# This code is part of Qiskit.
#
# (C) Copyright IBM 2017, 2021.
#
# This code is licensed under the Apache License, Version 2.0. You may
# obtain a copy of this license in the LICENSE.txt file in the root directory
# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
#
# Any modifications or derivative works of this code must retain this
# copyright notice, and modified files need to carry a notice indicating
# that they have been altered from the originals.

"""
Mixin for linear operator interface.
"""

from abc import ABC, abstractmethod

from .multiply import MultiplyMixin


class LinearMixin(MultiplyMixin, ABC):
    """Abstract Mixin for linear operator.

    This class defines the following operator overloads:

        - ``+`` / ``__add__``
        - ``-`` / ``__sub__``
        - ``*`` / ``__rmul__`
        - ``/`` / ``__truediv__``
        - ``__neg__``

    The following abstract methods must be implemented by subclasses
    using this mixin

        - ``_add(self, other, qargs=None)``
        - ``_multiply(self, other)``
    """

    def __add__(self, other):
        # enable easy use of sum(...)
        if not isinstance(other, type(self)) and other == 0:
            return self

        qargs = getattr(other, "qargs", None)
        return self._add(other, qargs=qargs)

    def __radd__(self, other):
        # enable easy use of sum(...)
        if not isinstance(other, type(self)) and other == 0:
            return self

        qargs = getattr(other, "qargs", None)
        return self._add(other, qargs=qargs)

    def __sub__(self, other):
        qargs = getattr(other, "qargs", None)
        return self._add(-other, qargs=qargs)

    def __rsub__(self, other):
        qargs = getattr(other, "qargs", None)
        return (-self)._add(other, qargs=qargs)

    @abstractmethod
    def _add(self, other, qargs=None):
        """Return the CLASS self + other.

        If ``qargs`` are specified the other operator will be added
        assuming it is identity on all other subsystems.

        Args:
            other (CLASS): an operator object.
            qargs (None or list): optional subsystems to add on
                                  (Default: None)

        Returns:
            CLASS: the CLASS self + other.
        """