import requests.exceptions


def seek_delimiter(file, delimiter, blocksize):
    """ Seek current file to next byte after a delimiter bytestring

    This seeks the file to the next byte following the delimiter.  It does
    not return anything.  Use ``file.tell()`` to see location afterwards.

    Parameters
    ----------
    file: a file
    delimiter: bytes
        a delimiter like ``b'\n'`` or message sentinel
    blocksize: int
        Number of bytes to read from the file at once.
    """

    if file.tell() == 0:
        return

    last = b''
    while True:
        current = file.read(blocksize)
        if not current:
            return
        full = last + current
        try:
            i = full.index(delimiter)
            file.seek(file.tell() - (len(full) - i) + len(delimiter))
            return
        except ValueError:
            pass
        last = full[-len(delimiter):]


def read_block(f, offset, length, delimiter=None):
    """ Read a block of bytes from a file

    Parameters
    ----------
    fn: string
        Path to filename on S3
    offset: int
        Byte offset to start read
    length: int
        Number of bytes to read
    delimiter: bytes (optional)
        Ensure reading starts and stops at delimiter bytestring

    If using the ``delimiter=`` keyword argument we ensure that the read
    starts and stops at delimiter boundaries that follow the locations
    ``offset`` and ``offset + length``.  If ``offset`` is zero then we
    start at zero.  The bytestring returned WILL include the
    terminating delimiter string.

    Examples
    --------

    >>> from io import BytesIO  # doctest: +SKIP
    >>> f = BytesIO(b'Alice, 100\\nBob, 200\\nCharlie, 300')  # doctest: +SKIP
    >>> read_block(f, 0, 13)  # doctest: +SKIP
    b'Alice, 100\\nBo'

    >>> read_block(f, 0, 13, delimiter=b'\\n')  # doctest: +SKIP
    b'Alice, 100\\nBob, 200\\n'

    >>> read_block(f, 10, 10, delimiter=b'\\n')  # doctest: +SKIP
    b'Bob, 200\\nCharlie, 300'
    """
    if delimiter:
        f.seek(offset)
        seek_delimiter(f, delimiter, 2**16)
        start = f.tell()
        length -= start - offset

        f.seek(start + length)
        seek_delimiter(f, delimiter, 2**16)
        end = f.tell()
        eof = not f.read(1)

        offset = start
        length = end - start

    f.seek(offset)
    bytes = f.read(length)
    return bytes


class HtmlError(Exception):
    """Holds the message and code from cloud errors."""
    def __init__(self, error_response=None):
        if error_response:
            self.message = error_response.get('message', '')
            self.code = error_response.get('code', None)
        else:
            self.message = ''
            self.code = None
        # Call the base class constructor with the parameters it needs
        super(HtmlError, self).__init__(self.message)

RETRIABLE_EXCEPTIONS = (
    requests.exceptions.ChunkedEncodingError,
    requests.exceptions.ConnectionError,
    requests.exceptions.ReadTimeout,
    requests.exceptions.Timeout,
    requests.exceptions.ProxyError,
    requests.exceptions.SSLError,
    requests.exceptions.ContentDecodingError
)


def is_retriable(exception):
    """Returns True if this exception is retriable."""
    errs = list(range(500, 505))
    errs += [str(e) for e in errs]
    if isinstance(exception, HtmlError):
        return exception.code in errs
    return isinstance(exception, RETRIABLE_EXCEPTIONS)
