export interface PromoLike {
  kind: 'AGENCY_GIFT' | 'DISCOUNT';
  active: boolean;
  maxUses: number | null;
  usedCount: number;
  validFrom: Date | null;
  validUntil: Date | null;
}

export type PromoCheck =
  | { ok: true }
  | { ok: false; reason: 'INACTIVE' | 'NOT_STARTED' | 'EXPIRED' | 'EXHAUSTED' | 'ALREADY_USED_BY_CLIENT' };

export function checkPromo(promo: PromoLike, now: Date, usedByClient: boolean): PromoCheck {
  if (!promo.active) return { ok: false, reason: 'INACTIVE' };
  if (promo.validFrom && now < promo.validFrom) return { ok: false, reason: 'NOT_STARTED' };
  if (promo.validUntil && now > promo.validUntil) return { ok: false, reason: 'EXPIRED' };
  if (promo.maxUses !== null && promo.usedCount >= promo.maxUses) return { ok: false, reason: 'EXHAUSTED' };
  if (usedByClient) return { ok: false, reason: 'ALREADY_USED_BY_CLIENT' };
  return { ok: true };
}

/** Промокод-скидка не суммируется с Barev+: берётся большая из двух */
export function combineDiscounts(barevPlusPct: number, promoPct: number | null): number {
  return Math.max(barevPlusPct, promoPct ?? 0);
}