# This code is part of Qiskit.
#
# (C) Copyright IBM 2020, 2023.
#
# 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.

"""ConverterBase Class"""

from abc import ABC, abstractmethod

from qiskit.opflow.operator_base import OperatorBase
from qiskit.utils.deprecation import deprecate_func


class ConverterBase(ABC):
    r"""
    Deprecated: Converters take an Operator and return a new Operator, generally isomorphic
    in some way with the first, but with certain desired properties. For example,
    a converter may accept ``CircuitOp`` and return a ``SummedOp`` of
    ``PauliOps`` representing the circuit unitary. Converters may not
    have polynomial space or time scaling in their operations. On the contrary, many
    converters, such as a ``MatrixExpectation`` or ``MatrixEvolution``, which convert
    ``PauliOps`` to ``MatrixOps`` internally, will require time or space exponential
    in the number of qubits unless a clever trick is known (such as the use of sparse
    matrices)."""

    @deprecate_func(
        since="0.24.0",
        additional_msg="For code migration guidelines, visit https://qisk.it/opflow_migration.",
    )
    def __init__(self) -> None:
        pass

    @abstractmethod
    def convert(self, operator: OperatorBase) -> OperatorBase:
        """Accept the Operator and return the converted Operator

        Args:
            operator: The Operator to convert.

        Returns:
            The converted Operator.

        """
        raise NotImplementedError
