// Singleton, remember when first referenced, key = DatedConversionRate.id
private static Map<ID,DatedConversionRate>              dcrIdToDcrMap;                      

//  ----------------------------------------
//  convertToOrgCurrency
//  ----------------------------------------
public static Decimal convertToOrgCurrency(String currencyIso, Decimal currencyVal, Date currencyDate) {
    Decimal res;
    if (currencyIso == null || currencyVal == null) {}
    else {
        if (dcrIdToDcrMap == null)              // build the map only once for life of the transaoction
            dcrIdToDcrMap = new Map<ID,DatedConversionRate> ([select id, isoCode, conversionrate, nextStartDate, startDate from DatedConversionRate]);
        for (DatedConversionRate dcr: dcrIdToDcrMap.values()) {
            if (dcr.isoCode != currencyIso)                                         continue;   // look for a match in table against input currency
            if (currencyDate < dcr.startDate || currencyDate >= dcr.nextStartDate)  continue;   // not in the range of effective dates
            System.debug(LoggingLevel.INFO,'Converting using dcr: ' + dcr);
            res =  currencyVal / dcr.conversionRate;
            break;  
        }
    }
    System.debug(LoggingLevel.INFO,'convertToOrgCurrency res= ' + res);
    return res;
}

// Singleton, remember when first referenced, key = DatedConversionRate.id
private static Map<ID,DatedConversionRate>              dcrIdToDcrMap;                      

//  ----------------------------------------
//  convertToOrgCurrency - convert value in some ISO currency to org currency as of given date
//  ----------------------------------------
public static Decimal convertToOrgCurrency(String currencyIso, Decimal currencyVal, Date currencyDate) {
    Decimal res;
    if (currencyIso == null || currencyVal == null) {}
    else {
        if (dcrIdToDcrMap == null)              // build the map only once for life of the transaoction
            dcrIdToDcrMap = new Map<ID,DatedConversionRate> ([select id, isoCode, conversionrate, nextStartDate, startDate from DatedConversionRate]);
        for (DatedConversionRate dcr: dcrIdToDcrMap.values()) {
            if (dcr.isoCode != currencyIso)                                         continue;   // look for a match in table against input currency
            if (currencyDate < dcr.startDate || currencyDate >= dcr.nextStartDate)  continue;   // not in the range of effective dates
            System.debug(LoggingLevel.INFO,'Converting using dcr: ' + dcr);
            res =  currencyVal / dcr.conversionRate;
            break;  
        }
    }
    System.debug(LoggingLevel.INFO,'convertToOrgCurrency res= ' + res);
    return res;
}

//  ----------------------------------------
//  convertFromOrgCurrency - convert value in org currency to some iso currency as of given date 
//  ----------------------------------------
public static Decimal convertFromOrgCurrency(String currencyIso, Decimal currencyVal, Date currencyDate) {
    Decimal res;
    if (currencyIso == null || currencyVal == null) {}
    else {
        if (dcrIdToDcrMap == null)              // build the map only once for life of the transaoction
            dcrIdToDcrMap = new Map<ID,DatedConversionRate> ([select id, isoCode, conversionrate, nextStartDate, startDate from DatedConversionRate]);
        for (DatedConversionRate dcr: dcrIdToDcrMap.values()) {
            if (dcr.isoCode != currencyIso)                                         continue;   // look for a match in table against input currency
            if (currencyDate < dcr.startDate || currencyDate >= dcr.nextStartDate)  continue;   // not in the range of effective dates
            System.debug(LoggingLevel.INFO,'Converting using dcr: ' + dcr);
            res =  currencyVal * dcr.conversionRate;
            break;  
        }
    }
    System.debug(LoggingLevel.INFO,'convertFromOrgCurrency res= ' + res);
    return res;
}