//// A worker pool for Gleam that distributes work across a limited number of
//// worker processes. Workers are pooled to avoid the overhead of
//// spawning and killing processes for each task.
////
//// The pool manages a queue of work items and distributes them to idle workers.
//// When no workers are available, work is queued until a worker becomes free.
//// The pool handles worker crashes gracefully and automatically manages the
//// worker lifecycle.
////
//// ## Example
////
//// ```gleam
//// import crew
//// import gleam/erlang/process
////
//// pub fn main() {
////   // Create a pool name
////   let pool_name = process.new_name("image_downloader")
////
////   // Start an unsupervised pool
////   let assert Ok(_) =
////       crew.new(pool_name, download_image)
////       |> crew.fixed_size(4)
////       |> crew.start
////
////   // Execute work on the pool
////   let result = crew.run(pool_name, 5000, "tasteful-ramen.jpeg")
//// }
//// ```

// -- IMPORTS -----------------------------------------------------------------

import gleam/bool
import gleam/deque.{type Deque}
import gleam/dict.{type Dict}
import gleam/erlang/process.{
  type Monitor, type Name, type Pid, type Selector, type Subject,
}
import gleam/list
import gleam/option.{type Option, None, Some}
import gleam/otp/actor.{type Next, type StartError}
import gleam/otp/static_supervisor.{type Supervisor}
import gleam/otp/supervision.{type ChildSpecification}
import gleam/result
import gleam/set.{type Set}
import gleam/string

// TODO: - min/max dynamic scaling? what (if any) is the load metric?
// TODO: fairer scheduler? group work by caller and round-robin maybe?

// poolboy: https://github.com/devinus/poolboy
// lifeguard: https://github.com/Pevensie/lifeguard
// gen_stage: https://github.com/elixir-lang/gen_stage

// -- BUILDER -----------------------------------------------------------------

/// A builder for configuring a worker pool before starting it.
pub opaque type Builder(state, work, result) {
  Builder(
    name: Name(PoolMsg(work, result)),
    size: Int,
    max_queue_length: Option(Int),
    init: fn() -> state,
    init_timeout: Int,
    work: fn(state, work) -> result,
  )
}

/// Create a new worker pool builder with the given name.
///
/// The name is used to register the pool so that work can be sent to it.
///
/// By default, the pool will have a number of workers equal to the number of
/// scheduler threads available on the system (typically the number of CPU cores).
///
/// ## Example
///
/// ```gleam
/// let pool_name = process.new_name("pool")
/// let builder = crew.new(pool_name, worker)
/// ```
pub fn new(
  name: Name(PoolMsg(work, result)),
  work: fn(work) -> result,
) -> Builder(Nil, work, result) {
  new_with_state(name, Nil, fn(_state, input) { work(input) })
}

/// Create a new worker pool builder with the given name and state.
///
/// The name is used to register the pool so that work can be sent to it.
///
/// By default, the pool will have a number of workers equal to the number of
/// scheduler threads available on the system (typically the number of CPU cores).
pub fn new_with_state(
  name: Name(PoolMsg(work, result)),
  state: state,
  work: fn(state, work) -> result,
) -> Builder(state, work, result) {
  new_with_initialiser(name, 1000, fn() { state }, work)
}

/// Create a new worker pool builder with the given name and initialiser.
///
/// The name is used to register the pool so that work can be sent to it.
/// The initialiser will run on the worker process before it registers itself
/// as a worker.
///
/// By default, the pool will have a number of workers equal to the number of
/// scheduler threads available on the system (typically the number of CPU cores).
pub fn new_with_initialiser(
  name: Name(PoolMsg(work, result)),
  timeout init_timeout: Int,
  init init: fn() -> state,
  run work: fn(state, work) -> result,
) -> Builder(state, work, result) {
  let size = scheduler_count()
  Builder(name:, size:, max_queue_length: None, init_timeout:, init:, work:)
}

/// Set the number of worker processes in the pool to a fixed number.
///
/// When set to less than 1 starting the pool will fail.
///
/// ## Example
///
/// ```gleam
/// crew.new(pool_name)
/// |> crew.fixed_size(8)  // Use 8 workers regardless of CPU count
/// ```
pub fn fixed_size(
  builder: Builder(state, work, result),
  size: Int,
) -> Builder(state, work, result) {
  Builder(..builder, size:)
}

/// To avoid overloading the system, the internal work queue is limited.
///
/// If this limit is reached, callers of `enqueue` have to wait until enough
/// work has been done before continuing.
///
/// By default, the `max_queue_size` depends on the number of workers in the pool.
///
/// ## Example
///
/// ```gleam
/// crew.new(pool_name, worder)
/// |> crew.max_queue_length(100)
/// ```
pub fn max_queue_length(
  builder: Builder(state, work, result),
  max_queue_length: Int,
) -> Builder(state, work, result) {
  Builder(..builder, max_queue_length: Some(max_queue_length))
}

// -- START -------------------------------------------------------------------

/// Start an unsupervised worker pool from the given builder.
///
/// Returns a supervisor that manages the pool and its workers. In most cases,
/// you should use `supervised` instead to get a child specification that can
/// be added to your application's supervision tree.
///
/// ## Panics
/// This function will exit the process if any workers fail to start, similar
/// to `static_supervisor.start`.
///
/// ## Example
///
/// ```gleam
/// let assert Ok(pool_supervisor) =
///   crew.new(pool_name)
///   |> crew.start
/// ```
pub fn start(
  builder: Builder(state, work, result),
) -> Result(Supervisor, StartError) {
  use result <- result.map(start_tree(builder))
  result.data
}

/// Create a child specification for a supervised worker pool.
///
/// This is the recommended way to start a worker pool as part of your
/// application's supervision tree. The returned child specification can be
/// added to a supervisor using `static_supervisor.add`.
///
/// ## Example
///
/// ```gleam
/// let pool_spec =
///   crew.new(pool_name)
///   |> crew.fixed_size(4)
///   |> crew.supervised
///
/// let assert Ok(_) =
///   supervisor.new(supervisor.OneForOne)
///   |> supervisor.add(pool_spec)
///   |> supervisor.start
/// ```
pub fn supervised(
  builder: Builder(state, work, result),
) -> ChildSpecification(Supervisor) {
  use <- supervision.supervisor
  start_tree(builder)
}

fn start_tree(
  builder: Builder(state, work, result),
) -> Result(actor.Started(_), StartError) {
  use <- bool.guard(
    when: builder.size <= 0,
    return: Error(actor.InitFailed("pool size must be greater than zero")),
  )

  let main_supervisor = static_supervisor.new(static_supervisor.RestForOne)
  let worker_supervisor = static_supervisor.new(static_supervisor.OneForOne)

  let pool_spec = {
    use <- supervision.worker

    actor.new_with_initialiser(1000, init_pool(builder, _))
    |> actor.named(builder.name)
    |> actor.on_message(pool)
    |> actor.start
  }

  let worker_spec = {
    use <- supervision.worker
    start_worker(builder)
  }

  let worker_supervisor_spec = {
    use <- supervision.supervisor

    worker_supervisor
    |> repeat(times: builder.size, with: static_supervisor.add(_, worker_spec))
    |> static_supervisor.start
  }

  main_supervisor
  |> static_supervisor.add(pool_spec)
  |> static_supervisor.add(worker_supervisor_spec)
  |> static_supervisor.start()
}

// -- CALL --------------------------------------------------------------------

/// Send a single piece of work to one of the workers and wait for the result.
///
/// This function blocks until the work is completed or the timeout is reached.
/// If no worker is available, the message will be queued and sent to the next
/// free worker.
///
/// ## Parameters
/// - `pool` - The name of the pool to execute work on
/// - `timeout` - Maximum time to wait for completion in milliseconds
/// - `work` - The message to send to the worker function
///
/// ## Panics
/// - If the pool does not complete the work within the specified timeout
/// - If the pool is not running
/// - If the worker crashes while executing the work
pub fn call(
  in pool: Name(PoolMsg(work, result)),
  timeout timeout: Int,
  msg work: work,
) -> result {
  let assert [result] = do_call(pool, timeout, [work])
  result
}

/// Send multiple pieces of work concurrently to the pool workers, without
/// ordering guarantees.
///
/// Work is distributed among the available workers and executed concurrently.
/// Results are returned in the order they complete, not the order they were
/// submitted.
///
/// ## Parameters
/// - `pool` - The name of the pool to execute work on
/// - `timeout` - Maximum time to wait for all work to complete in milliseconds
/// - `work` - A list of messages describing the work to be done
///
/// ## Panics
/// - If the pool does not complete all work within the specified timeout
/// - If the pool is not running
/// - If any worker crashes while executing work
pub fn call_parallel(
  in pool: Name(PoolMsg(work, result)),
  timeout timeout: Int,
  msg work: List(work),
) -> List(result) {
  list.reverse(do_call(pool, timeout, work))
}

@internal
pub fn do_call(
  pool: Name(PoolMsg(work, result)),
  timeout: Int,
  work: List(work),
) -> List(result) {
  let timeout_end = system_time() + timeout
  let assert Ok(pool_pid) = process.named(pool) as "Pool is not running"

  let monitor = process.monitor(pool_pid)
  let receive = process.new_subject()

  // we can always cast here because we will be blocking below anyways
  actor.send(process.named_subject(pool), Enqueue(receive, work, None))

  let selector =
    process.new_selector()
    |> process.select_specific_monitor(monitor, fn(down) {
      let msg =
        "Pool exited while waiting for work to complete: "
        <> string.inspect(down)

      panic as msg
    })
    |> process.select_map(receive, fn(result) {
      case result {
        Ok(value) -> value
        Error(down) -> {
          // demonitor here in case the panic is rescued
          process.demonitor_process(monitor)
          let msg = "Worker exited: " <> string.inspect(down)
          panic as msg
        }
      }
    })

  let result = receive_loop(work, selector, timeout_end, [])

  process.demonitor_process(monitor)
  cancel(pool, receive)

  case result {
    Ok(value) -> value
    Error(_) -> panic as "Pool did not complete work in time"
  }
}

fn receive_loop(
  work: List(work),
  selector: Selector(result),
  timeout_end: Int,
  state: List(result),
) -> Result(List(result), Nil) {
  case work {
    [] -> Ok(state)
    [_, ..work] -> {
      let timeout = timeout_end - system_time()
      case process.selector_receive(selector, timeout) {
        Ok(result) ->
          receive_loop(work, selector, timeout_end, [result, ..state])
        Error(Nil) -> Error(Nil)
      }
    }
  }
}

// -- ASYNC API ---------------------------------------------------------------

/// The first time `enqueue*` is called from a process, the pool starts to monitor
/// that process and cancels all ongoing work in case it goes down.
///
/// Sometimes it is useful to manually unsubscribe and cancel all ongoing work
/// for a subject. Doing so will also remove the monitor added in the pool once
/// the last `receive` subject got cancelled.
///
/// Note that finished work might still arrive on this selector after
/// `cancel` got called.
pub fn cancel(
  pool: Name(PoolMsg(work, result)),
  receive: Subject(Result(result, process.ExitReason)),
) -> Nil {
  actor.send(process.named_subject(pool), Cancel(receive))
}

/// Submit a single piece of work to the pool using a subscription channel.
/// The work will be done asynchronously and the result will be sent back
/// to the provided subject. The timeout controls how long to wait for the
/// work to be added to the queue successfully.
///
/// This is a lower-level function for submitting work.  It is the callers
/// responsibility to handle timeouts, submission order and failures.
/// Most users should prefer `call`.
///
/// The first time you `enqueue` is called from a process, the pool sets up
/// a monitor making sure work is cancelled when the process no longer exists
/// to receive a result. You can clean up this monitor early by using `cancel`.
pub fn submit(
  pool: Name(PoolMsg(work, result)),
  timeout: Int,
  receive: Subject(Result(result, process.ExitReason)),
  work: work,
) -> Nil {
  submit_all(pool, timeout, receive, [work])
}

/// Submit multiple pieces of work to the pool using a subscription channel.
/// The work will be done asynchronously and the result will be sent back
/// to the provided subject. The timeout controls how long to wait for the
/// work to be added to the queue successfully.
///
/// This is a lower-level function for submitting work.  It is the callers
/// responsibility to handle timeouts, submission order and failures.
/// Most users should prefer the `call_parallel` function.
///
/// The first time you `enqueue` is called from a process, the pool sets up
/// a monitor making sure work is cancelled when the process no longer exists
/// to receive a result. You can clean up this monitor early by using `cancel`.
pub fn submit_all(
  pool: Name(PoolMsg(work, result)),
  timeout: Int,
  receive: Subject(Result(result, process.ExitReason)),
  work: List(work),
) -> Nil {
  let counter = get_counter(pool)
  let subject = process.named_subject(pool)

  case work {
    [] -> Nil
    [_, ..] if counter > 0 -> {
      actor.send(subject, Enqueue(receive:, work:, enqueued: None))
    }
    [_, ..] -> {
      actor.call(subject, timeout, fn(enqueued) {
        Enqueue(receive:, work:, enqueued: Some(enqueued))
      })
    }
  }
}

// -- POOL --------------------------------------------------------------------

pub opaque type PoolMsg(work, result) {
  WorkerStarted(pid: Pid, send: Subject(Work(work, result)))
  WorkerIdle(pid: Pid)
  MonitoredProcessExited(reason: process.Down)
  //
  GetWorkerCount(reply_to: Subject(Int))
  Enqueue(
    receive: Receiver(result),
    work: List(work),
    enqueued: Option(Subject(Nil)),
  )
  Cancel(receive: Receiver(result))
}

type State(work, result) {
  State(
    name: Name(PoolMsg(work, result)),
    worker_count: Int,
    idle_workers: List(Worker(work, result)),
    active_workers: Dict(Pid, ActiveWorker(work, result)),
    //
    callers: Dict(Pid, Caller(result)),
    channels: Dict(Receiver(result), Channel(work, result)),
    //
    queue: Deque(Work(work, result)),
    overflow_queue: Deque(Request(work, result)),
  )
}

type Worker(work, result) {
  Worker(pid: Pid, send: Subject(Work(work, result)), monitor: Monitor)
}

type ActiveWorker(work, result) {
  ActiveWorker(worker: Worker(work, result), work: Work(work, result))
}

type Caller(result) {
  Caller(pid: Pid, monitor: Monitor, channels: Set(Receiver(result)))
}

type Channel(work, result) {
  Channel(from: Pid, workers: Set(Pid), receive: Receiver(result))
}

type Request(work, result) {
  Request(
    caller: Pid,
    receive: Receiver(result),
    work: List(work),
    enqueued: Option(Subject(Nil)),
  )
}

fn init_pool(
  builder: Builder(state, work, result),
  self: Subject(PoolMsg(work, result)),
) {
  let selector =
    process.new_selector()
    |> process.select(self)
    |> process.select_monitors(MonitoredProcessExited)

  // it's big, but it will provide backpressure eventually.
  let max_queue_length =
    builder.max_queue_length
    |> option.unwrap(builder.size * 100)

  set_counter(builder.name, max_queue_length)

  let state =
    State(
      name: builder.name,
      worker_count: 0,
      idle_workers: [],
      active_workers: dict.new(),
      callers: dict.new(),
      channels: dict.new(),
      queue: deque.new(),
      overflow_queue: deque.new(),
    )

  actor.initialised(state)
  |> actor.selecting(selector)
  |> actor.returning(self)
  |> Ok
}

fn pool(
  state: State(work, result),
  msg: PoolMsg(work, result),
) -> Next(State(work, result), PoolMsg(work, result)) {
  let next_state = case msg {
    WorkerStarted(pid:, send:) -> {
      let monitor = process.monitor(pid)

      let worker = Worker(pid:, send:, monitor:)
      State(..state, worker_count: state.worker_count + 1)
      |> try_dequeue_work(worker)
    }

    MonitoredProcessExited(process.ProcessDown(pid:, ..)) -> {
      case dict.get(state.callers, pid), dict.get(state.active_workers, pid) {
        // Caller down -
        // We will get a second Down message for when the process is caller and worker.
        Ok(_), _ -> {
          // we do not have an active caller, maybe we still got the cancel just before?
          use caller <- try_(dict.get(state.callers, pid), state)

          let channels =
            set.fold(caller.channels, state.channels, fn(channels, receive) {
              use channel <- try_(dict.get(state.channels, receive), channels)
              // we kill all workers currently working on things for this caller.
              // the WorkerStopped messages will then clean up the workers.
              set.each(channel.workers, process.kill)

              dict.delete(channels, receive)
            })

          // the queue is cleaned up while consuming it if the caller is missing.
          let callers = dict.delete(state.callers, pid)

          State(..state, callers:, channels:)
        }

        // Active worker down
        Error(_), Ok(_) -> {
          let active_workers = dict.delete(state.active_workers, pid)
          let worker_count = state.worker_count - 1
          State(..state, active_workers:, worker_count:)
        }

        // idle worker down
        Error(_), Error(_) -> {
          let idle_workers =
            list.filter(state.idle_workers, fn(worker) { worker.pid != pid })
          let worker_count = state.worker_count - 1
          State(..state, idle_workers:, worker_count:)
        }
      }
    }

    // we only monitor processes and can ignore all other Down messages
    MonitoredProcessExited(_) -> state

    WorkerIdle(pid:) -> {
      // got an idle message from a worker that is not active
      use ActiveWorker(worker:, work: Work(receive:, ..)) <- try_(
        dict.get(state.active_workers, pid),
        state,
      )

      let active_workers = dict.delete(state.active_workers, pid)

      case dict.get(state.channels, receive) {
        Ok(channel) -> {
          let channel =
            Channel(..channel, workers: set.delete(channel.workers, pid))
          let channels = dict.insert(state.channels, receive, channel)

          State(..state, channels:, active_workers:)
          |> try_dequeue_work(worker)
        }

        // the channels got cancelled while we were still working.
        // we sent a kill channel while unsubscribing, so we drop the worker here.
        Error(_) ->
          State(..state, active_workers:, worker_count: state.worker_count - 1)
      }
    }

    GetWorkerCount(reply_to:) -> {
      process.send(reply_to, state.worker_count)
      state
    }

    Enqueue(receive: _, work: [], enqueued: None) -> state
    Enqueue(receive: _, work: [], enqueued: Some(enqueued)) -> {
      process.send(enqueued, Nil)
      state
    }
    Enqueue(receive:, work:, enqueued:) -> {
      // caller already exited, do not queue their work.
      use pid <- try_(process.subject_owner(receive), state)

      // get the channel data or construct a new entry if needed.
      let channel = case dict.get(state.channels, receive) {
        Ok(channel) -> channel
        Error(_) -> Channel(from: pid, workers: set.new(), receive:)
      }

      // get the caller data and insert the new channel, starting to monitor if needed.
      let caller = case dict.get(state.callers, pid) {
        Ok(caller) ->
          Caller(..caller, channels: set.insert(caller.channels, receive))

        Error(_) -> {
          let monitor = process.monitor(pid)
          Caller(pid: pid, monitor:, channels: set.new() |> set.insert(receive))
        }
      }

      // enqueue_loop will insert the channel into the state.
      State(..state, callers: dict.insert(state.callers, pid, caller))
      |> enqueue_loop(channel, work, enqueued)
    }

    Cancel(receive:) -> {
      // if the receiver does not exist, we don't need to do anything
      use channel <- try_(dict.get(state.channels, receive), state)

      let channels = dict.delete(state.channels, receive)

      // cancel all running workers
      set.each(channel.workers, process.kill)

      use caller <- try_(
        dict.get(state.callers, channel.from),
        State(..state, channels:),
      )

      let caller =
        Caller(..caller, channels: set.delete(caller.channels, receive))

      // if this channel was the last one for the caller, demonitor and remove it.
      let callers = case set.is_empty(caller.channels) {
        True -> {
          process.demonitor_process(caller.monitor)
          dict.delete(state.callers, caller.pid)
        }
        False -> {
          dict.insert(state.callers, caller.pid, caller)
        }
      }

      State(..state, channels:, callers:)
    }
  }

  actor.continue(next_state)
}

fn enqueue_loop(
  state: State(work, result),
  channel: Channel(work, result),
  work: List(work),
  enqueued: Option(Subject(Nil)),
) -> State(work, result) {
  let Channel(from: caller, receive:, workers:) = channel

  case work, state.idle_workers {
    [work, ..rest], [worker, ..idle_workers] -> {
      // got a work item and a worker - it's a match!
      let work = Work(work:, caller:, receive:)
      process.send(worker.send, work)

      let active_worker = ActiveWorker(worker:, work:)
      let active_workers =
        dict.insert(state.active_workers, worker.pid, active_worker)

      let channel = Channel(..channel, workers: set.insert(workers, worker.pid))

      State(..state, active_workers:, idle_workers:)
      |> enqueue_loop(channel, rest, enqueued)
    }

    [_, ..], [] -> {
      let capacity = get_counter(state.name)

      let #(queue, overflow, new_capacity) =
        enqueue_loop2(caller, receive, work, state.queue, capacity)

      decrement_counter(state.name, capacity - new_capacity)

      // if we have overflow, push it onto the channels overflow queue
      // if we don't, send back that we enqueued successfully.
      let overflow_queue = case overflow {
        [] -> {
          case enqueued {
            Some(enqueued) -> process.send(enqueued, Nil)
            None -> Nil
          }

          state.overflow_queue
        }

        work -> {
          let request = Request(caller:, receive:, work:, enqueued:)
          deque.push_back(state.overflow_queue, request)
        }
      }

      let channels = dict.insert(state.channels, receive, channel)

      State(..state, queue:, channels:, overflow_queue:)
    }

    [], _ -> {
      case enqueued {
        Some(enqueued) -> process.send(enqueued, Nil)
        None -> Nil
      }

      State(..state, channels: dict.insert(state.channels, receive, channel))
    }
  }
}

fn enqueue_loop2(
  caller: Pid,
  receive: Receiver(result),
  work: List(work),
  queue: Deque(Work(work, result)),
  capacity: Int,
) {
  case work {
    [work, ..rest] if capacity > 0 -> {
      let queue = deque.push_back(queue, Work(work:, caller:, receive:))
      let capacity = capacity - 1
      enqueue_loop2(caller, receive, rest, queue, capacity)
    }

    _ -> #(queue, work, capacity)
  }
}

fn try_dequeue_work(
  state: State(work, result),
  worker: Worker(work, result),
) -> State(work, result) {
  use #(work, state) <- try(pop_work(state), fn(_) {
    // the queue is empty, this worker becomes idle.
    State(..state, idle_workers: [worker, ..state.idle_workers])
  })

  use channel <- try(dict.get(state.channels, work.receive), fn(_) {
    // this channel got cancelled, try again
    try_dequeue_work(state, worker)
  })

  process.send(worker.send, work)

  let active_worker = ActiveWorker(worker:, work:)
  let active_workers =
    dict.insert(state.active_workers, worker.pid, active_worker)

  let channel =
    Channel(..channel, workers: set.insert(channel.workers, worker.pid))
  let channels = dict.insert(state.channels, work.receive, channel)

  State(..state, active_workers:, channels:)
}

fn pop_work(
  state: State(work, result),
) -> Result(#(Work(work, result), State(work, result)), Nil) {
  use #(work, queue) <- result.try(deque.pop_front(state.queue))

  use #(Request(caller:, receive:, work: request, enqueued:), overflow_queue) <- try(
    deque.pop_front(state.overflow_queue),
    fn(_) {
      // overflow queue is empty, the queue got smaller and we can increase the capacity
      increment_counter(state.name, 1)
      Ok(#(work, State(..state, queue:)))
    },
  )

  case request {
    [first, ..rest] -> {
      // we can push one overflow item onto the queue as a replacement for the
      // item that just got popped off.
      let queue = deque.push_back(queue, Work(work: first, caller:, receive:))

      // push the request back if there is still work to do.
      // if there is none, send the enqueued signal.
      let overflow_queue = case rest {
        [_, ..] -> {
          let request = Request(caller:, receive:, work: rest, enqueued:)
          deque.push_front(overflow_queue, request)
        }

        [] -> {
          case enqueued {
            Some(enqueued) -> process.send(enqueued, Nil)
            None -> Nil
          }

          overflow_queue
        }
      }

      Ok(#(work, State(..state, queue:, overflow_queue:)))
    }

    [] -> {
      // got an empty work request - this should not happen but I guess we can handle it
      case enqueued {
        Some(enqueued) -> process.send(enqueued, Nil)
        None -> Nil
      }

      pop_work(State(..state, overflow_queue:))
    }
  }
}

// -- WORKER ------------------------------------------------------------------

type Receiver(result) =
  Subject(Result(result, process.ExitReason))

type Work(work, result) {
  Work(work: work, caller: Pid, receive: Receiver(result))
}

fn start_worker(
  builder: Builder(state, work, result),
) -> Result(actor.Started(_), actor.StartError) {
  let initialised = process.new_subject()
  let pid = process.spawn(fn() { worker(builder, initialised) })

  let monitor = process.monitor(pid)

  let selector =
    process.new_selector()
    |> process.select_map(initialised, Ok)
    |> process.select_specific_monitor(monitor, Error)

  let result = case process.selector_receive(selector, builder.init_timeout) {
    Ok(Ok(Nil)) -> Ok(actor.Started(pid, Nil))
    Ok(Error(down)) -> Error(actor.InitExited(down.reason))
    Error(Nil) -> {
      process.unlink(pid)
      process.kill(pid)
      Error(actor.InitTimeout)
    }
  }

  process.demonitor_process(monitor)
  result
}

fn worker(builder: Builder(state, work, result), initialised: Subject(Nil)) {
  let self = process.self()
  let subject = process.new_subject()
  let pool_subject = process.named_subject(builder.name)

  let state = builder.init()
  process.send(initialised, Nil)
  process.send(pool_subject, WorkerStarted(self, subject))

  worker_loop(pool_subject, self, subject, state, builder.work)
}

fn worker_loop(pool, self, subject, state, do_work) -> Nil {
  let Work(work:, receive:, caller: _) = process.receive_forever(subject)

  let result = do_work(state, work)

  process.send(pool, WorkerIdle(self))
  process.send(receive, Ok(result))

  worker_loop(pool, self, subject, state, do_work)
}

// -- HELPERS -----------------------------------------------------------------

fn repeat(
  times times: Int,
  from state: state,
  with f: fn(state) -> state,
) -> state {
  case times > 0 {
    True -> repeat(times - 1, f(state), f)
    False -> state
  }
}

fn try(result: Result(a, x), or: fn(x) -> b, then: fn(a) -> b) {
  case result {
    Ok(x) -> then(x)
    Error(x) -> or(x)
  }
}

fn try_(result: Result(a, _), or: b, then: fn(a) -> b) {
  case result {
    Ok(x) -> then(x)
    Error(_) -> or
  }
}

@external(erlang, "crew_ffi", "scheduler_count")
fn scheduler_count() -> Int

@external(erlang, "crew_ffi", "system_time")
fn system_time() -> Int

@external(erlang, "crew_ffi", "get_counter")
fn get_counter(name: Name(a)) -> Int

@external(erlang, "crew_ffi", "increment_counter")
fn increment_counter(name: Name(a), amount: Int) -> Nil

@external(erlang, "crew_ffi", "decrement_counter")
fn decrement_counter(name: Name(a), amount: Int) -> Nil

@external(erlang, "crew_ffi", "set_counter")
fn set_counter(name: Name(a), value: Int) -> Nil