// Service defines exposed apis about billing system.
type Service struct {
	dbType string
	opUser *user.User

	cashService       *CashService
	contractService   *ContractService
	couponService     *CouponService
	eventService      *EventService
	ledgerService     *LedgerService
	paymentService    *PaymentService
	receivableService *ReceivableService
	profitService     *ProfitService
}

// NewService creates service.
func NewService(dbType string, repo *resource.Repository) *Service {
	return &Service{
		dbType:            dbType,
		cashService:       NewCashService(&dbCashSource{}),
		contractService:   NewContractService(&dbContractSource{}),
		couponService:     NewCouponService(&dbCouponSource{}),
		eventService:      NewEventService(&dbEventSource{}),
		ledgerService:     NewLedgerService(&dbLedgerSource{}),
		paymentService:    NewPaymentService(&dbPaymentSource{}),
		receivableService: NewReceivableService(&dbReceivableSource{}),
		profitService:     NewProfitService(&dbProfitSource{}, NewProfitCalculator(repo)),
	}
}

database.Transation(db, func(tx *gorm.DB) error {
  serviceA.DoSomething()
  ...
  serviceB.DoSomething()
  ...
  serviceC.DoSomething()
  return nil
})

adjFunc := defaultCouponAdjectFunc
if couponType == CouponQuota {
	adjFunc = s.GetQuotaCouponAdjectFunc(rates)
}
switch couponType {
case CouponQuota, CouponAmount:
	quotaUsed := s.GetCouponUsed(ledgers, coupon.ID, adjFunc)
	...
case CouponLessRate:
	...
case CouponPercent:
	return nil, errors.Wrap(app.ErrUnexpected, "TBD")
}

// GetCouponUsed return coupon usage.
func (s *Service) GetCouponUsed(ledgers []Ledger, couponID uuid.UUID,
	adjFunc func(x float64, month string) float64) float64 {
	var total float64
	for _, ledger := range ledgers {
		if ledger.Subject != SubjectDiscount.String() {
			continue
		}

		if ledger.CouponID == couponID {
			total += adjFunc(-ledger.Amount, ledger.Date)
		}
	}
	return total
}