of_bounds(
position, bounds
)
# Set indices that are greater than bounds
new_pos = position
new_pos[greater_than_bound[0]] = np.array(
[
np.array([u - l for u, l in zip(ub, lb)])
* np.random.random_sample((position.shape[1],))
+ lb
]
)
new_pos[lower_than_bound[0]] = np.array(
[
np.array([u - l for u, l in zip(ub, lb)])
* np.random.random_sample((position.shape[1],))
+ lb
]
)
return new_pos
[docs] def periodic(self, position, bounds, **kwargs):
r"""Sets the particles a periodic fashion
This method resets the particles that exeed the bounds by using the
modulo function to cut down the position. This creates a virtual,
periodic plane which is tiled with the search space.
The following equation describtes this strategy:
.. math::
:nowrap:
\begin{gather*}
x_{i, t, d} = \begin{cases}
ub_d - (lb_d - x_{i, t, d}) \mod s_d & \quad \text{if }x_{i, t, d} < lb_d \\
lb_d + (x_{i, t, d} - ub_d) \mod s_d & \quad \text{if }x_{i, t, d} > ub_d \\
x_{i, t, d} & \quad \text{otherwise}
\end{cases}\\
\\
\text{with}\\
\\
s_d = |ub_d - lb_d|
\end{gather*}
"""
lb, ub = bounds
lower_than_bound, greater_than_bound = self._out_of_bounds(
position, bounds
)
bound_d = np.tile(
np.abs(np.array(ub) - np.array(lb)), (position.shape[0], 1)
)
ub = np.tile(ub, (position.shape[0], 1))
lb = np.tile(lb, (position.shape[0], 1))
new_pos = position
if lower_than_bound[0].size != 0 and lower_than_bound[1].size != 0:
new_pos[lower_than_bound] = ub[lower_than_bound] - np.mod(
(lb[lower_than_bound] - new_pos[lower_than_bound]),
bound_d[lower_than_bound],
)
if greater_than_bound[0].size != 0 and greater_than_bound[1].size != 0:
new_pos[greater_than_bound] = lb[greater_than_bound] + np.mod(
(new_pos[greater_than_bound] - ub[greater_than_bound]),
bound_d[greater_than_bound],
)
return new_pos
[docs]class VelocityHandler(HandlerMixin):
[docs] def __init__(self, strategy):
""" A VelocityHandler class
This class offers a way to handle velocities. It contains
methods to repair the velocities of particles that exceeded the
defined boundaries. Following strategies are available for the handling:
* Unmodified:
Returns the unmodified velocites.
* Adjust
Returns the velocity that is adjusted to be the distance between the current
and the previous position.
* Invert
Inverts and shrinks the velocity by the factor :code:`-z`.
* Zero
Sets the velocity of out-of-bounds particles to zero.
"""
self.strategy = strategy
self.strategies = self._get_all_strategies()
self.rep = Reporter(logger=logging.getLogger(__name__))
self.memory = None
[docs] def __call__(self, velocity, clamp, **kwargs):
"""Apply the selected strategy to the velocity-matrix given the bounds
Parameters
----------
velocity : np.ndarray
The swarm position to be handled
clamp : tuple of :code:`np.ndarray` or list
a tuple of size 2 where the first entry is the minimum clamp while
the second entry is the maximum clamp. Each array must be of shape
:code:`(dimensions,)`
kwargs : dict
Returns
-------
numpy.ndarray
the adjusted positions of the swarm
"""
try:
new_position = self.strategies[self.strategy](
velocity, clamp, **kwargs
)
except KeyError:
message = "Unrecognized strategy: {}. Choose one among: " + str(
[strat for strat in self.strategies.keys()]
)
self.rep.logger.exception(message.format(self.strategy))
raise
else:
return new_position
def __apply_clamp(self, velocity, clamp):
"""Helper method to apply a clamp to a velocity vector"""
clamped_vel = velocity
min_velocity, max_velocity = clamp
lower_than_clamp = clamped_vel <= min_velocity
greater_than_clamp = clamped_vel >= max_velocity
clamped_vel = np.where(lower_than_clamp, min_velocity, clamped_vel)
clamped_vel = np.where(greater_than_clamp, max_velocity, clamped_vel)
return clamped_vel
[docs] def unmodified(self, velocity, clamp=None, **kwargs):
"""Leaves the velocity unchanged"""
if clamp is None:
new_vel = velocity
else:
if clamp is not None:
new_vel = self.__apply_clamp(velocity, clamp)
return new_vel
[docs] def adjust(self, velocity, clamp=None, **kwargs):
r"""Adjust the velocity to the new position
The velocity is adjusted such that the following equation holds:
.. math::
\mathbf{v_{i,t}} = \mathbf{x_{i,t}} - \mathbf{x_{i,t-1}}
.. note::
This method should only be used in combination with a position handling
operation.
"""
try:
if self.memory is None:
new_vel = velocity
self.memory = kwargs["position"]
else:
new_vel = kwargs["position"] - self.memory
self.memory = kwargs["position"]
if clamp is not None:
new_vel = self.__apply_clamp(new_vel, clamp)
except KeyError:
self.rep.logger.exception("Keyword 'position' missing")
raise
else:
return new_vel
[docs] def invert(self, velocity, clamp=None, **kwargs):
r"""Invert the velocity if the particle is out of bounds
The velocity is inverted and shrinked. The shrinking is determined by the
kwarg :code:`z`. The default shrinking factor is :code:`0.5`. For all
velocities whose particles are out of bounds the following equation is
applied:
.. math::
\mathbf{v_{i,t}} = -z\mathbf{v_{i,t}}
"""
try:
# Default for the shrinking factor
if "z" not in kwargs:
z = 0.5
else:
z = kwargs["z"]
lower_than_bound, greater_than_bound = self._out_of_bounds(
kwargs["position"], kwargs["bounds"]
)
new_vel = velocity
new_vel[lower_than_bound[0]] = (-z) * new_vel[lower_than_bound[0]]
new_vel[greater_than_bound[0]] = (-z) * new_vel[
greater_than_bound[0]
]
if clamp is not None:
new_vel = self.__apply_clamp(new_vel, clamp)
except KeyError:
self.rep.logger.exception("Keyword 'position' or 'bounds' missing")
raise
else:
return new_vel
[docs] def zero(self, velocity, clamp=None, **kwargs):
"""Set velocity to zero if the particle is out of bounds"""
try:
lower_than_bound, greater_than_bound = self._out_of_bounds(
kwargs["position"], kwargs["bounds"]
)
new_vel = velocity
new_vel[lower_than_bound[0]] = np.zeros(velocity.shape[1])
new_vel[greater_than_bound[0]] = np.zeros(velocity.shape[1])
except KeyError:
self.rep.logger.exception("Keyword 'position' or 'bounds' missing")
raise
else:
return new_vel