"""
:mod:`websockets.legacy.framing` reads and writes WebSocket frames.

It deals with a single frame at a time. Anything that depends on the sequence
of frames is implemented in :mod:`websockets.legacy.protocol`.

See `section 5 of RFC 6455`_.

.. _section 5 of RFC 6455: http://tools.ietf.org/html/rfc6455#section-5

"""

import struct
from typing import Any, Awaitable, Callable, Optional, Sequence

from ..exceptions import PayloadTooBig, ProtocolError
from ..frames import Frame as NewFrame, Opcode


try:
    from ..speedups import apply_mask
except ImportError:  # pragma: no cover
    from ..utils import apply_mask


class Frame(NewFrame):
    @classmethod
    async def read(
        cls,
        reader: Callable[[int], Awaitable[bytes]],
        *,
        mask: bool,
        max_size: Optional[int] = None,
        extensions: Optional[Sequence["extensions.Extension"]] = None,
    ) -> "Frame":
        """
        Read a WebSocket frame.

        :param reader: coroutine that reads exactly the requested number of
            bytes, unless the end of file is reached
        :param mask: whether the frame should be masked i.e. whether the read
            happens on the server side
        :param max_size: maximum payload size in bytes
        :param extensions: list of classes with a ``decode()`` method that
            transforms the frame and return a new frame; extensions are applied
            in reverse order
        :raises ~websockets.exceptions.PayloadTooBig: if the frame exceeds
            ``max_size``
        :raises ~websockets.exceptions.ProtocolError: if the frame
            contains incorrect values

        """

        # Read the header.
        data = await reader(2)
        head1, head2 = struct.unpack("!BB", data)

        # While not Pythonic, this is marginally faster than calling bool().
        fin = True if head1 & 0b10000000 else False
        rsv1 = True if head1 & 0b01000000 else False
        rsv2 = True if head1 & 0b00100000 else False
        rsv3 = True if head1 & 0b00010000 else False

        try:
            opcode = Opcode(head1 & 0b00001111)
        except ValueError as exc:
            raise ProtocolError("invalid opcode") from exc

        if (True if head2 & 0b10000000 else False) != mask:
            raise ProtocolError("incorrect masking")

        length = head2 & 0b01111111
        if length == 126:
            data = await reader(2)
            (length,) = struct.unpack("!H", data)
        elif length == 127:
            data = await reader(8)
            (length,) = struct.unpack("!Q", data)
        if max_size is not None and length > max_size:
            raise PayloadTooBig(f"over size limit ({length} > {max_size} bytes)")
        if mask:
            mask_bits = await reader(4)

        # Read the data.
        data = await reader(length)
        if mask:
            data = apply_mask(data, mask_bits)

        frame = cls(fin, opcode, data, rsv1, rsv2, rsv3)

        if extensions is None:
            extensions = []
        for extension in reversed(extensions):
            frame = cls(*extension.decode(frame, max_size=max_size))

        frame.check()

        return frame

    def write(
        self,
        write: Callable[[bytes], Any],
        *,
        mask: bool,
        extensions: Optional[Sequence["extensions.Extension"]] = None,
    ) -> None:
        """
        Write a WebSocket frame.

        :param frame: frame to write
        :param write: function that writes bytes
        :param mask: whether the frame should be masked i.e. whether the write
            happens on the client side
        :param extensions: list of classes with an ``encode()`` method that
            transform the frame and return a new frame; extensions are applied
            in order
        :raises ~websockets.exceptions.ProtocolError: if the frame
            contains incorrect values

        """
        # The frame is written in a single call to write in order to prevent
        # TCP fragmentation. See #68 for details. This also makes it safe to
        # send frames concurrently from multiple coroutines.
        write(self.serialize(mask=mask, extensions=extensions))


# Backwards compatibility with previously documented public APIs
from ..frames import parse_close  # isort:skip # noqa
from ..frames import prepare_ctrl as encode_data  # isort:skip # noqa
from ..frames import prepare_data  # isort:skip # noqa
from ..frames import serialize_close  # isort:skip # noqa


# at the bottom to allow circular import, because Extension depends on Frame
from .. import extensions  # isort:skip # noqa