package org.apidesign.apifest08.currency;

 import java.lang.ref.WeakReference;

 import java.math.BigDecimal;

 import java.util.Currency;

 import java.util.Map;

 import java.util.WeakHashMap;

 /**

  * Create convertors using a flyweight to reduce the number of repetative creations of the same convertor.

  * 

  * @author D'Arcy Smith

  * @version 1.0

  */

 public final class ConvertorFactory

 {

     /**

      * flyweight so that only one vestion of each converter is created at a time.

      */

     private final static Map<String, WeakReference<Convertor>> convertors;

     static

     {

         convertors = new WeakHashMap<String, WeakReference<Convertor>>();

     }

     /** 

      * Prevent accidental construction.

      */

     private ConvertorFactory()

     {        

     }

     /**

      * Get the convertor for the specified currencies.  The currency name format

      * must be acceptable to java.util.Currency.getInstance(String)

      * 

      * @param a the currency to convert from.

      * @param aRate the exchange rate for a to b.

      * @param b the currency to convert to.

      * @param bRate the echante rate for b to a.

      * @return the convertor for the specified currencies.

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

      */

     public static Convertor getConvertor(final String     a,

                                          final BigDecimal aRate,

                                          final String     b,

                                          final BigDecimal bRate)

     {

         final Currency  currencyA;

         final Currency  currencyB;

         final Convertor convertor;

         currencyA = Currency.getInstance(a);

         currencyB = Currency.getInstance(b);        

         convertor = getConvertor(currencyA, aRate, currencyB, bRate);

         return (convertor);

     }

     /**

      * Get the convertor for the specified currencies.

      * 

      * @param a the currency to convert from.

      * @param aRate the exchange rate for a to b.

      * @param b the currency to convert to.

      * @param bRate the echante rate for b to a.

      * @return the convertor for the specified currencies.

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

      */

     public static Convertor getConvertor(final Currency   a,

                                          final BigDecimal aRate,

                                          final Currency   b,

                                          final BigDecimal bRate)

     {

         final String key;        

         Convertor    convertor;

         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");

         }

         key = a.getCurrencyCode() + aRate + b.getCurrencyCode() + bRate;

         // make sure that we don't try to overwrite one

         synchronized(convertors)

         {

             if(!(convertors.containsKey(key)))

             {        

                 convertor = new ConvertorImpl(a, aRate, b, bRate);

                 convertors.put(key, new WeakReference(convertor));

             }

         }

         convertor = convertors.get(key).get();

         return (convertor);

     }

 }