# Copyright 2019 D-Wave Systems Inc.
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# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
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# http://www.apache.org/licenses/LICENSE-2.0
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# =============================================================================
"""A composite that uses the roof duality algorithm [#bht]_ [#bh]_ to fix some
variables in the binary quadratic model before passing it on to its child
sampler.
.. [#bht] Boros, E., P.L. Hammer, G. Tavares. Preprocessing of Unconstrained
Quadratic Binary Optimization. Rutcor Research Report 10-2006, April, 2006.
.. [#bh] Boros, E., P.L. Hammer. Pseudo-Boolean optimization. Discrete Applied
Mathematics 123, (2002), pp. 155-225
"""
from dimod.reference.composites.fixedvariable import FixedVariableComposite
from dimod.roof_duality import fix_variables
__all__ = ['RoofDualityComposite']
[docs]class RoofDualityComposite(FixedVariableComposite):
"""Uses roof duality to assign some variables before invoking child sampler.
Uses the :func:`~dimod.roof_duality.fix_variables` function to determine
variable assignments, then fixes them before calling the child sampler.
Returned samples include the fixed variables.
Args:
child (:obj:`dimod.Sampler`):
A dimod sampler. Used to sample the bqm after variables have been
fixed.
"""
@property
def parameters(self):
params = self.child.parameters.copy()
params['sampling_mode'] = []
return params
[docs] def sample(self, bqm, sampling_mode=True, **parameters):
"""Sample from the provided binary quadratic model.
Uses the :func:`~dimod.roof_duality.fix_variables` function to determine
which variables to fix.
Args:
bqm (:obj:`dimod.BinaryQuadraticModel`):
Binary quadratic model to be sampled from.
sampling_mode (bool, optional, default=True):
In sampling mode, only roof-duality is used. When
`sampling_mode` is false, strongly connected components are used
to fix more variables, but in some optimal solutions these
variables may take different values.
**parameters:
Parameters for the child sampler.
Returns:
:obj:`dimod.SampleSet`
"""
# use roof-duality to decide which variables to fix
parameters['fixed_variables'] = fix_variables(bqm, sampling_mode=sampling_mode)
return super(RoofDualityComposite, self).sample(bqm, **parameters)