# Copyright (c) 2020 Paddle Quantum Authors. All Rights Reserved.
#
# 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.

"""
HGenerator
"""

from numpy import array, kron, trace
import scipy

__all__ = ["H_generator", ]


def H_generator():
    """
    Generate a Hamiltonian with trivial descriptions
    Returns: A Hamiltonian
    """

    beta = 1
    sigma_I = array([[1, 0], [0, 1]])
    sigma_Z = array([[1, 0], [0, -1]])

    H = (-kron(kron(sigma_Z, sigma_Z), sigma_I) - kron(
        kron(sigma_I, sigma_Z), sigma_Z) - kron(
            kron(sigma_Z, sigma_I), sigma_Z))

    rho = scipy.linalg.expm(-1 * beta *
                            H) / trace(scipy.linalg.expm(-1 * beta * H))

    return H.astype("complex64"), rho.astype("complex64")