package org.apidesign.apifest08.currency;

import java.math.BigDecimal;

import java.math.MathContext;

import java.math.RoundingMode;

import java.util.Currency;

/**

 * Convert between two currencies.

 *

 * @author D'Arcy Smith

 * @version 1.0

 */

final class ConvertorImpl

    implements Convertor

{

    /**

     * The currency to cvonvert from.

     */

    private final Currency currencyA;

    /**

     * The currency to cvonvert from.

     */

    private final Currency currencyB;

    /**

     * The echange rate between a and b.

     */

    private final BigDecimal currencyARate;

    /**

     * The echange rate between b and a.

     */

    private final BigDecimal currencyBRate;

    /**

     * Constructs a convertor with the specified currencies.

     * 

     * @param a the currency to convert from.

     * @param aRate the exchage rage between from and to.

     * @param b the currency to convert to.

     * @param bRate the exchage rage between to and from.

     * @throws IllegalArgumentException if either any of the arguments are null or if either rate <= 0.

     */

    public ConvertorImpl(final Currency   a,

                         final BigDecimal aRate,

                         final Currency   b,

                         final BigDecimal bRate)

    {

        if(a == null)

        {

            throw new IllegalArgumentException("a cannot be null");

        }

        if(b == null)

        {

            throw new IllegalArgumentException("b cannot be null");

        }

        if(aRate == null)

        {

            throw new IllegalArgumentException("aRate cannot be null");

        }

        if(bRate == null)

        {

            throw new IllegalArgumentException("bRate cannot be null");

        }

        if(aRate.compareTo(BigDecimal.ZERO) <= 0)

        {

            throw new IllegalArgumentException("aRate must be > 0, was: " + aRate);

        }

        if(bRate.compareTo(BigDecimal.ZERO) <= 0)

        {

            throw new IllegalArgumentException("bRate must be > 0, was: " + bRate);

        }

        currencyA     = a;

        currencyB     = b;

        currencyARate = aRate;

        currencyBRate = bRate;

    }

    /**

     * Convert an amount from one currency to another.

     * 

     * @param from the currency to convert from.

     * @param to the currency to convert to.

     * @param amount the amount to convert.

     * @return the converted amount.

     * @throws IllegalArgumentException if any of the arguments are null.

     * @throws InvalidConversionException if either from or to are not equal to the currencies passed to the constructor.

     */

    public BigDecimal convert(final Currency   from,

                              final Currency   to,

                              final BigDecimal amount)

        throws InvalidConversionException

    {

        final BigDecimal result;

        if(amount == null)

        {

            throw new IllegalArgumentException("amount cannot be null");

        }

        if(from == null)

        {

            throw new IllegalArgumentException("from cannot be null");

        }

        if(to == null)

        {

            throw new IllegalArgumentException("to cannot be null");

        }

        if(!(from.equals(currencyA)) && (!(from.equals(currencyB))))

        {

            throw new InvalidConversionException("cannot convert from: " + from.getCurrencyCode(), from, currencyA, currencyB);

        }

        if(!(to.equals(currencyA)) && (!(to.equals(currencyB))))

        {

            throw new InvalidConversionException("cannot convert to: " + to.getCurrencyCode(), to, currencyA, currencyB);

        }

        // converting between the same currency is no converstion at all.

        if(from.equals(to))

        {

            result = amount;

        }

        else

        {

            final BigDecimal rateX;

            final BigDecimal rateY;

            final BigDecimal temp;

            if(from.equals(currencyA))

            {

                rateX = currencyARate;

                rateY = currencyBRate;

            }

            else

            {

                rateX = currencyBRate;

                rateY = currencyARate;

            }

            temp   = amount.divide(rateX, MathContext.DECIMAL32);

            result = temp.multiply(rateY);

        }

        return (result.setScale(2, RoundingMode.HALF_DOWN));

    }

}