import gleam/erlang/process
import gleam/otp/actor
import gleam/otp/supervision
import gleam/result
import mala
import simplifile

// --- Public API ---

/// A reference to the process file manager. This can be used to register
/// ownership of file paths using the `register` function.
///
pub opaque type ProcessFileManager {
  ProcessFileManager(
    table: mala.BagTable(process.Pid, String),
    subject: process.Subject(Message),
  )
}

/// Create a new process file manager. Typically you should use the
/// `supervised` function instead of this one, adding it to your application's
/// supervision tree.
///
pub fn start() -> actor.StartResult(ProcessFileManager) {
  actor.new_with_initialiser(1000, initialise)
  |> actor.on_message(handle_message)
  |> actor.start
}

/// A supervision specification for a new process file manager.
///
pub fn supervised() -> supervision.ChildSpecification(ProcessFileManager) {
  supervision.worker(start)
}

/// Register a file system path as being owned by the process that calls this
/// function, so when the process exits the process file manager will delete
/// any file or directory at the path. If there is no file at that location
/// then nothing happens.
///
/// The first time a process calls this function they will be registered by the
/// manager process, but subsequent calls work directly with ETS and do not
/// produce any work for the manager.
///
pub fn register(
  manager: ProcessFileManager,
  ownership_of new_path: String,
) -> Result(Nil, Nil) {
  let owner = process.self()
  let table = manager.table
  use already_registered <- result.try(mala.has_key(table, owner))

  case already_registered {
    // If this process is already registered then all that needs to be done is
    // to insert the new file path into the table so it will be deleted later.
    True -> mala.insert(table, owner, new_path)

    // If this is a new process then it has to ask the manager process to
    // register it.
    False -> Ok(actor.call(manager.subject, 1000, Registration(new_path, _)))
  }
}

// --- Actor implementation ---

pub opaque type Message {
  /// A new process is registering itself as an owner of files.
  Registration(path: String, reply: process.Subject(Nil))
  /// A file-owning process has exited, meaning any files it owns now need to
  /// be deleted to clean-up.
  MonitoredProcessDown(pid: process.Pid)
}

type State {
  State(table: mala.BagTable(process.Pid, String))
}

fn handle_message(
  state: State,
  message: Message,
) -> actor.Next(State, Message) {
  case message {
    MonitoredProcessDown(pid:) -> handle_monitored_process_down(state, pid)
    Registration(path:, reply:) -> handle_registration(state, path, reply)
  }
}

fn handle_registration(
  state: State,
  path: String,
  reply: process.Subject(Nil),
) -> actor.Next(State, Message) {
  // Monitor the process, so we get informed later when the process exits.
  let assert Ok(pid) = process.subject_owner(reply) as "registee must be alive"
  let _monitor = process.monitor(pid)

  // Record this initial file in the table, both so it can be cleaned up and so
  // the code can tell that this process is already registered without going
  // via the manager actor again.
  let assert Ok(_) = mala.insert(state.table, pid, path) as "ETS table exists"

  // Inform the registee their registration is complete
  process.send(reply, Nil)

  // Onwards!
  actor.continue(state)
}

fn handle_monitored_process_down(
  state: State,
  pid: process.Pid,
) -> actor.Next(State, Message) {
  let table = state.table
  // Find and un-register all the files owned by this process
  let assert Ok(files) = mala.get(table, pid) as "ETS table exists"
  let assert Ok(_) = mala.delete_key(table, pid) as "ETS table exists"

  // Delete them all. This function does not return an error for absent paths,
  // so asserting panics only for when files exist but could not be deleted.
  let assert Ok(_) = simplifile.delete_all(files) as "deletion must succeed"

  // Let's go!
  actor.continue(state)
}

fn initialise(
  subject: process.Subject(Message),
) -> Result(actor.Initialised(State, Message, ProcessFileManager), b) {
  let table = mala.new()
  let handle = ProcessFileManager(table:, subject:)
  let selector =
    process.new_selector()
    |> process.select(subject)
    |> process.select_monitors(convert_down_message)

  actor.initialised(State(table))
  |> actor.selecting(selector)
  |> actor.returning(handle)
  |> Ok
}

fn convert_down_message(down: process.Down) -> Message {
  let assert process.ProcessDown(pid:, ..) = down
    as "process_file manager actor should never be monitoring a port"
  MonitoredProcessDown(pid:)
}