//// A task is a kind of process that computes a value and then sends the result back
//// to its parent. Commonly multiple tasks are used to compute multiple things at
//// once.
////
//// If you do not care to receive a result back at the end then you should not
//// use this module, `actor` or `process` are likely more suitable.
////
//// ```gleam
//// let task = task.async(fn() { do_some_work() })
//// let value = do_some_other_work()
//// value + task.await(task, 100)
//// ```
////
//// Tasks spawned with async can be awaited on by their caller process (and
//// only their caller) as shown in the example above. They are implemented by
//// spawning a process that sends a message to the caller once the given
//// computation is performed.
////
//// There are some important things to consider when using tasks:
////
//// 1. If you are using async tasks, you must await a reply as they are always
////    sent.
////
//// 2. Tasks link the caller and the spawned process. This means that,
////    if the caller crashes, the task will crash too and vice-versa. This is
////    on purpose: if the process meant to receive the result no longer
////    exists, there is no purpose in completing the computation.
////
//// 3. A task's callback function must complete by returning or panicking.
////    It must not `exit` with the reason "normal".
////
//// This module is inspired by Elixir's [Task module][1].
////
//// [1]: https://hexdocs.pm/elixir/master/Task.html
////

import gleam/dict
import gleam/dynamic.{type Dynamic}
import gleam/erlang/process.{type Pid, type Selector, type Subject}
import gleam/function
import gleam/list
import gleam/option.{type Option, None, Some}

pub opaque type Task(value) {
  Task(owner: Pid, pid: Pid, subject: Subject(value))
}

// TODO: test
/// Spawn a task process that calls a given function in order to perform some
/// work. The result of this function is sent back to the parent and can be
/// received using the `await` function.
///
/// See the top level module documentation for more information on async/await.
///
pub fn async(work: fn() -> value) -> Task(value) {
  let owner = process.self()
  let subject = process.new_subject()
  let pid =
    process.start(linked: True, running: fn() { process.send(subject, work()) })
  Task(owner: owner, pid: pid, subject: subject)
}

pub type AwaitError {
  Timeout
  Exit(reason: Dynamic)
}

// We can only wait on a task if we are the owner of it so crash if we are
// waiting on a task we don't own.
fn assert_owner(task: Task(a)) -> Nil {
  let self = process.self()
  case task.owner == self {
    True -> Nil
    False ->
      process.send_abnormal_exit(
        self,
        "awaited on a task that does not belong to this process",
      )
  }
}

// TODO: test
/// Wait for the value computed by a task.
///
/// If the a value is not received before the timeout has elapsed then an error
/// is returned.
///
pub fn try_await(task: Task(value), timeout: Int) -> Result(value, AwaitError) {
  assert_owner(task)
  let selector =
    process.new_selector()
    |> process.selecting(task.subject, function.identity)
  case process.select(selector, timeout) {
    // The task process has sent back a value
    Ok(x) -> Ok(x)

    // The task process is alive but has not sent a value yet
    Error(Nil) -> Error(Timeout)
  }
}

/// Wait for the value computed by a task.
///
/// If the a value is not received before the timeout has elapsed or if the
/// task process crashes then this function crashes.
///
pub fn await(task: Task(value), timeout: Int) -> value {
  let assert Ok(value) = try_await(task, timeout)
  value
}

/// Get the `Pid` for a task.
///
pub fn pid(task: Task(value)) -> Pid {
  task.pid
}

/// Wait endlessly for the value computed by a task.
///
/// Be Careful! Like `try_await_forever`, this function does not return until
/// there is a value to receive.
///
/// If the task process crashes then this function crashes.
///
pub fn await_forever(task: Task(value)) -> value {
  assert_owner(task)
  let selector =
    process.new_selector()
    |> process.selecting(task.subject, function.identity)
  process.select_forever(selector)
}

type Message2(t1, t2) {
  M2FromSubject1(t1)
  M2FromSubject2(t2)
  M2Timeout
}

/// Wait for the values computed by multiple tasks.
///
/// For each task, if the a value is not received before the timeout has
/// elapsed then an error is returned.
///
pub fn try_await2(
  task1: Task(t1),
  task2: Task(t2),
  timeout: Int,
) -> #(Result(t1, AwaitError), Result(t2, AwaitError)) {
  assert_owner(task1)
  assert_owner(task2)

  let timeout_subject = process.new_subject()
  let timer = process.send_after(timeout_subject, timeout, M2Timeout)

  process.new_selector()
  |> process.selecting(task1.subject, M2FromSubject1)
  |> process.selecting(task2.subject, M2FromSubject2)
  |> process.selecting(timeout_subject, function.identity)
  |> try_await2_loop(None, None, timer)
}

fn try_await2_loop(
  selector: Selector(Message2(t1, t2)),
  t1: Option(Result(t1, AwaitError)),
  t2: Option(Result(t2, AwaitError)),
  timer: process.Timer,
) -> #(Result(t1, AwaitError), Result(t2, AwaitError)) {
  case t1, t2 {
    Some(t1), Some(t2) -> {
      process.cancel_timer(timer)
      #(t1, t2)
    }

    _, _ -> {
      case process.select_forever(selector) {
        // The task process has sent back a value
        M2FromSubject1(x) -> {
          let t1 = Some(Ok(x))
          try_await2_loop(selector, t1, t2, timer)
        }
        M2FromSubject2(x) -> {
          let t2 = Some(Ok(x))
          try_await2_loop(selector, t1, t2, timer)
        }

        M2Timeout -> {
          #(
            option.unwrap(t1, Error(Timeout)),
            option.unwrap(t2, Error(Timeout)),
          )
        }
      }
    }
  }
}

type Message3(t1, t2, t3) {
  M3FromSubject1(t1)
  M3FromSubject2(t2)
  M3FromSubject3(t3)
  M3Timeout
}

/// Wait for the values computed by multiple tasks.
///
/// For each task, if the a value is not received before the timeout has
/// elapsed then an error is returned.
///
pub fn try_await3(
  task1: Task(t1),
  task2: Task(t2),
  task3: Task(t3),
  timeout: Int,
) -> #(Result(t1, AwaitError), Result(t2, AwaitError), Result(t3, AwaitError)) {
  assert_owner(task1)
  assert_owner(task2)
  assert_owner(task3)

  let timeout_subject = process.new_subject()
  let timer = process.send_after(timeout_subject, timeout, M3Timeout)

  process.new_selector()
  |> process.selecting(task1.subject, M3FromSubject1)
  |> process.selecting(task2.subject, M3FromSubject2)
  |> process.selecting(task3.subject, M3FromSubject3)
  |> process.selecting(timeout_subject, function.identity)
  |> try_await3_loop(None, None, None, timer)
}

fn try_await3_loop(
  selector: Selector(Message3(t1, t2, t3)),
  t1: Option(Result(t1, AwaitError)),
  t2: Option(Result(t2, AwaitError)),
  t3: Option(Result(t3, AwaitError)),
  timer: process.Timer,
) -> #(Result(t1, AwaitError), Result(t2, AwaitError), Result(t3, AwaitError)) {
  case t1, t2, t3 {
    Some(t1), Some(t2), Some(t3) -> {
      process.cancel_timer(timer)
      #(t1, t2, t3)
    }

    _, _, _ -> {
      case process.select_forever(selector) {
        // The task process has sent back a value
        M3FromSubject1(x) -> {
          let t1 = Some(Ok(x))
          try_await3_loop(selector, t1, t2, t3, timer)
        }
        M3FromSubject2(x) -> {
          let t2 = Some(Ok(x))
          try_await3_loop(selector, t1, t2, t3, timer)
        }
        M3FromSubject3(x) -> {
          let t3 = Some(Ok(x))
          try_await3_loop(selector, t1, t2, t3, timer)
        }

        M3Timeout -> {
          #(
            option.unwrap(t1, Error(Timeout)),
            option.unwrap(t2, Error(Timeout)),
            option.unwrap(t3, Error(Timeout)),
          )
        }
      }
    }
  }
}

type Message4(t1, t2, t3, t4) {
  M4FromSubject1(t1)
  M4FromSubject2(t2)
  M4FromSubject3(t3)
  M4FromSubject4(t4)
  M4Timeout
}

/// Wait for the values computed by multiple tasks.
///
/// For each task, if the a value is not received before the timeout has
/// elapsed then an error is returned.
///
pub fn try_await4(
  task1: Task(t1),
  task2: Task(t2),
  task3: Task(t3),
  task4: Task(t4),
  timeout: Int,
) -> #(
  Result(t1, AwaitError),
  Result(t2, AwaitError),
  Result(t3, AwaitError),
  Result(t4, AwaitError),
) {
  assert_owner(task1)
  assert_owner(task2)
  assert_owner(task3)

  let timeout_subject = process.new_subject()
  let timer = process.send_after(timeout_subject, timeout, M4Timeout)

  process.new_selector()
  |> process.selecting(task1.subject, M4FromSubject1)
  |> process.selecting(task2.subject, M4FromSubject2)
  |> process.selecting(task3.subject, M4FromSubject3)
  |> process.selecting(task4.subject, M4FromSubject4)
  |> process.selecting(timeout_subject, function.identity)
  |> try_await4_loop(None, None, None, None, timer)
}

fn try_await4_loop(
  selector: Selector(Message4(t1, t2, t3, t4)),
  t1: Option(Result(t1, AwaitError)),
  t2: Option(Result(t2, AwaitError)),
  t3: Option(Result(t3, AwaitError)),
  t4: Option(Result(t4, AwaitError)),
  timer: process.Timer,
) -> #(
  Result(t1, AwaitError),
  Result(t2, AwaitError),
  Result(t3, AwaitError),
  Result(t4, AwaitError),
) {
  case t1, t2, t3, t4 {
    Some(t1), Some(t2), Some(t3), Some(t4) -> {
      process.cancel_timer(timer)
      #(t1, t2, t3, t4)
    }

    _, _, _, _ -> {
      case process.select_forever(selector) {
        // The task process has sent back a value
        M4FromSubject1(x) -> {
          let t1 = Some(Ok(x))
          try_await4_loop(selector, t1, t2, t3, t4, timer)
        }
        M4FromSubject2(x) -> {
          let t2 = Some(Ok(x))
          try_await4_loop(selector, t1, t2, t3, t4, timer)
        }
        M4FromSubject3(x) -> {
          let t3 = Some(Ok(x))
          try_await4_loop(selector, t1, t2, t3, t4, timer)
        }
        M4FromSubject4(x) -> {
          let t4 = Some(Ok(x))
          try_await4_loop(selector, t1, t2, t3, t4, timer)
        }

        M4Timeout -> {
          #(
            option.unwrap(t1, Error(Timeout)),
            option.unwrap(t2, Error(Timeout)),
            option.unwrap(t3, Error(Timeout)),
            option.unwrap(t4, Error(Timeout)),
          )
        }
      }
    }
  }
}

type Message(t) {
  Message(from: Int, value: t)
  MessageTimeout
}

/// Wait for the values computed by multiple tasks.
///
/// For each task, if the a value is not received before the timeout has
/// elapsed then an error is returned.
///
pub fn try_await_all(
  tasks: List(Task(t)),
  timeout: Int,
) -> List(Result(t, AwaitError)) {
  let #(selector, tasks_count) = {
    let acc = #(process.new_selector(), 0)
    // We need to do a couple of things before we start listening:
    // - we have to check the owner of every task
    // - we have to count the tasks to know when we can stop waiting for results
    // - we have to map each task's message to keep track of its position in the
    //   original list
    //
    // Instead of iterating through the tasks list three times we do everything
    // in a single fold that has to go through the list just once!
    use #(selector, tasks_count), task, index <- list.index_fold(tasks, acc)
    assert_owner(task)
    let selector = process.selecting(selector, task.subject, Message(index, _))
    #(selector, tasks_count + 1)
  }

  let timeout_subject = process.new_subject()
  let timer = process.send_after(timeout_subject, timeout, MessageTimeout)
  let selector = process.selecting(selector, timeout_subject, function.identity)

  try_await_all_loop(dict.new(), tasks_count, timer, selector)
}

fn try_await_all_loop(
  values: dict.Dict(Int, Result(b, AwaitError)),
  tasks_count: Int,
  timer: process.Timer,
  selector: Selector(Message(b)),
) -> List(Result(b, AwaitError)) {
  case dict.size(values) == tasks_count {
    // If there's no more values to receive then we can return the list...
    True -> {
      process.cancel_timer(timer)
      dict_to_list(values, tasks_count, Error(Timeout))
    }
    // Otherwise we wait to receive another message (or the timeout) from one of
    // the tasks.
    False ->
      case process.select_forever(selector) {
        MessageTimeout -> dict_to_list(values, tasks_count, Error(Timeout))
        Message(from: index, value:) -> {
          let values = dict.insert(values, index, Ok(value))
          try_await_all_loop(values, tasks_count, timer, selector)
        }
      }
  }
}

/// Given a dict returns a list with size `sized` where each item at index `i`
/// is `dict.get(dict, i) |> result.unwrap(default)`.
///
/// ## Examples
///
/// ```gleam
/// dict.from_list([#(1, "a"), #(4, "b")])
/// |> dict_to_list(5, " ")
/// // -> [" ", "a", " ", " ", "b"]
/// ```
///
fn dict_to_list(dict: dict.Dict(Int, a), sized: Int, default: a) -> List(a) {
  dict_to_list_loop(dict, default, sized - 1, [])
}

fn dict_to_list_loop(
  dict: dict.Dict(Int, a),
  default: a,
  index: Int,
  list: List(a),
) -> List(a) {
  case index < 0 {
    True -> list
    False -> {
      let value = case dict.get(dict, index) {
        Error(_) -> default
        Ok(value) -> value
      }
      dict_to_list_loop(dict, default, index - 1, [value, ..list])
    }
  }
}