RxJS feedback loop using Rx.Subject

feedback.ts

// DO NOT SUBMIT: Code for teaching ariels to feed state back through
// RxJS.

import { test } from 'ava';
import * as Rx from 'rxjs';
import { List } from 'immutable-sorted';

class State {
  private constructor(private lastN: List<number>, private num: number) {}

  public static make(num: number): State { return new State(List(), num); }

  public push(a: number): State {
    const newLast = List(this.lastN.push(a).slice(-this.num));
    return new State(newLast, this.num);
  }

  public avg(): number {
    const sum = this.lastN.reduce((a, b) => a! + b!, 0);
    return sum / this.num;
  }
}

test('feedback', async (t) => {
  const state = new Rx.Subject<State>();

  const avgs = [] as number[];

  const c = Rx.Observable.interval(100)
    .withLatestFrom(state)
    .map(([n, s]) => [n, s.push(n)] as [number, State])
    .do(([_, s]) => state.next(s))
      .take(8)
    .forEach(([_, s]) => avgs.push(s.avg()));
  state.next(State.make(4));
  await c;

  t.deepEqual([0, 0.25, 0.75, 1.5, 2.5, 3.5, 4.5, 5.5], avgs);
});