# Copyright 2019 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 composite that breaks the problem into sub-problems corresponding to the
connected components of the binary
quadratic model graph before sending to its child sampler.
"""
from dimod.core.composite import ComposedSampler
from dimod.sampleset import SampleSet
from dimod.traversal import connected_components
__all__ = ['ConnectedComponentsComposite']
[docs]class ConnectedComponentsComposite(ComposedSampler):
"""Composite to decompose a problem to the connected components
and solve each.
Connected components of a bqm graph are computed (if not provided),
and each subproblem is passed to the child sampler.
Returned samples from each child sampler are merged. Only the best solution
of each response is pick and merge with others
(i.e. this composite returns a single solution).
Args:
sampler (:obj:`dimod.Sampler`):
A dimod sampler
Examples:
This example uses :class:`.ConnectedComponentsComposite` to instantiate a
composed sampler that submits a simple Ising problem to a sampler.
The composed sampler finds the connected components and solves each.
>>> h = {1: -1.3, 2: 2.3, 3:-1.2, 4: -0.5}
>>> J = {(1, 4): -0.6, (1, 3): 0.6, (3, 4): 1.0, (2, 3): -1.0}
>>> sampler = dimod.ConnectedComponentsComposite(dimod.ExactSolver())
>>> sampleset = sampler.sample_ising(h, J)
"""
def __init__(self, child_sampler):
self._children = [child_sampler]
@property
def children(self):
return self._children
@property
def parameters(self):
params = self.child.parameters.copy()
return params
@property
def properties(self):
return {'child_properties': self.child.properties.copy()}
[docs] def sample(self, bqm, components=None, **parameters):
"""Sample from the provided binary quadratic model.
Args:
bqm (:obj:`dimod.BinaryQuadraticModel`):
Binary quadratic model to be sampled from.
components (list(set)):
A list of disjoint set of variables that fully partition the variables
**parameters:
Parameters for the sampling method, specified by the child sampler.
Returns:
:obj:`dimod.SampleSet`
"""
# solve the problem on the child system
child = self.child
variables = bqm.variables
if components is None:
components = list(connected_components(bqm))
if isinstance(components, set):
components = [components]
sampleset = None
fixed_value = min(bqm.vartype.value)
for component in components:
bqm_copy = bqm.copy()
bqm_copy.fix_variables({i: fixed_value for i in (variables - component)})
if sampleset is None:
# here .truncate(1) is used to pick the best solution only. The other options
# for future development is to combine all sample with all.
# This way you'd get the same behaviour as the ExactSolver
sampleset = child.sample(bqm_copy, **parameters).truncate(1)
else:
sampleset = sampleset.truncate(1).append_variables(child.sample(bqm_copy, **parameters).truncate(1))
if sampleset is None:
return SampleSet.from_samples_bqm({}, bqm)
else:
return SampleSet.from_samples_bqm(sampleset, bqm)