//! The Logical Link Control and Adaptation Protocol (L2CAP).
//!
//! Note that LE and Classic Bluetooth differ quite a bit on this layer, even though they're
//! supposed to share L2CAP. We're only implementing the LE bits.
//!
//! L2CAP provides "channels" to the upper layers that are mapped to the physical transport below
//! the L2CAP layer (the LE Link Layer or whatever Classic Bluetooth does). A channel is identified
//! by a 16-bit ID (also see [`Channel`]), a few of which are reserved.
//!
//! A minimal implementation for Classic Bluetooth must support the L2CAP signaling channel
//! (`0x0001`). A minimal implementation for BLE has to support the L2CAP LE signaling channel
//! (`0x0005`), the Attribute Protocol channel (`0x0004`), and the LE Security Manager channel
//! (`0x0006`).
//!
//! Establishing new connection-oriented channels, as well as transferring data over the
//! connectionless channel (`0x0002`) makes use of *Protocol/Service Multiplexers* (PSMs), which are
//! numbers identifying the protocol or service to use. These numbers are either defined by the
//! Bluetooth SIG or allocated dynamically for use with the Service Discovery Protocol (SDP). The
//! preallocated numbers are hosted online [here][l2c].
//!
//! [l2c]: https://www.bluetooth.com/specifications/assigned-numbers/logical-link-control

mod signaling;

use self::signaling::SignalingState;
use crate::att::{self, AttributeProvider, AttributeServer, NoAttributes};
use crate::link::queue::{Consume, Producer};
use crate::link::{data::Llid, MIN_DATA_PAYLOAD_BUF};
use crate::security::{NoSecurity, SecurityLevel, SecurityManager};
use crate::{bytes::*, utils::HexSlice, Error};
use core::fmt;
use core::ops::{Deref, DerefMut};

/// An L2CAP channel identifier (CID).
///
/// Channels are basically like TCP ports. A `Protocol` can listen on a channel and is connected to
/// a channel on the other device to which all responses are addressed.
///
/// A number of channel identifiers are reserved for predefined functions:
///
/// * `0x0000`: The null identifier. Must never be used as a destination endpoint.
/// * `0x0001`: L2CAP signaling channel (Classic Bluetooth only).
/// * `0x0002`: Connectionless channel (Classic Bluetooth only).
/// * `0x0003`: AMP manager (not relevant for Classic and LE Bluetooth).
/// * `0x0004`: Attribute protocol (ATT). BLE only.
/// * `0x0005`: LE L2CAP signaling channel.
/// * `0x0006`: LE Security Manager protocol.
/// * `0x0007`: Classic Bluetooth Security Manager protocol.
/// * `0x0008`-`0x003E`: Reserved.
/// * `0x003F`: AMP test manager (not relevant for Classic and LE Bluetooth).
///
/// For BLE, channels `0x0040`-`0x007F` are dynamically allocated, while `0x0080` and beyond are
/// reserved and should not be used (as of *Bluetooth 4.2*).
///
/// For classic Bluetooth, all channels `0x0040`-`0xFFFF` are available for dynamic allocation.
#[derive(PartialEq, Eq, Hash, Copy, Clone)]
pub struct Channel(u16);

impl Channel {
    /// The null channel identifier. Must not be used as a destination endpoint.
    pub const NULL: Self = Channel(0x0000);

    /// The channel used by the Attribute Protocol (ATT).
    pub const ATT: Self = Channel(0x0004);

    /// LE L2CAP signaling channel (connectionless).
    pub const LE_SIGNALING: Self = Channel(0x0005);

    /// LE Security Manager channel.
    pub const LE_SECURITY_MANAGER: Self = Channel(0x0006);

    /// Returns the channel identifier (CID) as a raw `u16`.
    pub fn as_raw(&self) -> u16 {
        self.0
    }

    /// Returns whether this channel is connection-oriented.
    ///
    /// L2CAP PDUs addressed to connection-oriented channels are called *B-frames* if the channel is
    /// in "Basic Mode", and can be either *S-frames* or *I-frames* if the channel is in
    /// retransmission/flow control/streaming modes.
    pub fn is_connection_oriented(&self) -> bool {
        !self.is_connectionless()
    }

    /// Returns whether this channel is connectionless.
    ///
    /// L2CAP PDUs addressed to connectionless channels are called *G-frames*.
    pub fn is_connectionless(&self) -> bool {
        matches!(self.0, 0x0002 | 0x0001 | 0x0005)
    }
}

impl fmt::Debug for Channel {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{:#06X}", self.0)
    }
}

impl defmt::Format for Channel {
    fn format(&self, fmt: defmt::Formatter<'_>) {
        defmt::write!(fmt, "{:#06X}", self.0)
    }
}

impl FromBytes<'_> for Channel {
    fn from_bytes(bytes: &mut ByteReader<'_>) -> Result<Self, Error> {
        Ok(Channel(bytes.read_u16_le()?))
    }
}

impl ToBytes for Channel {
    fn to_bytes(&self, writer: &mut ByteWriter<'_>) -> Result<(), Error> {
        writer.write_u16_le(self.0)
    }
}

/// Trait for L2CAP channel mappers that provide access to the protocol or service behind a CID.
pub trait ChannelMapper {
    /// The attribute provider used by the ATT server.
    type AttributeProvider: AttributeProvider;

    /// Look up what's connected to `channel` (eg. the `Protocol` to which to forward).
    fn lookup(&mut self, channel: Channel) -> Option<ChannelData<'_, dyn ProtocolObj + '_>>;

    /// Returns information about the Attribute Protocol on channel `0x0004`.
    fn att(&mut self) -> ChannelData<'_, AttributeServer<Self::AttributeProvider>>;
}

/// Data associated with a connected L2CAP channel.
pub struct ChannelData<'a, P: ?Sized> {
    /// Channel to which responses should be addressed.
    ///
    /// For fixed, predefined channels, this is always the same value for both devices, but
    /// dynamically allocated channels can have different CIDs on both devices.
    response_channel: Channel,

    /// The protocol listening on this channel.
    protocol: &'a mut P,

    /// Outgoing PDU size of the protocol.
    ///
    /// This is the number of bytes that must be available to the protocol in the TX buffer to
    /// guarantee that all of the protocol's PDUs will fit.
    pdu: u8,
}

impl<'a> ChannelData<'a, dyn ProtocolObj + 'a> {
    /// Creates a `ChannelData` carrying a dynamically-dispatched `dyn ProtocolObj` from a concrete
    /// `Protocol` implementor `T`.
    fn new_dyn<T: Protocol + 'a>(response_channel: Channel, protocol: &'a mut T) -> Self {
        assert!(
            usize::from(T::RSP_PDU_SIZE + Header::SIZE) <= MIN_DATA_PAYLOAD_BUF,
            "protocol min PDU is smaller than data channel PDU (L2CAP reassembly NYI)"
        );

        ChannelData {
            response_channel,
            pdu: T::RSP_PDU_SIZE,
            protocol,
        }
    }
}

impl<'a, P: Protocol> ChannelData<'a, P> {
    fn new(response_channel: Channel, protocol: &'a mut P) -> Self {
        assert!(
            usize::from(P::RSP_PDU_SIZE + Header::SIZE) <= MIN_DATA_PAYLOAD_BUF,
            "protocol min PDU is smaller than data channel PDU (L2CAP reassembly NYI)"
        );

        ChannelData {
            response_channel,
            pdu: P::RSP_PDU_SIZE,
            protocol,
        }
    }
}

impl<'a, P: ?Sized> ChannelData<'a, P> {
    /// Returns the `Channel` to which the response should be sent.
    pub fn response_channel(&self) -> Channel {
        self.response_channel
    }

    /// Returns the PDU size required by the protocol.
    ///
    /// This is the minimum size in Bytes the protocol needs to have provided for any of its
    /// outgoing PDUs. `Protocol` implementations may make use of additional space as well, but this
    /// is the very minimum.
    ///
    /// The L2CAP implementation will not forward incoming PDUs to the protocol unless this amount
    /// of space is available in the TX buffer. This guarantees that the response will always fit.
    pub fn pdu_size(&self) -> u8 {
        self.pdu
    }

    /// Returns the protocol connected to the channel.
    pub fn protocol(&mut self) -> &mut P {
        self.protocol
    }

    /// Consumes `self` and returns the protocol connected to the channel, with lifetime `'a`.
    ///
    /// This can be useful when the lifetime of the reference must not be tied to `self`, as would
    /// be the case with `protocol()`.
    pub fn into_protocol(self) -> &'a mut P {
        self.protocol
    }
}

/// A fixed BLE channel map that provides only the required channel endpoints and does not allow
/// dynamic channels.
///
/// The channels are mapped as follows (no other channels are supported):
///
/// * `0x0004`: Attribute protocol (ATT).
/// * `0x0005`: LE L2CAP signaling channel.
/// * `0x0006`: LE Security Manager protocol.
pub struct BleChannelMap<A: AttributeProvider, S: SecurityLevel> {
    att: AttributeServer<A>,
    signaling: SignalingState,
    sm: SecurityManager<S>,
}

impl BleChannelMap<NoAttributes, NoSecurity> {
    /// Creates a new channel map with no backing data for the connected protocols.
    ///
    /// This means:
    /// * The attribute server on channel `0x0004` will host an empty attribute set.
    /// * The security manager on channel `0x0006` will not support pairing or any security.
    pub fn empty() -> Self {
        Self {
            att: AttributeServer::new(NoAttributes),
            signaling: SignalingState::new(),
            sm: SecurityManager::no_security(),
        }
    }
}

impl<A: AttributeProvider> BleChannelMap<A, NoSecurity> {
    pub fn with_attributes(att: A) -> Self {
        Self {
            att: AttributeServer::new(att),
            signaling: SignalingState::new(),
            sm: SecurityManager::no_security(),
        }
    }

    /// Provides mutable access to the underlying `AttributeProvider`.
    pub fn attribute_provider(&mut self) -> &mut A {
        self.att.provider()
    }
}

impl<A: AttributeProvider, S: SecurityLevel> ChannelMapper for BleChannelMap<A, S> {
    type AttributeProvider = A;

    fn lookup(&mut self, channel: Channel) -> Option<ChannelData<'_, dyn ProtocolObj + '_>> {
        match channel {
            Channel::ATT => Some(ChannelData::new_dyn(channel, &mut self.att)),
            Channel::LE_SIGNALING => Some(ChannelData::new_dyn(channel, &mut self.signaling)),
            Channel::LE_SECURITY_MANAGER => Some(ChannelData::new_dyn(channel, &mut self.sm)),
            _ => None,
        }
    }

    fn att(&mut self) -> ChannelData<'_, AttributeServer<Self::AttributeProvider>> {
        ChannelData::new(Channel::ATT, &mut self.att)
    }
}

/// Trait for protocols that sit on top of L2CAP (object-safe part).
///
/// A protocol can be connected to an L2CAP channel via a `ChannelMapper`.
pub trait ProtocolObj {
    /// Process a message sent to the protocol.
    ///
    /// The message is reassembled by L2CAP already, and the `responder` is guaranteed to fit a
    /// protocol payload of at least `Protocol::RSP_PDU_SIZE` Bytes, as defined by the protocol.
    ///
    /// # Errors
    ///
    /// This method should only return an error when a critical problem occurs that can not be
    /// recovered from and that can not be reported back to the connected device using the protocol.
    /// This means that only things like unrecoverable protocol parsing errors should return an
    /// error here.
    fn process_message(&mut self, message: &[u8], responder: Sender<'_>) -> Result<(), Error>;
}

/// Trait for protocols that sit on top of L2CAP (non-object-safe part).
///
/// This extends the `ProtocolObj` trait with other protocol properties.
pub trait Protocol: ProtocolObj {
    /// Minimum size needed by PDUs sent by this protocol.
    ///
    /// Incoming PDUs will only be forwarded to the protocol if there is at least this much space in
    /// the TX buffer.
    const RSP_PDU_SIZE: u8;
}

/// Header used by *all* L2CAP PDUs.
#[derive(Debug)]
struct Header {
    /// Length of the payload following the length and channel fields (after reassembly).
    length: u16,
    /// Destination endpoint of the PDU.
    channel: Channel,
}

impl Header {
    /// The size of an L2CAP message header in Bytes.
    const SIZE: u8 = 2 + 2;
}

impl<'a> FromBytes<'a> for Header {
    fn from_bytes(bytes: &mut ByteReader<'a>) -> Result<Self, Error> {
        let length = bytes.read_u16_le()?;
        let channel = Channel::from_bytes(bytes)?;
        Ok(Self { length, channel })
    }
}

impl ToBytes for Header {
    fn to_bytes(&self, writer: &mut ByteWriter<'_>) -> Result<(), Error> {
        writer.write_u16_le(self.length)?;
        writer.write_u16_le(self.channel.as_raw())?;
        Ok(())
    }
}

struct Message<P> {
    header: Header,
    payload: P,
}

impl<'a, P: FromBytes<'a>> FromBytes<'a> for Message<P> {
    fn from_bytes(bytes: &mut ByteReader<'a>) -> Result<Self, Error> {
        let header = Header::from_bytes(bytes)?;
        assert_eq!(
            header.length as usize,
            bytes.bytes_left(),
            "L2CAP reassembly not yet implemented"
        );

        Ok(Self {
            header,
            payload: P::from_bytes(bytes)?,
        })
    }
}

impl<P: ToBytes> ToBytes for Message<P> {
    fn to_bytes(&self, writer: &mut ByteWriter<'_>) -> Result<(), Error> {
        self.header.to_bytes(writer)?;
        self.payload.to_bytes(writer)?;
        Ok(())
    }
}

/// L2CAP channel manager and responder.
#[derive(Debug)]
pub struct L2CAPState<M: ChannelMapper> {
    mapper: M,
}

impl<M: ChannelMapper> L2CAPState<M> {
    /// Creates a new L2CAP state using the given channel configuration.
    pub fn new(mapper: M) -> Self {
        Self { mapper }
    }

    /// Gives this instance the ability to transmit packets.
    pub fn tx<'a, P: Producer>(&'a mut self, tx: &'a mut P) -> L2CAPStateTx<'a, M, P> {
        L2CAPStateTx { l2cap: self, tx }
    }

    /// Provides mutable access to the underlying `ChannelMapper`.
    pub fn channel_mapper(&mut self) -> &mut M {
        &mut self.mapper
    }
}

/// Provides a way to send a L2CAP message with preallocated storage.
///
/// This can be done either in response to an incoming packet (via `ProtocolObj::process_msg`), or
/// as a device-initiated packet (eg. an attribute notification).
pub struct Sender<'a> {
    /// The protocol's max. outgoing PDU size.
    pdu: u8,

    /// Data PDU channel.
    tx: &'a mut dyn Producer,

    /// Channel to which the response will be addressed.
    channel: Channel,
}

impl<'a> Sender<'a> {
    /// Creates a `Sender` from a `Producer`, ensuring that sufficient free space is available to
    /// fit a PDU described by `chdata`.
    ///
    /// If there is not enough space in `tx`, returns `None`.
    fn new<T: ?Sized>(chdata: &ChannelData<'_, T>, tx: &'a mut dyn Producer) -> Option<Self> {
        let free = tx.free_space();
        let needed = chdata.pdu_size() + Header::SIZE;
        if free < needed {
            debug!("{} free bytes, need {}", free, needed);
            return None;
        }

        let resp_channel = chdata.response_channel();
        let pdu = chdata.pdu_size();
        Some(Sender {
            pdu,
            tx,
            channel: resp_channel,
        })
    }

    /// Enqueues an L2CAP message to be sent over the data connection.
    ///
    /// L2CAP header (including the destination endpoint's channel) and the data channel PDU header
    /// will be added automatically.
    ///
    /// This will fail if there's not enough space left in the TX queue.
    pub fn send<P: ToBytes>(&mut self, payload: P) -> Result<(), Error> {
        self.send_with(|writer| payload.to_bytes(writer))
    }

    /// Enqueues an L2CAP message encoded by a closure.
    ///
    /// L2CAP header and data channel PDU header will be added automatically. The closure `f` only
    /// has to write the protocol PDU to transmit over L2CAP.
    ///
    /// The L2CAP implementation will ensure that there are exactly `Protocol::RSP_PDU_SIZE` Bytes
    /// available in the `ByteWriter` passed to the closure.
    pub fn send_with<T, E>(
        &mut self,
        f: impl FnOnce(&mut ByteWriter<'_>) -> Result<T, E>,
    ) -> Result<T, E>
    where
        E: From<Error>,
    {
        // FIXME automatic fragmentation is not implemented

        // The payload length goes into the header, so we have to skip that part and write it later
        let mut f = Some(f);
        let channel = self.channel;
        let pdu = self.pdu;
        let mut r = None;
        let r2 = self.tx.produce_dyn(
            pdu + Header::SIZE,
            &mut |writer: &mut ByteWriter<'_>| -> Result<_, Error> {
                let mut header_writer = writer.split_off(usize::from(Header::SIZE))?;

                // The PDU size is determined based on how much space is left in `writer`, so we can't
                // just `split_off` the protocol's PDU size.
                assert!(writer.space_left() >= pdu.into());
                let mut payload_writer = ByteWriter::new(&mut writer.rest()[..pdu.into()]);
                let left = payload_writer.space_left();
                let result = f.take().unwrap()(&mut payload_writer);
                let is_ok = result.is_ok();
                r = Some(result);
                let used = left - payload_writer.space_left();
                writer.skip(used).unwrap();

                assert!(used < 0xFFFF);
                Header {
                    length: used as u16,
                    channel,
                }
                .to_bytes(&mut header_writer)?;

                assert_eq!(header_writer.space_left(), 0);

                if is_ok {
                    Ok(Llid::DataStart)
                } else {
                    // The error returned doesn't matter much. It just has to be an `Err` so the PDU
                    // isn't actually sent.
                    Err(Error::InvalidValue)
                }
            },
        );

        if let Err(e) = r2 {
            if e != Error::InvalidValue {
                // Legitimate error
                return Err(e.into());
            }
        }

        r.unwrap()
    }
}

/// An `L2CAPState` with the ability to transmit packets.
///
/// Derefs to the underlying `L2CAPState`.
pub struct L2CAPStateTx<'a, M: ChannelMapper, P: Producer> {
    l2cap: &'a mut L2CAPState<M>,
    tx: &'a mut P,
}

impl<'a, M: ChannelMapper, P: Producer> L2CAPStateTx<'a, M, P> {
    /// Process the start of a new L2CAP message (or a complete, unfragmented message).
    ///
    /// If the incoming message is unfragmented, it will be forwarded to the protocol listening on
    /// the addressed channel, and a response may be sent.
    pub fn process_start(&mut self, message: &[u8]) -> Consume<()> {
        let msg = match Message::<&[u8]>::from_bytes(&mut ByteReader::new(message)) {
            Ok(msg) => msg,
            Err(e) => return Consume::always(Err(e)),
        };

        if usize::from(msg.header.length) != msg.payload.len() {
            // Lengths mismatch => Reassembly needed
            unimplemented!("L2CAP reassembly");
        }

        self.dispatch(msg.header.channel, msg.payload)
    }

    /// Process continuation of an L2CAP message.
    ///
    /// This is not yet implemented and will always panic.
    pub fn process_cont(&mut self, _data: &[u8]) -> Consume<()> {
        unimplemented!("reassembly")
    }

    /// Dispatches a fully reassembled L2CAP message to the protocol listening on the addressed
    /// channel.
    fn dispatch(&mut self, channel: Channel, payload: &[u8]) -> Consume<()> {
        if let Some(mut chdata) = self.l2cap.mapper.lookup(channel) {
            let sender = if let Some(sender) = Sender::new(&chdata, self.tx) {
                sender
            } else {
                return Consume::never(Ok(()));
            };

            Consume::always(chdata.protocol().process_message(payload, sender))
        } else {
            warn!(
                "ignoring message sent to unconnected channel {:?}: {:?}",
                channel,
                HexSlice(payload)
            );
            Consume::always(Ok(()))
        }
    }

    /// Prepares for sending data using the Attribute Protocol.
    ///
    /// This will reserve sufficient space in the outgoing PDU buffer to send any ATT PDU, and then
    /// return an `AttributeServerTx` instance that can be used to initiate an ATT-specific
    /// procedure.
    ///
    /// Returns `None` if there's not enough space in the TX packet queue to send an ATT PDU. If
    /// that happens, calling this method again at a later time (after the Link-Layer had time to
    /// transmit more packets) might succeed.
    pub fn att(&mut self) -> Option<att::AttributeServerTx<'_, M::AttributeProvider>> {
        let att = self.l2cap.mapper.att();
        Sender::new(&att, self.tx).map(move |sender| att.into_protocol().with_sender(sender))
    }
}

impl<'a, M: ChannelMapper, P: Producer> Deref for L2CAPStateTx<'a, M, P> {
    type Target = L2CAPState<M>;

    fn deref(&self) -> &Self::Target {
        &self.l2cap
    }
}

impl<'a, M: ChannelMapper, P: Producer> DerefMut for L2CAPStateTx<'a, M, P> {
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.l2cap
    }
}