Refactor exchange-rate conversion for reuse

pkg/api/impl/coingecko/coingecko.go

@@ -146,6 +146,33 @@ func (s *Service) GetCoinGraphData(convert, symbol, name string, start, end int6
 	return ret, nil
 }
 
+// GetExchangeRate gets the current excange rate between two currencies
+func (s *Service) GetExchangeRate(convertFrom string, convertTo string) (float64, error) {
+	convertFrom = strings.ToLower(convertFrom)
+	convertTo = strings.ToLower(convertTo)
+	if convertFrom == convertTo {
+		return 1.0, nil
+	}
+	rates, err := s.client.ExchangeRates()
+	if err != nil {
+		return 0, err
+	}
+	if rates == nil {
+		return 0, fmt.Errorf("expected rates, received nil")
+	}
+	// Combined rate is convertFrom->BTC->convertTo
+	fromRate, found := (*rates)[convertFrom]
+	if !found {
+		return 0, fmt.Errorf("unsupported currency conversion: %s", convertFrom)
+	}
+	toRate, found := (*rates)[convertTo]
+	if !found {
+		return 0, fmt.Errorf("unsupported currency conversion: %s", convertTo)
+	}
+	rate := toRate.Value / fromRate.Value
+	return rate, nil
+}
+
 // GetGlobalMarketGraphData gets global market graph data
 func (s *Service) GetGlobalMarketGraphData(convert string, start int64, end int64) (apitypes.MarketGraph, error) {
 	days := strconv.Itoa(util.CalcDays(start, end))
@@ -160,25 +187,10 @@ func (s *Service) GetGlobalMarketGraphData(convert string, start int64, end int6
 	}
 
 	// This API does not appear to support vs_currency and only returns USD, so use ExchangeRates to convert
-	rate := 1.0
-	if convertTo != "usd" {
-		rates, err := s.client.ExchangeRates()
-		if err != nil {
-			return ret, err
-		}
-		if rates == nil {
-			return ret, fmt.Errorf("expected rates, received nil")
-		}
-		// Combined rate is USD->BTC->other
-		btcRate, found := (*rates)[convertTo]
-		if !found {
-			return ret, fmt.Errorf("unsupported currency conversion: %s", convertTo)
-		}
-		usdRate, found := (*rates)["usd"]
-		if !found {
-			return ret, fmt.Errorf("unsupported currency conversion: usd")
-		}
-		rate = btcRate.Value / usdRate.Value
+	// TODO: watch out - this is not cached, so we hit the backend every time!
+	rate, err := s.GetExchangeRate("usd", convertTo)
+	if err != nil {
+		return ret, err
 	}
 
 	var marketCapUSD [][]float64