# Copyright 2020 D-Wave Systems Inc.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
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"""A dimod composite that uses the steepest gradient descent to post-process
samples.
"""
import dimod
import greedy
__all__ = ['SteepestDescentComposite']
[docs]class SteepestDescentComposite(dimod.ComposedSampler):
"""Runs greedy local optimization (steepest descent) on input problem,
seeded with samples from the sampler.
Args:
child_sampler (:class:`dimod.Sampler`):
A dimod sampler, such as a :class:`~dwave.system.samplers.DWaveSampler`.
Examples:
>>> from dwave.system import DWaveSampler
>>> from greedy import SteepestDescentComposite
...
>>> sampler = SteepestDescentComposite(DWaveSampler())
>>> h = {0: -1, 4: 2}
>>> J = {(0, 4): 1.5}
>>> sampleset = sampler.sample_ising(h, J)
>>> sampleset.first.energy
-4.5
"""
def __init__(self, child_sampler):
self.children = [child_sampler]
# set the parameters
self.parameters = child_sampler.parameters.copy()
# set the properties
self.properties = dict(child_properties=child_sampler.properties.copy())
parameters = None # overwritten by init
"""dict[str, list]: Parameters in the form of a dict.
For an instantiated composed sampler, keys are the keyword parameters
accepted by the child sampler and parameters added by the composite.
"""
children = None # overwritten by init
"""list [child_sampler]: List containing the structured sampler."""
properties = None # overwritten by init
"""dict: Properties in the form of a dict.
Contains the properties of the child sampler.
"""
[docs] def sample(self, bqm, **parameters):
"""Sample from the provided binary quadratic model.
Args:
bqm (:class:`dimod.BinaryQuadraticModel`):
Binary quadratic model to be sampled from.
**parameters:
Parameters for the sampling method, specified by the child
sampler.
Returns:
:class:`dimod.SampleSet`
"""
# solve the problem on the child system
child = self.child
sampleset = child.sample(bqm, **parameters)
greedy_sampler = greedy.SteepestDescentSolver()
return greedy_sampler.sample(bqm, initial_states=sampleset)