export type Sm2Grade = 1 | 2 | 3 | 4

export interface Sm2State {
  easinessFactor: number
  interval: number
}

export interface Sm2UpdateResult extends Sm2State {
  nextReviewAt: Date
}

/** SM-2 update per US-FLASHCARDS spec (grades 1–4). */
export function computeSm2Update(
  grade: number,
  previous: Sm2State,
): Sm2UpdateResult {
  const g = Math.min(4, Math.max(1, Math.round(grade))) as Sm2Grade
  const ef = previous.easinessFactor
  const newEF = Math.max(
    1.3,
    ef + 0.1 - (5 - g) * (0.08 + (5 - g) * 0.02),
  )

  const nextInterval =
    g <= 2 ? 1 : Math.max(1, Math.round(previous.interval * newEF))

  const nextReviewAt = new Date()
  nextReviewAt.setDate(nextReviewAt.getDate() + nextInterval)

  return {
    easinessFactor: newEF,
    interval: nextInterval,
    nextReviewAt,
  }
}

export function isCardMastered(interval: number): boolean {
  return interval >= 21
}