//// The state of the session that persists over connections.
//// 
//// From the specs:
//// The Session state in the Client consists of:
//// * QoS 1 and QoS 2 messages which have been sent to the Server, but have not been completely acknowledged.
//// * QoS 2 messages which have been received from the Server, but have not been completely acknowledged. 
//// 
//// Also, to simplify things, we keep the packet id also.

import gleam/bool
import gleam/dict.{type Dict}
import gleam/list
import gleam/set.{type Set}
import spoke/core/session_state.{
  type SessionState, type StorageUpdate, ClearPacketState, ClearSession,
  ReceivedQoS2, UnackedQoS1, UnreceivedQoS2, UnreleasedQoS2, UpdatePacketState,
}
import spoke/packet
import spoke/packet/client/outgoing

pub opaque type Session {
  Session(
    clean_session: Bool,
    packet_id: Int,
    /// Published QoS 1 messages waiting for PubAck,
    /// used for re-transmission of Publish.
    unacked_qos1: Dict(Int, packet.MessageData),
    /// Published QoS 2 messages waiting for PubRec,
    /// used for re-transmission of Publish.
    unreceived_qos2: Dict(Int, packet.MessageData),
    /// Published QoS 2 message ids waiting for PubComp,
    /// used for re-transmission of PubRel.
    unreleased_qos2: Set(Int),
    /// Received QoS 2 message ids waiting for PubRel from publisher,
    /// used for de-duplication.
    incomplete_qos2_in: Set(Int),
  )
}

pub type PubAckResult {
  PublishFinished(Session, List(StorageUpdate))
  InvalidPubAckId
}

pub fn new(clean_session: Bool) -> Session {
  Session(
    clean_session:,
    packet_id: 1,
    unacked_qos1: dict.new(),
    unreceived_qos2: dict.new(),
    unreleased_qos2: set.new(),
    incomplete_qos2_in: set.new(),
  )
}

pub fn from_state(state: SessionState) -> Session {
  let session = new(False)
  let session = Session(..session, packet_id: state.packet_id)

  let session = {
    use session, packet_state <- list.fold(state.packet_states, session)
    case packet_state {
      #(id, UnackedQoS1(message)) ->
        Session(
          ..session,
          unacked_qos1: dict.insert(session.unacked_qos1, id, message),
        )
      #(id, UnreceivedQoS2(message)) ->
        Session(
          ..session,
          unreceived_qos2: dict.insert(session.unreceived_qos2, id, message),
        )
      #(id, ReceivedQoS2) ->
        Session(
          ..session,
          unreleased_qos2: set.insert(session.unreleased_qos2, id),
        )
      #(id, UnreleasedQoS2) ->
        Session(
          ..session,
          incomplete_qos2_in: set.insert(session.incomplete_qos2_in, id),
        )
    }
  }

  session
}

// [MQTT-3.1.2-6]
// If CleanSession is set to 1,
// the Client and Server MUST discard any previous Session and start a new one.
// This Session lasts as long as the Network Connection.
// State data associated with this Session MUST NOT be reused in any subsequent Session.
pub fn connect(
  session: Session,
  clean_session: Bool,
) -> #(Session, List(StorageUpdate)) {
  // If the current session is not persisted, just return a new session
  use <- bool.guard(
    when: session.clean_session,
    return: #(new(clean_session), []),
  )

  case clean_session {
    // If not cleaning the session, continue current session
    False -> #(session, [])

    // Else clear persistent storage and start new session
    True -> #(new(True), [ClearSession])
  }
}

pub fn pending_publishes(session: Session) -> Int {
  dict.size(session.unacked_qos1)
  + dict.size(session.unreceived_qos2)
  + set.size(session.unreleased_qos2)
}

pub fn reserve_packet_id(
  session: Session,
) -> #(Session, Int, List(StorageUpdate)) {
  let id = session.packet_id

  let next_id = case id {
    65_535 -> 1
    id -> id + 1
  }

  use storage_updates <- store_if_persistent(session, [
    session_state.StoreNextPacketId(next_id),
  ])

  #(Session(..session, packet_id: next_id), id, storage_updates)
}

pub fn start_qos1_publish(
  session: Session,
  message: packet.MessageData,
) -> #(Session, outgoing.Packet, List(StorageUpdate)) {
  let #(session, id, id_updates) = reserve_packet_id(session)
  let unacked_qos1 = dict.insert(session.unacked_qos1, id, message)

  let packet =
    outgoing.Publish(packet.PublishDataQoS1(message, dup: False, packet_id: id))

  use storage_updates <- store_if_persistent(session, [
    UpdatePacketState(id, UnackedQoS1(message)),
    ..id_updates
  ])

  #(Session(..session, unacked_qos1:), packet, storage_updates)
}

pub fn start_qos2_publish(
  session: Session,
  message: packet.MessageData,
) -> #(Session, outgoing.Packet, List(StorageUpdate)) {
  let #(session, id, id_updates) = reserve_packet_id(session)
  let unreceived_qos2 = dict.insert(session.unreceived_qos2, id, message)

  let packet =
    outgoing.Publish(packet.PublishDataQoS2(message, dup: False, packet_id: id))

  use storage_updates <- store_if_persistent(session, [
    UpdatePacketState(id, UnreceivedQoS2(message)),
    ..id_updates
  ])

  #(Session(..session, unreceived_qos2:), packet, storage_updates)
}

pub fn handle_pubrec(
  session: Session,
  id: Int,
) -> #(Session, List(StorageUpdate)) {
  let unreceived_qos2 = dict.delete(session.unreceived_qos2, id)
  let unreleased_qos2 = set.insert(session.unreleased_qos2, id)
  use storage_updates <- store_if_persistent(session, [
    UpdatePacketState(id, ReceivedQoS2),
  ])

  #(Session(..session, unreceived_qos2:, unreleased_qos2:), storage_updates)
}

pub fn handle_pubcomp(
  session: Session,
  id: Int,
) -> #(Session, List(StorageUpdate)) {
  let unreleased_qos2 = set.delete(session.unreleased_qos2, id)
  use storage_updates <- store_if_persistent(session, [ClearPacketState(id)])
  #(Session(..session, unreleased_qos2:), storage_updates)
}

/// Marks a received packet id as unreleased.
/// The second element in the return value indicates whether we should
/// publish the message to the client or not.
pub fn start_qos2_receive(
  session: Session,
  packet_id: Int,
) -> #(Session, Bool, List(StorageUpdate)) {
  case set.contains(session.incomplete_qos2_in, packet_id) {
    True -> #(session, False, [])
    False -> {
      let incomplete_qos2_in = set.insert(session.incomplete_qos2_in, packet_id)
      use storage_updates <- store_if_persistent(session, [
        UpdatePacketState(packet_id, UnreleasedQoS2),
      ])
      #(Session(..session, incomplete_qos2_in:), True, storage_updates)
    }
  }
}

pub fn handle_puback(session: Session, packet_id: Int) -> PubAckResult {
  case dict.has_key(session.unacked_qos1, packet_id) {
    True -> {
      let unacked_qos1 = dict.delete(session.unacked_qos1, packet_id)
      use storage_updates <- store_if_persistent(session, [
        ClearPacketState(packet_id),
      ])
      PublishFinished(Session(..session, unacked_qos1:), storage_updates)
    }
    False -> InvalidPubAckId
  }
}

pub fn handle_pubrel(
  session: Session,
  packet_id: Int,
) -> #(Session, List(StorageUpdate)) {
  let incomplete_qos2_in = set.delete(session.incomplete_qos2_in, packet_id)
  use storage_updates <- store_if_persistent(session, [
    ClearPacketState(packet_id),
  ])
  #(Session(..session, incomplete_qos2_in:), storage_updates)
}

pub fn packets_to_send_after_connect(session: Session) -> List(outgoing.Packet) {
  let packets = list.new()

  // Add unacked Qos1 messages
  let packets =
    list.fold(dict.keys(session.unacked_qos1), packets, fn(packets, id) {
      let assert Ok(message) = dict.get(session.unacked_qos1, id)
      let packet =
        outgoing.Publish(packet.PublishDataQoS1(
          message,
          dup: True,
          packet_id: id,
        ))
      [packet, ..packets]
    })

  // Add unreceived QoS2 messages
  let packets =
    list.fold(dict.keys(session.unreceived_qos2), packets, fn(packets, id) {
      let assert Ok(message) = dict.get(session.unreceived_qos2, id)
      let packet =
        outgoing.Publish(packet.PublishDataQoS2(
          message,
          dup: True,
          packet_id: id,
        ))
      [packet, ..packets]
    })

  // Add unreleased QoS2 messages
  let packets =
    list.fold(set.to_list(session.unreleased_qos2), packets, fn(packets, id) {
      [outgoing.PubRel(id), ..packets]
    })

  packets
}

fn store_if_persistent(
  session: Session,
  updates: List(StorageUpdate),
  operation: fn(List(StorageUpdate)) -> a,
) -> a {
  case session.clean_session {
    False -> operation(updates)
    True -> operation([])
  }
}