Source code for dimod.reference.composites.spin_transform

# Copyright 2018 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,
#    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
#    See the License for the specific language governing permissions and
#    limitations under the License.
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# ================================================================================================
"""
On the D-Wave system, coupling :math:`J_{i,j}` adds a small bias to qubits :math:`i` and
:math:`j` due to leakage. This can become significant for chained qubits. Additionally,
qubits are biased to some small degree in one direction or another.
Applying a spin-reversal transform can improve results by reducing the impact of possible
analog and systematic errors. A spin-reversal transform does not alter the Ising problem;
the transform simply amounts to reinterpreting spin up as spin down, and visa-versa, for
a particular spin.
"""
from random import random
import itertools

import numpy as np

from dimod.core.composite import Composite
from dimod.core.sampler import Sampler
from dimod.core.structured import Structured
from dimod.sampleset import SampleSet, concatenate
from dimod.vartypes import Vartype

__all__ = ['SpinReversalTransformComposite']


class SpinReversalTransformComposite(Sampler, Composite):
    """Composite for applying spin reversal transform preprocessing.

    Spin reversal transforms (or "gauge transformations") are applied
    by flipping the spin of variables in the Ising problem. After
    sampling the transformed Ising problem, the same bits are flipped in the
    resulting sample [#km]_.

    Args:
        sampler: A `dimod` sampler object.

    Examples:
        This example composes a dimod ExactSolver sampler with spin transforms then
        uses it to sample an Ising problem.

        >>> # Compose the sampler
        >>> base_sampler = dimod.ExactSolver()
        >>> composed_sampler = dimod.SpinReversalTransformComposite(base_sampler)
        >>> base_sampler in composed_sampler.children
        True
        >>> # Sample an Ising problem
        >>> response = composed_sampler.sample_ising({'a': -0.5, 'b': 1.0}, {('a', 'b'): -1})
        >>> print(next(response.data()))           # doctest: +SKIP
        Sample(sample={'a': 1, 'b': 1}, energy=-1.5)

    References
    ----------
    .. [#km] Andrew D. King and Catherine C. McGeoch. Algorithm engineering
        for a quantum annealing platform. https://arxiv.org/abs/1410.2628,
        2014.

    """
    children = None
    parameters = None
    properties = None

    def __init__(self, child):
        self.children = [child]

        if isinstance(child, Structured):
            # todo something like Structured.__init__(self)
            raise NotImplementedError

        self.parameters = parameters = {'spin_reversal_variables': []}
        parameters.update(child.parameters)

        self.properties = {'child_properties': child.properties}

    def sample(self, bqm, num_spin_reversal_transforms=2, spin_reversal_variables=None, **kwargs):
        """Sample from the binary quadratic model.

        Args:
            bqm (:obj:`~dimod.BinaryQuadraticModel`):
                Binary quadratic model to be sampled from.

            num_spin_reversal_transforms (integer, optional, default=2):
                Number of spin reversal transform runs.

            spin_reversal_variables (list/dict, optional):
                Deprecated and no longer functional.

        Returns:
            :obj:`.SampleSet`

        Examples:
            This example runs 100 spin reversals applied to one variable of a QUBO problem.

            >>> import dimod
            ...
            >>> base_sampler = dimod.ExactSolver()
            >>> composed_sampler = dimod.SpinReversalTransformComposite(base_sampler)
            >>> Q = {('a', 'a'): -1, ('b', 'b'): -1, ('a', 'b'): 2}
            >>> response = composed_sampler.sample_qubo(Q,
            ...               num_spin_reversal_transforms=100,
            ...               spin_reversal_variables={'a'})
            >>> len(response)
            400
            >>> print(next(response.data()))           # doctest: +SKIP
            Sample(sample={'a': 0, 'b': 1}, energy=-1.0)

        """

        if spin_reversal_variables is not None:
            # this kwarg does not actually make sense for multiple SRTs. To
            # get the same functionality a user should apply them by hand
            # to their BQM before submitting.
            import warnings
            warnings.warn("'spin_reversal_variables' kwarg is deprecated and no longer functions.",
                          DeprecationWarning)

        # make a main response
        responses = []

        flipped_bqm = bqm.copy()
        transform = {v: False for v in bqm.variables}

        for ii in range(num_spin_reversal_transforms):
            # flip each variable with a 50% chance
            for v in bqm:
                if random() > .5:
                    transform[v] = not transform[v]
                    flipped_bqm.flip_variable(v)

            flipped_response = self.child.sample(flipped_bqm, **kwargs)

            tf_idxs = [flipped_response.variables.index(v)
                       for v, flip in transform.items() if flip]

            if bqm.vartype is Vartype.SPIN:
                flipped_response.record.sample[:, tf_idxs] = -1 * flipped_response.record.sample[:, tf_idxs]
            else:
                flipped_response.record.sample[:, tf_idxs] = 1 - flipped_response.record.sample[:, tf_idxs]

            responses.append(flipped_response)

        return concatenate(responses)