// RxJS v6+
import { interval, fromEvent, combineLatest, BehaviorSubject } from 'rxjs';
import { scan, startWith, map, takeWhile, switchMap } from 'rxjs/operators';
import { State, Letter, Letters } from './interfaces';
​
const randomLetter = () =>
  String.fromCharCode(
    Math.random() * ('z'.charCodeAt(0) - 'a'.charCodeAt(0)) + 'a'.charCodeAt(0)
  );
const levelChangeThreshold = 20;
const speedAdjust = 50;
const endThreshold = 15;
const gameWidth = 30;
​
const intervalSubject = new BehaviorSubject(600);
​
const letters$ = intervalSubject.pipe(
  switchMap(i =>
    interval(i).pipe(
      scan<number, Letters>
        (letters => ({
          intrvl: i,
          ltrs: [
            {
              letter: randomLetter(),
              yPos: Math.floor(Math.random() * gameWidth)
            },
            ...letters.ltrs
          ]
        }),
        { ltrs: [], intrvl: 0 })
    )
  )
);
​
const keys$ = fromEvent(document, 'keydown').pipe(
  startWith({ key: '' }),
  map((e: KeyboardEvent) => e.key)
);
​
const renderGame = (state: State) => (
  (document.body.innerHTML = `Score: ${state.score}, Level: ${state.level} <br/>`),
  state.letters.forEach(
    l =>
      (document.body.innerHTML += '&nbsp'.repeat(l.yPos) + l.letter + '<br/>')
  ),
  (document.body.innerHTML +=
    '<br/>'.repeat(endThreshold - state.letters.length - 1) +
    '-'.repeat(gameWidth))
);
const renderGameOver = () => (document.body.innerHTML += '<br/>GAME OVER!');
const noop = () => {};
​
const game$ = combineLatest(keys$, letters$).pipe(
  scan<[string, Letters], State>
    ((state, [key, letters]) => (
      letters.ltrs[letters.ltrs.length - 1] &&
      letters.ltrs[letters.ltrs.length - 1].letter === key
        ? ((state.score = state.score + 1), letters.ltrs.pop())
        : noop,
      state.score > 0 && state.score % levelChangeThreshold === 0
        ? ((letters.ltrs = []),
          (state.level = state.level + 1),
          (state.score = state.score + 1),
          intervalSubject.next(letters.intrvl - speedAdjust))
        : noop,
      { score: state.score, letters: letters.ltrs, level: state.level }
    ),
    { score: 0, letters: [], level: 1 }),
  takeWhile(state => state.letters.length < endThreshold)
);
​
game$.subscribe(renderGame, noop, renderGameOver);

export interface Letter {
  letter: String;
  yPos: number;
}
export interface Letters {
  ltrs: Letter[];
  intrvl: number;
}
export interface State {
  score: number;
  letters: Letter[];
  level: number;
}