smut

import gleam/result
import gleam/string

import gleam/erlang/process.{type Subject}
import gleam/otp/actor.{type Next}

import smut/types.{
  type SmutError, type TableHandle, type ValueHandle, NewExited, NewFailed,
  ValueHandle,
}

const default_timeout: Int = 5000

fn from_start_error(error: actor.StartError) -> SmutError {
  case error {
    actor.InitTimeout -> NewFailed("timeout")
    actor.InitFailed(reason) -> NewFailed(reason)
    actor.InitExited(reason) ->
      case reason {
        process.Normal -> NewExited("process exited normally")
        process.Killed -> NewExited("process was killed")
        process.Abnormal(reason) -> string.inspect(reason) |> NewExited()
      }
  }
}

@external(erlang, "erl_shim", "get_table")
fn get_table() -> TableHandle

@external(erlang, "erl_shim", "lookup")
fn lookup_shim(table: TableHandle, key: Int) -> Result(a, Nil)

@external(erlang, "erl_shim", "insert_new")
fn insert_new_shim(table: TableHandle, val: a) -> Int

@external(erlang, "erl_shim", "replace")
fn replace_shim(table: TableHandle, key: Int, val: a) -> Nil

/// Create a table in which to store shared, mutable state, and return a
/// handle to it.
///
/// This should be called as early as possible in your program, and the
/// return value used to instantiate new state slots.
///
/// ```gleam
/// let global_table = smut.init()
/// let counter = smut.new(0)
/// smut.get(counter) |> echo
/// // Ok(0)
/// ```
pub fn init() -> TableHandle {
  get_table()
}

type Msg(state) {
  Replace(state)
  Update(fn(state) -> state)
  UpdateThenRetrieve(Subject(state), fn(state) -> state)
  RetrieveThenUpdate(Subject(state), fn(state) -> state)
}

/// A handle to an individual shared, mutable value of type `state`.
pub opaque type State(state) {
  State(pipe: Subject(Msg(state)), handle: ValueHandle(state), timeout: Int)
}

/// Change the default retrieval timeout for this `state` handle from the
/// default of 5000 ms to `ms` ms.
pub fn with_timeout(state: State(state), ms: Int) -> State(state) {
  State(..state, timeout: ms)
}

fn lookup(handle: ValueHandle(a)) -> Result(a, Nil) {
  let ValueHandle(table, key) = handle
  lookup_shim(table, key)
}

fn replace(handle: ValueHandle(a), new_val: a) -> Nil {
  let ValueHandle(table, key) = handle
  replace_shim(table, key, new_val)
}

fn insert_new(table: TableHandle, val: a) -> ValueHandle(a) {
  let key = insert_new_shim(table, val)
  ValueHandle(table, key)
}

fn run_actor(
  handle: ValueHandle(state),
  message: Msg(state),
) -> Next(ValueHandle(state), Msg(state)) {
  case message {
    Replace(new_val) -> {
      replace(handle, new_val)
      actor.continue(handle)
    }
    Update(f) ->
      case lookup(handle) {
        Ok(val) -> {
          f(val) |> replace(handle, _)
          actor.continue(handle)
        }
        Error(_) -> actor.continue(handle)
      }
    UpdateThenRetrieve(return, f) ->
      case lookup(handle) {
        Ok(val) -> {
          let new_val = f(val)
          replace(handle, new_val)
          process.send(return, new_val)
          actor.continue(handle)
        }
        Error(_) -> actor.continue(handle)
      }
    RetrieveThenUpdate(return, f) ->
      case lookup(handle) {
        Ok(val) -> {
          process.send(return, val)
          f(val) |> replace(handle, _)
          actor.continue(handle)
        }
        Error(_) -> actor.continue(handle)
      }
  }
}

/// Create a new slot for a shared, mutable value of type `state`, and
/// return a handle to it, through which reads, writes, and updates can
/// be performed.
pub fn new(
  table: TableHandle,
  initial_val: state,
) -> Result(State(state), SmutError) {
  let value_handle = insert_new(table, initial_val)
  use actor <- result.try(
    actor.new(value_handle)
    |> actor.on_message(run_actor)
    |> actor.start
    |> result.map_error(from_start_error),
  )
  State(actor.data, value_handle, default_timeout) |> Ok()
}

/// Get the current value.
pub fn get(state: State(state)) -> Result(state, Nil) {
  lookup(state.handle)
}

/// Set the value of the state to `new_val`.
///
/// The majority of the time, you will probably want to use one of
/// 
/// * `update()`
/// * `update_and_retrieve()`
/// * `retrieve_and_update()`
/// 
/// instead. An operation like
/// ```gleam
/// let assert Ok(cur_val) = smut.get(state)
/// let new_val = update_state(cur_val)
/// smut.set(state, new_val)
/// ```
/// is not guaranteed to be atomic. Another process could futz with the value
/// of `state` between the call to `smut.get()` and `smut.set()`. The `update`
/// family of functions ensure atomic updates.
pub fn set(new_val: state, state: State(state)) -> Nil {
  process.send(state.pipe, Replace(new_val))
}

/// Update the current value of `state` with the given update function.
///
/// This operation is guaranteed to be atomic; that is, no other process
/// can affect the value of `state` between its retrieval and replacement.
pub fn update(with: fn(state) -> state, state: State(state)) -> Nil {
  process.send(state.pipe, Update(with))
}

pub fn update_and_retrieve(
  with: fn(state) -> state,
  state: State(state),
) -> Result(state, Nil) {
  let return = process.new_subject()
  process.send(state.pipe, UpdateThenRetrieve(return, with))
  process.receive(return, state.timeout)
}

pub fn retrieve_and_update(
  with: fn(state) -> state,
  state: State(state),
) -> Result(state, Nil) {
  let return = process.new_subject()
  process.send(state.pipe, RetrieveThenUpdate(return, with))
  process.receive(return, state.timeout)
}