implement rudimentary caching for yahoo finance to avoid http 429 rate limiting

providers/staticjsonYahooFinance/chart.go

@@ -3,10 +3,11 @@ package staticjsonYahooFinance
 import (
 	"fmt"
 	"net/url"
+	"time"
 )
 
-// A chart response is what we get back from Yahoo Finance
-type chart struct {
+// A chartResponse response is what we get back from Yahoo Finance
+type chartResponse struct {
 	Chart struct {
 		Result []struct {
 			Meta struct {
@@ -69,8 +70,16 @@ type chart struct {
 	} `json:"chart"`
 }
 
+// getChart first checks if the symbol chartCacheEntry is valid,
+// and if not then it downloads fresh chart data
 func (c client) getChart(symbol string) (int, error) {
-	chartResponse := &chart{}
+	if cacheItem, ok := chartCache[symbol]; ok {
+		// if the cacheEntry is still valid, use it
+		if time.Now().Before(cacheItem.LastUpdated.Add(cacheAgeSeconds)) {
+			return int(cacheItem.Chart.Chart.Result[0].Meta.RegularMarketPrice * 1000), nil
+		}
+	}
+	chartResponse := &chartResponse{}
 	err := c.get(fmt.Sprintf("/chart/%s", symbol), chartResponse, url.Values{})
 	if err != nil {
 		return 0, fmt.Errorf("http get request error: %v", err)
@@ -80,5 +89,7 @@ func (c client) getChart(symbol string) (int, error) {
 		return 0, fmt.Errorf("unexpected length of results - expected 1 but got %d", len(chartResponse.Chart.Result))
 	}
 
+	chartCache[symbol] = chartCacheEntry{Chart: *chartResponse, LastUpdated: time.Now()}
+
 	return int(chartResponse.Chart.Result[0].Meta.RegularMarketPrice * 1000), nil
 }