Novault-backend/internal/service/exchange_rate_service.go

package service

import (
	"errors"
	"fmt"
	"time"

	"accounting-app/internal/models"
	"accounting-app/internal/repository"
)

// Common exchange rate service errors
var (
	ErrInvalidRate         = errors.New("exchange rate must be positive")
	ErrInvalidEffectiveDate = errors.New("effective date cannot be in the future")
)

// ExchangeRateService handles business logic for exchange rates
type ExchangeRateService struct {
	repo *repository.ExchangeRateRepository
}

// NewExchangeRateService creates a new ExchangeRateService instance
func NewExchangeRateService(repo *repository.ExchangeRateRepository) *ExchangeRateService {
	return &ExchangeRateService{repo: repo}
}

// CreateExchangeRate creates a new exchange rate
func (s *ExchangeRateService) CreateExchangeRate(rate *models.ExchangeRate) error {
	// Validate rate value
	if rate.Rate <= 0 {
		return ErrInvalidRate
	}

	// Validate effective date (should not be in the future)
	if rate.EffectiveDate.After(time.Now()) {
		return ErrInvalidEffectiveDate
	}

	// Validate currencies are different
	if rate.FromCurrency == rate.ToCurrency {
		return repository.ErrSameCurrency
	}

	return s.repo.Create(rate)
}

// GetExchangeRateByID retrieves an exchange rate by its ID
func (s *ExchangeRateService) GetExchangeRateByID(id uint) (*models.ExchangeRate, error) {
	return s.repo.GetByID(id)
}

// GetAllExchangeRates retrieves all exchange rates
func (s *ExchangeRateService) GetAllExchangeRates() ([]models.ExchangeRate, error) {
	return s.repo.GetAll()
}

// UpdateExchangeRate updates an existing exchange rate
func (s *ExchangeRateService) UpdateExchangeRate(rate *models.ExchangeRate) error {
	// Validate rate value
	if rate.Rate <= 0 {
		return ErrInvalidRate
	}

	// Validate effective date (should not be in the future)
	if rate.EffectiveDate.After(time.Now()) {
		return ErrInvalidEffectiveDate
	}

	// Validate currencies are different
	if rate.FromCurrency == rate.ToCurrency {
		return repository.ErrSameCurrency
	}

	return s.repo.Update(rate)
}

// DeleteExchangeRate deletes an exchange rate by its ID
func (s *ExchangeRateService) DeleteExchangeRate(id uint) error {
	return s.repo.Delete(id)
}

// GetExchangeRateByCurrencyPair retrieves the most recent exchange rate for a currency pair
func (s *ExchangeRateService) GetExchangeRateByCurrencyPair(fromCurrency, toCurrency models.Currency) (*models.ExchangeRate, error) {
	return s.repo.GetByCurrencyPair(fromCurrency, toCurrency)
}

// GetExchangeRateByCurrencyPairAndDate retrieves the exchange rate for a currency pair on a specific date
func (s *ExchangeRateService) GetExchangeRateByCurrencyPairAndDate(fromCurrency, toCurrency models.Currency, date time.Time) (*models.ExchangeRate, error) {
	return s.repo.GetByCurrencyPairAndDate(fromCurrency, toCurrency, date)
}

// GetLatestExchangeRates retrieves the most recent exchange rate for each currency pair
func (s *ExchangeRateService) GetLatestExchangeRates() ([]models.ExchangeRate, error) {
	return s.repo.GetLatestRates()
}

// ConvertCurrency converts an amount from one currency to another using the most recent exchange rate
func (s *ExchangeRateService) ConvertCurrency(amount float64, fromCurrency, toCurrency models.Currency) (float64, error) {
	// If currencies are the same, return the original amount
	if fromCurrency == toCurrency {
		return amount, nil
	}

	// Get the exchange rate
	rate, err := s.repo.GetByCurrencyPair(fromCurrency, toCurrency)
	if err != nil {
		// If direct rate not found, try inverse rate
		if errors.Is(err, repository.ErrExchangeRateNotFound) {
			inverseRate, inverseErr := s.repo.GetByCurrencyPair(toCurrency, fromCurrency)
			if inverseErr != nil {
				return 0, fmt.Errorf("no exchange rate found for %s to %s: %w", fromCurrency, toCurrency, err)
			}
			// Use inverse rate: 1 / rate
			if inverseRate.Rate == 0 {
				return 0, errors.New("invalid inverse exchange rate (zero)")
			}
			return amount / inverseRate.Rate, nil
		}
		return 0, err
	}

	return amount * rate.Rate, nil
}

// ConvertCurrencyOnDate converts an amount from one currency to another using the exchange rate on a specific date
func (s *ExchangeRateService) ConvertCurrencyOnDate(amount float64, fromCurrency, toCurrency models.Currency, date time.Time) (float64, error) {
	// If currencies are the same, return the original amount
	if fromCurrency == toCurrency {
		return amount, nil
	}

	// Get the exchange rate for the specific date
	rate, err := s.repo.GetByCurrencyPairAndDate(fromCurrency, toCurrency, date)
	if err != nil {
		// If direct rate not found, try inverse rate
		if errors.Is(err, repository.ErrExchangeRateNotFound) {
			inverseRate, inverseErr := s.repo.GetByCurrencyPairAndDate(toCurrency, fromCurrency, date)
			if inverseErr != nil {
				return 0, fmt.Errorf("no exchange rate found for %s to %s on %s: %w", fromCurrency, toCurrency, date.Format("2006-01-02"), err)
			}
			// Use inverse rate: 1 / rate
			if inverseRate.Rate == 0 {
				return 0, errors.New("invalid inverse exchange rate (zero)")
			}
			return amount / inverseRate.Rate, nil
		}
		return 0, err
	}

	return amount * rate.Rate, nil
}

// GetExchangeRateByCurrency retrieves all exchange rates involving a specific currency
func (s *ExchangeRateService) GetExchangeRateByCurrency(currency models.Currency) ([]models.ExchangeRate, error) {
	return s.repo.GetByCurrency(currency)
}

// SetExchangeRate creates or updates an exchange rate for a currency pair
// This is a convenience method for users to set rates without worrying about create vs update
func (s *ExchangeRateService) SetExchangeRate(fromCurrency, toCurrency models.Currency, rate float64, effectiveDate time.Time) error {
	// Validate rate value
	if rate <= 0 {
		return ErrInvalidRate
	}

	// Validate effective date
	if effectiveDate.After(time.Now()) {
		return ErrInvalidEffectiveDate
	}

	// Validate currencies are different
	if fromCurrency == toCurrency {
		return repository.ErrSameCurrency
	}

	// Create new exchange rate entry
	exchangeRate := &models.ExchangeRate{
		FromCurrency:  fromCurrency,
		ToCurrency:    toCurrency,
		Rate:          rate,
		EffectiveDate: effectiveDate,
	}

	return s.repo.Create(exchangeRate)
}