package org.apidesign.apifest08.currency;

import java.math.BigDecimal;

import java.math.MathContext;

import java.math.RoundingMode;

/** Convertor able to convert amount from one currency to other currency.

 * 

 * Exchange rate is provided by {@link ExchangeRateProvider}.

 */

public class Convertor {

    boolean remainderAllowed = false; //if false, remained is not allowed (should be true ideally, but can't handle it now)

    ExchangeRateProvider exchangeRateProvider; 

    private Convertor() {

    }

    /**

     * Static method used to create convertor.

     * 

     * @param exchangeRateProvider {@link ExchangeRateProvider} used to get exchange rate.

     * 

     * @return Returns <code>Convertor</code> which can be used to convert money.

     */

    public static Convertor createConvertor(ExchangeRateProvider exchangeRateProvider) {

        Convertor c = new Convertor();

        c.exchangeRateProvider = exchangeRateProvider;

        return c;

    }

    /**

     * Convert <code>amount</code> from <code>fromCurrency</code> to <code>toCurrency</code> as specified

     * in <code>ExchangeRateProvider</code>.

     * 

     * @param amount Amount which should be converted. Can't be negative value (can be zero or positive).

     * @return Return <code>ConversionResult</code> which holds conversion result.

     */

    public ConversionResult convert(BigDecimal amount) {

        return convertValue(amount, false);

    }

    /**

     * Convert <code>amount</code> from <code>toCurrency</code> to <code>fromCurrency</code> as specified

     * in <code>ExchangeRateProvider</code>. This is <em>reverted</em> order than suggested by  names of currency fields in <code>ExchangeRate</code>.

     * 

     * @param amount Amount which should be converted. Can't be negative value (can be zero or positive).

     * @return Return <code>ConversionResult</code> which holds conversion result.

     */

    public ConversionResult convertBack(BigDecimal amount) {

        return convertValue(amount, true);

    }

    private ConversionResult convertValue(BigDecimal amount, boolean convertBack) throws RuntimeException {

        ConversionResult result = new ConversionResult();

        ExchangeRate rate = exchangeRateProvider.getExchangeRate();

        int fromFranctionDigits = exchangeRateProvider.getFromCurrency().getDefaultFractionDigits();

        int toFractionDigits = exchangeRateProvider.getToCurrency().getDefaultFractionDigits();

        if (toFractionDigits!=2) {

            throw new RuntimeException("Can't process currency with defaultFractionDigits!=2, "+exchangeRateProvider.getToCurrency()+" has "+toFractionDigits+" defaultFractionDigits");

        }

        if (fromFranctionDigits!=2) {

            throw new RuntimeException("Can't process currency with defaultFractionDigits!=2, "+exchangeRateProvider.getFromCurrency()+" has "+fromFranctionDigits+" defaultFractionDigits");

        }

        if (amount.signum()==-1) {

            throw new RuntimeException("Can convert only non-negative value, current value is "+amount);

        }

        MathContext context = new MathContext(0, RoundingMode.DOWN);

        BigDecimal from;

        BigDecimal to;

        if (convertBack) {

            //converting in reverted way

            to = rate.getFromValue();

            from = rate.getToValue();

        } else {

            //converting in mornak way

            from = rate.getFromValue();

            to = rate.getToValue();

        }

        BigDecimal amountCent = amount.movePointRight(2);

        final BigDecimal multiplied = amountCent.multiply(to,context);

        BigDecimal[] division = multiplied.divideAndRemainder(from,context);

        if (!remainderAllowed && !(BigDecimal.ZERO.equals(division[1]))) {

            throw new RuntimeException("Remained is not allowed - remaining amount is " + division[1]+ " cents");

        } else {

            result.setRemainder(BigDecimal.ZERO);

        }

        final BigDecimal converted = division[0].movePointLeft(2);        

        result.setConverted(converted);

        //result.setRemainder(...);

        return result;

    }

}