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));

     }

 }