fiat: add Currency field to the Price struct

Add Currency field to the price struct so that Price can be used to
represent any currency.

fiat/coincap_api.go

@@ -15,6 +15,10 @@ import (
 const (
 	// coinCapHistoryAPI is the endpoint we hit for historical price data.
 	coinCapHistoryAPI = "https://api.coincap.io/v2/assets/bitcoin/history"
+
+	// coinCapDefaultCurrency is the currency that the price data returned
+	// by the Coin Cap API is quoted in.
+	coinCapDefaultCurrency = "USD"
 )
 
 // ErrQueryTooLong is returned when we cannot get a granularity level for a
@@ -196,6 +200,7 @@ func parseCoinCapData(data []byte) ([]*Price, error) {
 		usdRecords[i] = &Price{
 			Timestamp: time.Unix(0, ns.Nanoseconds()),
 			Price:     decPrice,
+			Currency:  coinCapDefaultCurrency,
 		}
 	}
 

fiat/coincap_api_test.go

@@ -193,10 +193,12 @@ func TestParseCoinCapData(t *testing.T) {
 		{
 			Price:     price1,
 			Timestamp: time1,
+			Currency:  coinCapDefaultCurrency,
 		},
 		{
 			Price:     price2,
 			Timestamp: time2,
+			Currency:  coinCapDefaultCurrency,
 		},
 	}
 

fiat/coindesk_api.go

@@ -17,6 +17,10 @@ const (
 
 	// coinDeskTimeFormat is the date format used by coindesk.
 	coinDeskTimeFormat = "2006-01-02"
+
+	// coinDeskDefaultCurrency is the default currency that the price data
+	// returned by the Coin Desk API is quoted in.
+	coinDeskDefaultCurrency = "USD"
 )
 
 // coinDeskAPI implements the fiatBackend interface.
@@ -65,6 +69,7 @@ func parseCoinDeskData(data []byte) ([]*Price, error) {
 		usdRecords = append(usdRecords, &Price{
 			Timestamp: timestamp,
 			Price:     decimal.NewFromFloat(price),
+			Currency:  coinDeskDefaultCurrency,
 		})
 	}
 

fiat/coindesk_api_test.go

@@ -59,11 +59,13 @@ func TestParseCoinDeskData(t *testing.T) {
 				{
 					Price:     price,
 					Timestamp: timestamp,
+					Currency:  coinDeskDefaultCurrency,
 				},
 			}
 
 			require.True(t, expectedPrices[0].Price.Equal(prices[0].Price))
 			require.True(t, expectedPrices[0].Timestamp.Equal(prices[0].Timestamp))
+			require.Equal(t, expectedPrices[0].Currency, prices[0].Currency)
 		})
 	}
 }

fiat/fiat.go

@@ -23,7 +23,7 @@ var (
 	errRetriesFailed = errors.New("could not get data within max retries")
 )
 
-// Price represents the Bitcoin price in USD at a certain time.
+// Price represents the Bitcoin price in the given currency at a certain time.
 type Price struct {
 	// Timestamp is the time at which the BTC price is quoted.
 	Timestamp time.Time
@@ -31,6 +31,9 @@ type Price struct {
 	// Price is the fiat price for the given currency for 1 BTC at the
 	// given timestamp.
 	Price decimal.Decimal
+
+	// Currency is the code of the currency that the Price is quoted in.
+	Currency string
 }
 
 // retryQuery calls an api until it succeeds, or we hit our maximum retries.