func updateExchangeRate(code string)error{
    var rate float64
    switch (code){
        case "USD", "EUR", "CHF":
            resp, err := http.Get("http://currency-api.appspot.com/api/GBP/"+code+".json?key="+currency_api_key)
            if err != nil{
                return err
            }
            defer resp.Body.Close()
            body, err := ioutil.ReadAll(resp.Body)
            if err != nil {
                return err
            }
            var data struct{
                Success bool
                Source string
                Target string
                Rate float64
                Message string
            }
            err = json.Unmarshal(body, data)
            if err != nil {
                return err
            }
            if data.Success != true {
                err = errors.New(data.Message)
                return err
            }
            rate = data.Rate
            break
        default:
            err := errors.New("Unsupported currency code")
            return err
    }
    CurrencyRates[code] = rate_obj{
        Rate: rate,
        Expiry: time.Now().Unix() + (12*60*60),//12 hour expiry
    }
    return nil
}

func GetExchangeRate(code string)(float64, error){
    if CurrencyRates[code].Rate == 0 || CurrencyRates[code].Expiry >= time.Now().Unix() {
        err := updateExchangeRate(code)
        return 0, err
    }
    return CurrencyRates[code].Rate, nil
}

func ConvertCurrency(source, target string, value float64) (amount float64, err error){
    var src_rate, tgt_rate float64
    if source != "GBP"{
        src_rate, err = GetExchangeRate(source)
        if err != nil {
            return
        }
    }else{
        src_rate = 1.0
    }
    if target != "GBP"{
        tgt_rate, err = GetExchangeRate(source)
        if err != nil {
            return
        }
    }else{
        tgt_rate = 1.0
    }
    amount = value * (1/src_rate) * tgt_rate
    return
}