implement amount.go:Amount:FormatUSD with localizedLanguagePrinter

digits-work-sample-go.d/amount.go

@@ -52,7 +52,14 @@ func (amount Amount) Rounded(places int) Amount {
 
 // FormatUSD formats Amount as a string in US dollars.
 func (amount Amount) FormatUSD() string {
-	// TODO: Not implemented.
-	// Hint: Number localization gets tricky. Can you find a library to help?
-	return ""
+	rounded := amount.Rounded(2)
+	prefix := ""
+	if rounded < 0 {
+		prefix = "-"
+		rounded *= -1.0
+	}
+
+	roundedAsCurrency := currency.NarrowSymbol(currency.USD.Amount(float32(rounded)))
+	resultWithSpaceAfterDollarSign := localizedLanguagePrinter.Sprintf("%s%.2f", prefix, roundedAsCurrency)
+	return strings.Replace(resultWithSpaceAfterDollarSign, "$ ", "$", 1)
 }