import gleam/erlang/process.{type Subject}

type Msg(state) {
  Assign(state)
  Update(Subject(state), fn(state) -> state)
  Retrieve(Subject(state))
}

pub opaque type State(state) {
  Handle(Subject(Msg(state)))
}

fn run(state: state, pipe: Subject(Msg(state))) -> Nil {
  case process.receive_forever(pipe) {
    Assign(new_state) -> run(new_state, pipe)
    Update(return_pipe, with) -> {
      process.send(return_pipe, state)
      with(state) |> run(pipe)
    }
    Retrieve(return_pipe) -> {
      process.send(return_pipe, state)
      run(state, pipe)
    }
  }
}

pub fn new(initial_state: state) -> State(state) {
  let back_channel: Subject(Subject(Msg(state))) = process.new_subject()

  process.spawn(fn() {
    let pipe: Subject(Msg(state)) = process.new_subject()
    process.send(back_channel, pipe)
    run(initial_state, pipe)
  })

  let pipe = process.receive_forever(back_channel)
  Handle(pipe)
}

pub fn set(new: state, handle: State(state)) {
  let Handle(send) = handle
  process.send(send, Assign(new))
}

pub fn get(handle: State(state)) -> state {
  let return: Subject(state) = process.new_subject()
  let Handle(send) = handle
  process.send(send, Retrieve(return))
  process.receive_forever(return)
}