attempt at sm4, likely inconsistent but sm5 is significantly different so no dwelling

sm4.ts

const answers: CardAnswer[] = [
      {
        "hesitationDetected": true,
        "correct": true,
        "timeSpent": 5000,
        "date": new Date("2018-11-01T15:13:00.000Z")
      },
      {
        "hesitationDetected": false,
        "correct": false,
        "timeSpent": 10000,
        "date": new Date("2018-11-02T06:30:00.000Z")
      },
      {
        "hesitationDetected": true,
        "correct": true,
        "timeSpent": 1000,
        "date": new Date("2018-11-02T07:01:00.000Z")
      }
    ];
interface CardAnswer {
  [key: string]: (boolean | number | date)
  hesitationDetected: boolean;
  correct: boolean;
  timeSpent: number;
  date: date;
  ef?: EasinessFactor;
  interval?: Interval;
  oi: OptimalInterval;
}
type Quality = 0|1|2|3|4|5;
type EasinessFactor = number;
type Interval = number;
type OptimalInterval = number;

const interv = (rep: number, ef: EasinessFactor /* 1.3 <= easinessFactor <= 2.5 */): Interval => 
  rep === 1 ? 1
  : rep === 2 ? 6
  : interv(rep - 1, ef) * ef;

const ef = (prevEf: EasinessFactor, q: Quality): EasinessFactor => 
  Math.max(1.3, Math.min(2.5, 
        prevEf + (0.1 - (5 - q) * (0.08 + (5 - q) * 0.02))))

const efFromCards = (as: CardAnswer[]): EasinessFactor =>
  as.length > 1
  ? ef(efFromCards(as.slice(0, as.length - 1)), qual(as[as.length - 1]))
  : 2.5;

const qual = (a: CardAnswer): Quality => 
  a.correct
  ? (!a.hesitationDetected && a.timeSpent < 5000
     ? 5
     : !a.hesitationDetected || a.timeSpent < 5000
     ? 4
     : 3)
  : (!a.hesitationDetected && a.timeSpent < 5000
     ? 2
     : !a.hesitationDetected || a.timeSpent < 5000
     ? 1
     : 0);

const nextOI = (prevOI: OptimalInterval, interimOI: OptimalInterval, frac: number): OptimalInterval =>
  (1 - frac) * prevOI + frac * interimOI;

const primeOI = (i: interval, ef: EasinessFactor, q: Quality): OptimalInterval =>
  i * i * (1 - 1 / ef) / 2 * (0.2 * q - 1);
  
const calcOI = (as: CardAnswers[]): OptimalInterval =>
  as.length > 1
  ? nextOI(
      calcOI(as.slice(0, as.length - 1)),
      primeOI(as[as.length - 1].interval, as[as.length - 1].ef, qual(as[as.length - 1])),
      .5)
  : 2.5;

console.log(
  answers
    .reduce((as: CardAnswer[], a: CardAnswer): CardAnswer[] =>
            [...as, {...a, ef: efFromCards([...as, a, ]) }], [])
    .map((a: CardAnswer, x: number): CardAnswer => 
         ({ ...a, interval: interv(x + 1, a.ef, qual(a))}))
    .reduce((as: CardAnswer[], a: CardAnswer): CardAnswer => 
            [...as, {...a, oi: calcOI([...as, a, ])}], [])
    .map(({ef, interval, oi}) => ({ ef, interval, oi,})),
     
);