# Copyright 2020 Google LLC
#
# 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
#
#     https://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.

import struct

# NOTE: ``__config__`` **must** be the first import because it (may)
#       modify the search path used to locate shared libraries.
import google_crc32c.__config__
import google_crc32c._crc32c_cffi
from google_crc32c._checksum import CommonChecksum


def extend(crc, chunk):
    """Update an existing CRC checksum with new chunk of data.

    Args
        crc (int): An existing CRC check sum.
        chunk (Union[bytes, List[int], Tuple[int]]): A new chunk of data.
            Intended to be a byte string or similar.

    Returns
        int: New CRC checksum computed by extending existing CRC
        with ``chunk``.
    """
    return google_crc32c._crc32c_cffi.lib.crc32c_extend(crc, chunk, len(chunk))


def value(chunk):
    """Compute a CRC checksum for a chunk of data.

    Args
        chunk (Union[bytes, List[int], Tuple[int]]): A new chunk of data.
            Intended to be a byte string or similar.

    Returns
        int: New CRC checksum computed for ``chunk``.
    """
    return google_crc32c._crc32c_cffi.lib.crc32c_value(chunk, len(chunk))


class Checksum(CommonChecksum):
    """Hashlib-alike helper for CRC32C operations.

    Args:
        initial_value (Optional[bytes]): the initial chunk of data from
            which the CRC32C checksum is computed.  Defaults to b''.
    """

    __slots__ = ("_crc",)

    def __init__(self, initial_value=b""):
        self._crc = value(initial_value)

    def update(self, chunk):
        """Update the checksum with a new chunk of data.

        Args:
            chunk (Optional[bytes]): a chunk of data used to extend
                the CRC32C checksum.
        """
        self._crc = extend(self._crc, chunk)
