converter/app/convert_currency.go

package app

import (
	"context"
	"converter/domain"
	"converter/infrastructure/coinmarketcap"
	"fmt"
	"strings"

	"github.com/shopspring/decimal"
)

// Constants
const (
	CurrencyPrecision = 8 // All currencies use 8 decimal precision
)

// RateProvider defines the contract for fetching exchange rates
// Note: The returned RateDTO may contain a reversed rate (e.g., if requesting USD->BTC
// but only BTC->USD is available), indicated by FromCode/ToCode being swapped from the request
type RateProvider interface {
	GetRate(ctx context.Context, fromCode, toCode string) (coinmarketcap.RateDTO, error)
}

// ConvertCurrencyUseCase handles currency conversion operations
type ConvertCurrencyUseCase struct {
	rateProvider RateProvider
	converter    *domain.CurrencyConverter
}

// NewConvertCurrencyUseCase creates a new ConvertCurrencyUseCase
func NewConvertCurrencyUseCase(rateProvider RateProvider, converter *domain.CurrencyConverter) *ConvertCurrencyUseCase {
	return &ConvertCurrencyUseCase{
		rateProvider: rateProvider,
		converter:    converter,
	}
}

// Execute performs currency conversion
func (uc *ConvertCurrencyUseCase) Execute(ctx context.Context, amount, fromCode, toCode string) (domain.Money, error) {
	// Normalize currency codes to uppercase for consistency
	fromCode = strings.ToUpper(strings.TrimSpace(fromCode))
	toCode = strings.ToUpper(strings.TrimSpace(toCode))

	// Create currencies with uniform precision
	fromCurrency, err := domain.NewCurrency(fromCode, fromCode, CurrencyPrecision)
	if err != nil {
		return domain.Money{}, err
	}

	// Create money object from input
	money, err := domain.NewMoney(amount, fromCurrency)
	if err != nil {
		return domain.Money{}, err
	}

	// Get exchange rate DTO from provider
	rateDTO, err := uc.rateProvider.GetRate(ctx, fromCode, toCode)
	if err != nil {
		return domain.Money{}, err
	}

	// Convert DTO to domain Rate (domain logic)
	rate, err := uc.createDomainRate(rateDTO, fromCode, toCode)
	if err != nil {
		return domain.Money{}, err
	}

	// Convert using domain service
	result, err := uc.converter.Convert(money, rate)
	if err != nil {
		return domain.Money{}, err
	}

	return result, nil
}

// createDomainRate converts RateDTO to domain.Rate with proper domain validation
func (uc *ConvertCurrencyUseCase) createDomainRate(dto coinmarketcap.RateDTO, requestedFromCode, requestedToCode string) (domain.Rate, error) {
	// Convert price to decimal with proper precision first
	priceStr := fmt.Sprintf("%.*f", CurrencyPrecision, dto.Price)
	rateValue, err := decimal.NewFromString(priceStr)
	if err != nil {
		return domain.Rate{}, fmt.Errorf("invalid price format: %v", err)
	}

	// Check if we got a reversed rate and handle accordingly
	if dto.FromCode == requestedToCode && dto.ToCode == requestedFromCode {
		// Reversed: we got BTC->USD but requested USD->BTC
		// dto.FromName always contains the crypto name (Bitcoin)
		if rateValue.IsZero() {
			return domain.Rate{}, fmt.Errorf("cannot invert zero price")
		}

		fromCurrency, err := domain.NewCurrency(requestedFromCode, dto.ToName, CurrencyPrecision)
		if err != nil {
			return domain.Rate{}, err
		}

		toCurrency, err := domain.NewCurrency(requestedToCode, dto.FromName, CurrencyPrecision)
		if err != nil {
			return domain.Rate{}, err
		}

		invertedRate := decimal.NewFromInt(1).Div(rateValue)
		return domain.NewRate(fromCurrency, toCurrency, invertedRate, dto.Source)
	}

	// Direct: we got exactly what we requested
	// dto.FromName contains the crypto name
	fromCurrency, err := domain.NewCurrency(dto.FromCode, dto.FromName, CurrencyPrecision)
	if err != nil {
		return domain.Rate{}, err
	}

	toCurrency, err := domain.NewCurrency(dto.ToCode, dto.ToName, CurrencyPrecision)
	if err != nil {
		return domain.Rate{}, err
	}

	return domain.NewRate(fromCurrency, toCurrency, rateValue, dto.Source)
}