from __future__ import annotations

import codecs
import queue
import threading
from typing import Any, Callable, Iterable, Iterator, Literal, overload

from ..exceptions import ConcurrencyError
from ..frames import BINARY, CONT, TEXT, Frame
from ..typing import Data
from .utils import Deadline


__all__ = ["Assembler"]

UTF8Decoder = codecs.getincrementaldecoder("utf-8")


class Assembler:
    """
    Assemble messages from frames.

    :class:`Assembler` expects only data frames. The stream of frames must
    respect the protocol; if it doesn't, the behavior is undefined.

    Args:
        pause: Called when the buffer of frames goes above the high water mark;
            should pause reading from the network.
        resume: Called when the buffer of frames goes below the low water mark;
            should resume reading from the network.

    """

    def __init__(
        self,
        high: int | None = None,
        low: int | None = None,
        pause: Callable[[], Any] = lambda: None,
        resume: Callable[[], Any] = lambda: None,
    ) -> None:
        # Serialize reads and writes — except for reads via synchronization
        # primitives provided by the threading and queue modules.
        self.mutex = threading.Lock()

        # Queue of incoming frames.
        self.frames: queue.SimpleQueue[Frame | None] = queue.SimpleQueue()

        # We cannot put a hard limit on the size of the queue because a single
        # call to Protocol.data_received() could produce thousands of frames,
        # which must be buffered. Instead, we pause reading when the buffer goes
        # above the high limit and we resume when it goes under the low limit.
        if high is not None and low is None:
            low = high // 4
        if high is None and low is not None:
            high = low * 4
        if high is not None and low is not None:
            if low < 0:
                raise ValueError("low must be positive or equal to zero")
            if high < low:
                raise ValueError("high must be greater than or equal to low")
        self.high, self.low = high, low
        self.pause = pause
        self.resume = resume
        self.paused = False

        # This flag prevents concurrent calls to get() by user code.
        self.get_in_progress = False

        # This flag marks the end of the connection.
        self.closed = False

    def get_next_frame(self, timeout: float | None = None) -> Frame:
        # Helper to factor out the logic for getting the next frame from the
        # queue, while handling timeouts and reaching the end of the stream.
        if self.closed:
            try:
                frame = self.frames.get(block=False)
            except queue.Empty:
                raise EOFError("stream of frames ended") from None
        else:
            try:
                # Check for a frame that's already received if timeout <= 0.
                # SimpleQueue.get() doesn't support negative timeout values.
                if timeout is not None and timeout <= 0:
                    frame = self.frames.get(block=False)
                else:
                    frame = self.frames.get(block=True, timeout=timeout)
            except queue.Empty:
                raise TimeoutError(f"timed out in {timeout:.1f}s") from None
        if frame is None:
            raise EOFError("stream of frames ended")
        return frame

    def reset_queue(self, frames: Iterable[Frame]) -> None:
        # Helper to put frames back into the queue after they were fetched.
        # This happens only when the queue is empty. However, by the time
        # we acquire self.mutex, put() may have added items in the queue.
        # Therefore, we must handle the case where the queue is not empty.
        frame: Frame | None
        with self.mutex:
            queued = []
            try:
                while True:
                    queued.append(self.frames.get(block=False))
            except queue.Empty:
                pass
            for frame in frames:
                self.frames.put(frame)
            # This loop runs only when a race condition occurs.
            for frame in queued:  # pragma: no cover
                self.frames.put(frame)

    # This overload structure is required to avoid the error:
    # "parameter without a default follows parameter with a default"

    @overload
    def get(self, timeout: float | None, decode: Literal[True]) -> str: ...

    @overload
    def get(self, timeout: float | None, decode: Literal[False]) -> bytes: ...

    @overload
    def get(self, timeout: float | None = None, *, decode: Literal[True]) -> str: ...

    @overload
    def get(self, timeout: float | None = None, *, decode: Literal[False]) -> bytes: ...

    @overload
    def get(self, timeout: float | None = None, decode: bool | None = None) -> Data: ...

    def get(self, timeout: float | None = None, decode: bool | None = None) -> Data:
        """
        Read the next message.

        :meth:`get` returns a single :class:`str` or :class:`bytes`.

        If the message is fragmented, :meth:`get` waits until the last frame is
        received, then it reassembles the message and returns it. To receive
        messages frame by frame, use :meth:`get_iter` instead.

        Args:
            timeout: If a timeout is provided and elapses before a complete
                message is received, :meth:`get` raises :exc:`TimeoutError`.
            decode: :obj:`False` disables UTF-8 decoding of text frames and
                returns :class:`bytes`. :obj:`True` forces UTF-8 decoding of
                binary frames and returns :class:`str`.

        Raises:
            EOFError: If the stream of frames has ended.
            UnicodeDecodeError: If a text frame contains invalid UTF-8.
            ConcurrencyError: If two coroutines run :meth:`get` or
                :meth:`get_iter` concurrently.
            TimeoutError: If a timeout is provided and elapses before a
                complete message is received.

        """
        with self.mutex:
            if self.get_in_progress:
                raise ConcurrencyError("get() or get_iter() is already running")
            self.get_in_progress = True

        # Locking with get_in_progress prevents concurrent execution
        # until get() fetches a complete message or times out.

        try:
            deadline = Deadline(timeout)

            # Fetch the first frame.
            frame = self.get_next_frame(deadline.timeout(raise_if_elapsed=False))
            with self.mutex:
                self.maybe_resume()
            assert frame.opcode is TEXT or frame.opcode is BINARY
            if decode is None:
                decode = frame.opcode is TEXT
            frames = [frame]

            # Fetch subsequent frames for fragmented messages.
            while not frame.fin:
                try:
                    frame = self.get_next_frame(
                        deadline.timeout(raise_if_elapsed=False)
                    )
                except TimeoutError:
                    # Put frames already received back into the queue
                    # so that future calls to get() can return them.
                    self.reset_queue(frames)
                    raise
                with self.mutex:
                    self.maybe_resume()
                assert frame.opcode is CONT
                frames.append(frame)

        finally:
            self.get_in_progress = False

        # This converts frame.data to bytes when it's a bytearray.
        data = b"".join(frame.data for frame in frames)
        if decode:
            return data.decode()
        else:
            return data

    @overload
    def get_iter(self, decode: Literal[True]) -> Iterator[str]: ...

    @overload
    def get_iter(self, decode: Literal[False]) -> Iterator[bytes]: ...

    @overload
    def get_iter(self, decode: bool | None = None) -> Iterator[Data]: ...

    def get_iter(self, decode: bool | None = None) -> Iterator[Data]:
        """
        Stream the next message.

        Iterating the return value of :meth:`get_iter` yields a :class:`str` or
        :class:`bytes` for each frame in the message.

        The iterator must be fully consumed before calling :meth:`get_iter` or
        :meth:`get` again. Else, :exc:`ConcurrencyError` is raised.

        This method only makes sense for fragmented messages. If messages aren't
        fragmented, use :meth:`get` instead.

        Args:
            decode: :obj:`False` disables UTF-8 decoding of text frames and
                returns :class:`bytes`. :obj:`True` forces UTF-8 decoding of
                binary frames and returns :class:`str`.

        Raises:
            EOFError: If the stream of frames has ended.
            UnicodeDecodeError: If a text frame contains invalid UTF-8.
            ConcurrencyError: If two coroutines run :meth:`get` or
                :meth:`get_iter` concurrently.

        """
        with self.mutex:
            if self.get_in_progress:
                raise ConcurrencyError("get() or get_iter() is already running")
            self.get_in_progress = True

        # Locking with get_in_progress prevents concurrent execution
        # until get_iter() fetches a complete message or times out.

        # If get_iter() raises an exception e.g. in decoder.decode(),
        # get_in_progress remains set and the connection becomes unusable.

        # Yield the first frame.
        frame = self.get_next_frame()
        with self.mutex:
            self.maybe_resume()
        assert frame.opcode is TEXT or frame.opcode is BINARY
        if decode is None:
            decode = frame.opcode is TEXT
        if decode:
            decoder = UTF8Decoder()
            yield decoder.decode(frame.data, frame.fin)
        else:
            # Convert to bytes when frame.data is a bytearray.
            yield bytes(frame.data)

        # Yield subsequent frames for fragmented messages.
        while not frame.fin:
            frame = self.get_next_frame()
            with self.mutex:
                self.maybe_resume()
            assert frame.opcode is CONT
            if decode:
                yield decoder.decode(frame.data, frame.fin)
            else:
                # Convert to bytes when frame.data is a bytearray.
                yield bytes(frame.data)

        self.get_in_progress = False

    def put(self, frame: Frame) -> None:
        """
        Add ``frame`` to the next message.

        Raises:
            EOFError: If the stream of frames has ended.

        """
        with self.mutex:
            if self.closed:
                raise EOFError("stream of frames ended")

            self.frames.put(frame)
            self.maybe_pause()

    # put() and get/get_iter() call maybe_pause() and maybe_resume() while
    # holding self.mutex. This guarantees that the calls interleave properly.
    # Specifically, it prevents a race condition where maybe_resume() would
    # run before maybe_pause(), leaving the connection incorrectly paused.

    # A race condition is possible when get/get_iter() call self.frames.get()
    # without holding self.mutex. However, it's harmless — and even beneficial!
    # It can only result in popping an item from the queue before maybe_resume()
    # runs and skipping a pause() - resume() cycle that would otherwise occur.

    def maybe_pause(self) -> None:
        """Pause the writer if queue is above the high water mark."""
        # Skip if flow control is disabled.
        if self.high is None:
            return

        assert self.mutex.locked()

        # Check for "> high" to support high = 0.
        if self.frames.qsize() > self.high and not self.paused:
            self.paused = True
            self.pause()

    def maybe_resume(self) -> None:
        """Resume the writer if queue is below the low water mark."""
        # Skip if flow control is disabled.
        if self.low is None:
            return

        assert self.mutex.locked()

        # Check for "<= low" to support low = 0.
        if self.frames.qsize() <= self.low and self.paused:
            self.paused = False
            self.resume()

    def close(self) -> None:
        """
        End the stream of frames.

        Calling :meth:`close` concurrently with :meth:`get`, :meth:`get_iter`,
        or :meth:`put` is safe. They will raise :exc:`EOFError`.

        """
        with self.mutex:
            if self.closed:
                return

            self.closed = True

            if self.get_in_progress:
                # Unblock get() or get_iter().
                self.frames.put(None)

            if self.paused:
                # Unblock recv_events().
                self.paused = False
                self.resume()
