# Copyright 2021 The Pigweed Authors
#
# 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.
"""Provides a simple interface for transferring bulk data over pw_rpc."""

import abc
import asyncio
import logging
import threading
from typing import Any, Callable, Dict, Optional

from pw_rpc.callback_client import BidirectionalStream
from pw_status import Status
from pw_transfer.transfer_pb2 import Chunk

_LOG = logging.getLogger(__name__)


class _Timer:
    """A timer which invokes a callback after a certain timeout."""
    def __init__(self, callback: Callable[[], Any]):
        self._callback = callback
        self._task: Optional[asyncio.Task[Any]] = None

    def start(self, timeout_s: float):
        """Starts a new timer.

        If a timer is already running, it is stopped and a new timer started.
        This can be used to implement watchdog-like behavior, where a callback
        is invoked after some time without a kick.
        """
        self.stop()
        self._task = asyncio.create_task(self._run(timeout_s))

    def stop(self):
        """Terminates a running timer."""
        if self._task is not None:
            self._task.cancel()
            self._task = None

    async def _run(self, timeout_s: float):
        await asyncio.sleep(timeout_s)
        self._callback()
        self._task = None


class _Transfer(abc.ABC):
    """A client-side data transfer through a Manager.

    Subclasses are responsible for implementing all of the logic for their type
    of transfer, receiving messages from the server and sending the appropriate
    messages in response.
    """
    def __init__(self, transfer_id: int, data: bytes,
                 send_chunk: Callable[[Chunk], None],
                 end_transfer: Callable[['_Transfer', Status], None]):
        self.id = transfer_id
        self.status = Status.OK
        self.data = data
        self.done = threading.Event()
        self._send_chunk = send_chunk
        self._end_transfer = end_transfer

    @abc.abstractmethod
    async def begin(self) -> None:
        """Sends the initial chunk to notify the sever of the transfer."""

    @abc.abstractmethod
    async def handle_chunk(self, chunk: Chunk) -> bool:
        """Processes an incoming chunk from the server.

        Returns True if the transfer is complete or False otherwise.

        Only called for non-terminating chunks (i.e. those without a status).
        """

    def _send_error(self, error: Status) -> None:
        """Sends an error chunk to the server. Should only be called once."""
        self.status = error
        self._send_chunk(Chunk(transfer_id=self.id, status=error.value))


class _WriteTransfer(_Transfer):
    """A client -> server write transfer."""
    def __init__(
        self,
        transfer_id: int,
        data: bytes,
        send_chunk: Callable[[Chunk], None],
        end_transfer: Callable[[_Transfer, Status], None],
        response_timeout_s: float,
    ):
        super().__init__(transfer_id, data, send_chunk, end_transfer)
        self._offset = 0
        self._response_timeout_s = response_timeout_s
        self._response_timer = _Timer(
            lambda: self._end_transfer(self, Status.DEADLINE_EXCEEDED))

        self._max_bytes_to_send = 0
        self._max_chunk_size = 0
        self._chunk_delay_us: Optional[int] = None

    async def begin(self) -> None:
        """Sends the transfer ID, notifying the server that we want to write."""
        self._send_chunk(Chunk(transfer_id=self.id))
        self._response_timer.start(self._response_timeout_s)

    async def handle_chunk(self, chunk: Chunk) -> bool:
        """Processes an incoming chunk from the server.

        In a write transfer, the server only sends transfer parameter updates
        to the client. When a message is received, update local parameters and
        send data accordingly.
        """

        self._response_timer.stop()

        # Check whether the client has sent a previous data offset, which
        # indicates that some chunks were lost in transmission.
        if chunk.offset < self._offset:
            _LOG.debug('Write transfer %d rolling back to offset %d from %d',
                       self.id, chunk.offset, self._offset)

        self._offset = chunk.offset

        if self._offset > len(self.data):
            # Bad offset; terminate the transfer.
            _LOG.error(
                'Transfer %d: server requested invalid offset %d (size %d)',
                self.id, self._offset, len(self.data))

            self._send_error(Status.OUT_OF_RANGE)
            return True

        self._max_bytes_to_send = min(chunk.pending_bytes,
                                      len(self.data) - self._offset)

        if chunk.HasField('max_chunk_size_bytes'):
            self._max_chunk_size = chunk.max_chunk_size_bytes
        if chunk.HasField('min_delay_microseconds'):
            self._chunk_delay_us = chunk.min_delay_microseconds

        while self._max_bytes_to_send > 0:
            write_chunk = self._next_chunk()
            self._offset += len(write_chunk.data)
            self._max_bytes_to_send -= len(write_chunk.data)

            self._send_chunk(write_chunk)

            if self._chunk_delay_us:
                await asyncio.sleep(self._chunk_delay_us / 1e6)

        self._response_timer.start(self._response_timeout_s)
        return False

    def _next_chunk(self) -> Chunk:
        """Returns the next Chunk message to send in the data transfer."""
        chunk = Chunk(transfer_id=self.id, offset=self._offset)

        if len(self.data) - self._offset <= self._max_chunk_size:
            # Final chunk of the transfer.
            chunk.data = self.data[self._offset:]
            chunk.remaining_bytes = 0
        else:
            chunk.data = self.data[self._offset:self._offset +
                                   self._max_chunk_size]

        return chunk


class _ReadTransfer(_Transfer):
    """A client <- server read transfer.

    Although Python can effectively handle an unlimited transfer window, this
    client sets a conservative window and chunk size to avoid overloading the
    device. These are configurable in the constructor.
    """
    def __init__(self,
                 transfer_id: int,
                 send_chunk: Callable[[Chunk], None],
                 end_transfer: Callable[[_Transfer, Status], None],
                 response_timeout_s: float,
                 max_retries: int = 3,
                 max_bytes_to_receive: int = 8192,
                 max_chunk_size: int = 1024,
                 chunk_delay_us: int = None):
        super().__init__(transfer_id, bytes(), send_chunk, end_transfer)

        self._response_timeout_s = response_timeout_s
        self._response_timer = _Timer(self._on_timeout)
        self._chunk_timeout_count = 0
        self._max_retries = max_retries

        self._max_bytes_to_receive = max_bytes_to_receive
        self._max_chunk_size = max_chunk_size
        self._chunk_delay_us = chunk_delay_us

        self._remaining_transfer_size: Optional[int] = None
        self._offset = 0
        self._pending_bytes = max_bytes_to_receive

    async def begin(self) -> None:
        """Sends the initial transfer parameters for the read transfer."""
        self._send_transfer_parameters()

    async def handle_chunk(self, chunk: Chunk) -> bool:
        """Processes an incoming chunk from the server.

        In a read transfer, the client receives data chunks from the server.
        Once all pending data is received, the transfer parameters are updated.
        """

        self._response_timer.stop()
        self._chunk_timeout_count = 0

        if chunk.offset != self._offset:
            # Initially, the transfer service only supports in-order transfers.
            # If data is received out of order, request that the server
            # retransmit from the previous offset.
            self._pending_bytes = 0
            self._send_transfer_parameters()
            return False

        self.data += chunk.data
        self._pending_bytes -= len(chunk.data)
        self._offset += len(chunk.data)

        if chunk.HasField('remaining_bytes'):
            if chunk.remaining_bytes == 0:
                # No more data to read. Acknowledge receipt and finish.
                self._send_chunk(
                    Chunk(transfer_id=self.id, status=Status.OK.value))
                return True

            # The server may optionally indicate that it has a known size of
            # data available. This is not yet used.
            self._remaining_transfer_size = chunk.remaining_bytes

        if self._pending_bytes == 0:
            # All pending data was received. Send out a new parameters chunk for
            # the next block.
            # _send_transfer_parameters starts the response timer.
            self._send_transfer_parameters()
        else:
            # Wait for the next pending chunk.
            self._response_timer.start(self._response_timeout_s)

        return False

    def _send_transfer_parameters(self):
        """Sends an updated transfer parameters chunk to the server."""

        self._pending_bytes = self._max_bytes_to_receive

        chunk = Chunk(transfer_id=self.id,
                      pending_bytes=self._pending_bytes,
                      max_chunk_size_bytes=self._max_chunk_size,
                      offset=self._offset)

        if self._chunk_delay_us:
            chunk.min_delay_microseconds = self._chunk_delay_us

        self._send_chunk(chunk)

        # Start the timeout for the server to send a read chunk.
        self._response_timer.start(self._response_timeout_s)

    def _on_timeout(self) -> None:
        """Handles a timeout while waiting for a chunk.

        In a read transfer, if a chunk is not received by the timeout, the
        receiver resends the latest transfer parameters to prompt the server
        for more data. This is done several times, up to a specified number of
        retries, after which the transfer is terminated.
        """
        self._chunk_timeout_count += 1

        if self._chunk_timeout_count > self._max_retries:
            self._end_transfer(self, Status.DEADLINE_EXCEEDED)
        else:
            self._send_transfer_parameters()


_TransferDict = Dict[int, _Transfer]


class Manager:  # pylint: disable=too-many-instance-attributes
    """A manager for transmitting data through an RPC TransferService.

    This should be initialized with an active Manager over an RPC channel. Only
    one instance of this class should exist for a configured RPC TransferService
    -- the Manager supports multiple simultaneous transfers.

    When created, a Manager starts a separate thread in which transfer
    communications and events are handled.
    """
    def __init__(self,
                 rpc_transfer_service,
                 default_response_timeout_s: float = 2.0):
        """Initializes a Manager on top of a TransferService."""
        self._service: Any = rpc_transfer_service
        self._default_response_timeout_s = default_response_timeout_s

        # Ongoing transfers in the service by ID.
        self._read_transfers: _TransferDict = {}
        self._write_transfers: _TransferDict = {}

        # RPC streams for read and write transfers. These are shareable by
        # multiple transfers of the same type.
        self._read_stream: Optional[BidirectionalStream] = None
        self._write_stream: Optional[BidirectionalStream] = None

        self._loop = asyncio.new_event_loop()

        # Queues are used for communication between the Manager context and the
        # dedicated asyncio transfer thread.
        self._new_transfer_queue: asyncio.Queue = asyncio.Queue()
        self._read_chunk_queue: asyncio.Queue = asyncio.Queue()
        self._write_chunk_queue: asyncio.Queue = asyncio.Queue()
        self._quit_event = asyncio.Event()

        self._thread = threading.Thread(target=self._start_event_loop_thread,
                                        daemon=True)

        self._thread.start()

    def __del__(self):
        # Notify the thread that the transfer manager is being destroyed and
        # wait for it to exit.
        if self._thread.is_alive():
            self._loop.call_soon_threadsafe(self._quit_event.set)
            self._thread.join()

    def read(self, transfer_id: int) -> bytes:
        """Receives ("downloads") data from the server.

        Raises:
          Error: the transfer failed to complete
        """

        if transfer_id in self._read_transfers:
            raise ValueError(f'Read transfer {transfer_id} already exists')

        transfer = _ReadTransfer(transfer_id, self._send_read_chunk,
                                 self._end_read_transfer,
                                 self._default_response_timeout_s)
        self._start_read_transfer(transfer)

        transfer.done.wait()

        if not transfer.status.ok():
            raise Error(transfer.id, transfer.status)

        return transfer.data

    def write(self, transfer_id: int, data: bytes) -> None:
        """Transmits ("uploads") data to the server.

        Raises:
          Error: the transfer failed to complete
        """

        if transfer_id in self._write_transfers:
            raise ValueError(f'Write transfer {transfer_id} already exists')

        transfer = _WriteTransfer(transfer_id, data, self._send_write_chunk,
                                  self._end_write_transfer,
                                  self._default_response_timeout_s)
        self._start_write_transfer(transfer)

        transfer.done.wait()

        if not transfer.status.ok():
            raise Error(transfer.id, transfer.status)

    def _send_read_chunk(self, chunk: Chunk) -> None:
        assert self._read_stream is not None
        self._read_stream.send(chunk)

    def _send_write_chunk(self, chunk: Chunk) -> None:
        assert self._write_stream is not None
        self._write_stream.send(chunk)

    def _start_event_loop_thread(self):
        """Entry point for event loop thread that starts an asyncio context."""
        asyncio.set_event_loop(self._loop)

        # Recreate the async communication channels in the context of the
        # running event loop.
        self._new_transfer_queue = asyncio.Queue()
        self._read_chunk_queue = asyncio.Queue()
        self._write_chunk_queue = asyncio.Queue()
        self._quit_event = asyncio.Event()

        self._loop.create_task(self._transfer_event_loop())
        self._loop.run_forever()

    async def _transfer_event_loop(self):
        """Main async event loop."""
        exit_thread = self._loop.create_task(self._quit_event.wait())
        new_transfer = self._loop.create_task(self._new_transfer_queue.get())
        read_chunk = self._loop.create_task(self._read_chunk_queue.get())
        write_chunk = self._loop.create_task(self._write_chunk_queue.get())

        while not self._quit_event.is_set():
            # Perform a select(2)-like wait for one of several events to occur.
            done, _ = await asyncio.wait(
                (exit_thread, new_transfer, read_chunk, write_chunk),
                return_when=asyncio.FIRST_COMPLETED)

            if exit_thread in done:
                break

            if new_transfer in done:
                await new_transfer.result().begin()
                new_transfer = self._loop.create_task(
                    self._new_transfer_queue.get())

            if read_chunk in done:
                self._loop.create_task(
                    self._handle_chunk(self._read_transfers,
                                       self._end_read_transfer,
                                       read_chunk.result()))
                read_chunk = self._loop.create_task(
                    self._read_chunk_queue.get())

            if write_chunk in done:
                self._loop.create_task(
                    self._handle_chunk(self._write_transfers,
                                       self._end_write_transfer,
                                       write_chunk.result()))
                write_chunk = self._loop.create_task(
                    self._write_chunk_queue.get())

        self._loop.stop()

    @staticmethod
    async def _handle_chunk(
        transfers: _TransferDict,
        completion: Callable[[_Transfer, Status], None],
        chunk: Chunk,
    ) -> None:
        """Processes an incoming chunk from a stream.

        The chunk is dispatched to an active transfer based on its ID. If the
        transfer indicates that it is complete, the provided completion callback
        is invoked.
        """

        try:
            transfer = transfers[chunk.transfer_id]
        except KeyError:
            _LOG.error(
                'TransferManager received chunk for unknown transfer %d',
                chunk.transfer_id)
            # TODO(frolv): What should be done here, if anything?
            return

        should_exit = False

        # Status chunks are only used to terminate a transfer. They do not
        # contain any data that requires processing.
        if not chunk.HasField('status'):
            should_exit = await transfer.handle_chunk(chunk)

        if should_exit or chunk.HasField('status'):
            completion(transfer, Status(chunk.status))

    def _start_read_transfer(self, transfer: _Transfer) -> None:
        """Begins a new read transfer, opening the stream if it isn't."""

        self._read_transfers[transfer.id] = transfer

        if not self._read_stream:
            self._read_stream = self._service.Read(
                lambda _, chunk: self._loop.call_soon_threadsafe(
                    self._read_chunk_queue.put_nowait, chunk))

        _LOG.debug('Starting new read transfer %d', transfer.id)
        self._loop.call_soon_threadsafe(self._new_transfer_queue.put_nowait,
                                        transfer)

    def _end_read_transfer(self, transfer: _Transfer, status: Status) -> None:
        """Completes a read transfer."""
        del self._read_transfers[transfer.id]

        if not status.ok():
            _LOG.error('Read transfer %d terminated with status %s',
                       transfer.id, status)
            transfer.status = status

        # If no more transfers are using the read stream, close it.
        if not self._read_transfers and self._read_stream:
            self._read_stream.cancel()
            self._read_stream = None

        transfer.done.set()

    def _start_write_transfer(self, transfer: _Transfer) -> None:
        """Begins a new write transfer, opening the stream if it isn't."""

        self._write_transfers[transfer.id] = transfer

        if not self._write_stream:
            # TODO(frolv): Make the actual RPC call.
            self._write_stream = self._service.Write(
                lambda _, chunk: self._loop.call_soon_threadsafe(
                    self._write_chunk_queue.put_nowait, chunk))

        _LOG.debug('Starting new write transfer %d', transfer.id)
        self._loop.call_soon_threadsafe(self._new_transfer_queue.put_nowait,
                                        transfer)

    def _end_write_transfer(self, transfer: _Transfer, status: Status) -> None:
        """Completes a write transfer."""
        del self._write_transfers[transfer.id]

        if not status.ok():
            _LOG.error('Write transfer %d terminated with status %s',
                       transfer.id, status)
            transfer.status = status

        # If no more transfers are using the write stream, close it.
        if not self._write_transfers and self._write_stream:
            self._write_stream.cancel()
            self._write_stream = None

        transfer.done.set()


class Error(Exception):
    """Exception raised when a transfer fails.

    Stores the ID of the failed transfer and the error that occurred.
    """
    def __init__(self, transfer_id: int, status: Status):
        super().__init__(f'Transfer {transfer_id} failed with status {status}')
        self.transfer_id = transfer_id
        self.status = status