converter/infrastructure/coinmarketcap/client.go

package coinmarketcap

import (
	"context"
	"errors"
	"fmt"

	"github.com/go-resty/resty/v2"
)

// CoinMarketCap specific errors
var (
	ErrUnknownSymbol   = errors.New("unknown symbol")
	ErrInvalidConvert  = errors.New("invalid convert currency")
	ErrAPIError        = errors.New("coinmarketcap api error")
	ErrInvalidResponse = errors.New("invalid response format")
)

const (
	baseURL    = "https://pro-api.coinmarketcap.com"
	quotesPath = "/v1/cryptocurrency/quotes/latest"
)

// Client implements the RateProvider interface for CoinMarketCap API
type Client struct {
	httpClient *resty.Client
	apiKey     string
}

// NewClient creates a new CoinMarketCap API client
func NewClient(apiKey string) *Client {
	client := resty.New().
		SetBaseURL(baseURL).
		SetHeader("Accept", "application/json").
		SetHeader("X-CMC_PRO_API_KEY", apiKey)

	return &Client{
		httpClient: client,
		apiKey:     apiKey,
	}
}

// GetRate fetches exchange rate from CoinMarketCap API
func (c *Client) GetRate(ctx context.Context, fromCode, toCode string) (RateDTO, error) {
	// Try direct conversion first
	rateDTO, err := c.tryGetRate(ctx, fromCode, toCode)
	if err == nil {
		return rateDTO, nil
	}

	// If empty result but no other error, try reverse
	if err == ErrUnknownSymbol {
		reverseDTO, reverseErr := c.tryGetRate(ctx, toCode, fromCode)
		if reverseErr == nil {
			return reverseDTO, nil
		}

		// If reverse request failed with invalid convert, that confirms the original
		// "from" currency is indeed unknown (it's now the "convert" parameter)
		if reverseErr == ErrInvalidConvert {
			return RateDTO{}, err // Return original unknown symbol error
		}

		// Any other error from reverse request (network, auth, etc) should be surfaced
		return RateDTO{}, reverseErr
	}

	// Return original error (either non-unknown-symbol error or first currency not in crypto database)
	return RateDTO{}, err
}

// tryGetRate attempts to get rate in specified direction
func (c *Client) tryGetRate(ctx context.Context, fromCode, toCode string) (RateDTO, error) {
	var response APIResponse

	// Make API request
	resp, err := c.httpClient.R().
		SetContext(ctx).
		SetQueryParam("symbol", fromCode).
		SetQueryParam("convert", toCode).
		SetResult(&response).
		Get(quotesPath)

	if err != nil {
		return RateDTO{}, fmt.Errorf("%w: %v", ErrAPIError, err)
	}

	// Check HTTP status first
	if resp.StatusCode() != 200 {
		switch resp.StatusCode() {
		case 400:
			return RateDTO{}, fmt.Errorf("%w: %s", ErrInvalidConvert, toCode)
		case 401:
			return RateDTO{}, fmt.Errorf("%w: unauthorized - bad or missing API key", ErrAPIError)
		default:
			return RateDTO{}, fmt.Errorf("%w: HTTP %d", ErrAPIError, resp.StatusCode())
		}
	}

	// Check if data is empty - invalid currency
	if len(response.Data) == 0 {
		return RateDTO{}, fmt.Errorf("%w: %s", ErrUnknownSymbol, fromCode)
	}

	// Extract rate from response
	rateDTO, err := c.extractRateDTO(response, fromCode, toCode)
	if err != nil {
		return RateDTO{}, err
	}

	return rateDTO, nil
}


// extractRateDTO converts API response to RateDTO
func (c *Client) extractRateDTO(response APIResponse, fromCode, toCode string) (RateDTO, error) {
	// Get crypto data for the from currency
	cryptoData, exists := response.Data[fromCode]
	if !exists {
		return RateDTO{}, ErrInvalidResponse
	}

	// Get quote for the to currency
	quote, exists := cryptoData.Quote[toCode]
	if !exists {
		return RateDTO{}, ErrInvalidResponse
	}

	// Return the rate exactly as API provided it
	return RateDTO{
		FromCode: fromCode,
		ToCode:   toCode,
		FromName: cryptoData.Name,
		ToName:   toCode,
		Price:    quote.Price,
		Source:   "coinmarketcap",
	}, nil
}