Error: exchange rate from USD to GBX not found

We continue to hack here b/c we know that ECB provides everything to and
from EUR. However, when adding more providers, this will not work
anymore. We need something like Dijkstra's shortest path algorithm
implemented outside of rate provider. Adding some technical debt as
I do not have the time at the moment.

Issue: #618

name.abuchen.portfolio.tests/src/name/abuchen/portfolio/money/impl/GBXExchangeRateProviderTest.java

@@ -1,5 +1,6 @@
 package name.abuchen.portfolio.money.impl;
 
+import static org.hamcrest.number.IsCloseTo.closeTo;
 import static org.hamcrest.number.OrderingComparison.comparesEqualTo;
 import static org.junit.Assert.assertThat;
 
@@ -35,5 +36,22 @@ public void testIt()
         // GBP -> GBX
         ExchangeRateTimeSeries gbp_gbx = factory.getTimeSeries("GBP", "GBX");
         assertThat(gbp_gbx.lookupRate(LocalDate.now()).get().getValue(), comparesEqualTo(new BigDecimal(100.0)));
+
+        // GBX -> USD
+        // default value EUR -> GBP is 0.72666
+        // default value EUR -> USD is 1.0836
+        double calculatedRate = 0.01d * (1 / 0.72666d) * 1.0836d;
+
+        ExchangeRateTimeSeries gbx_usd = factory.getTimeSeries("GBX", "USD");
+        assertThat(gbx_usd.lookupRate(LocalDate.now()).get().getValue().doubleValue(),
+                        closeTo(calculatedRate, 0.00000001));
+
+        // USD -> GBX
+        calculatedRate = (1 / 1.0836d) * 0.72666d * 100;
+
+        ExchangeRateTimeSeries usd_gbx = factory.getTimeSeries("USD", "GBX");
+        assertThat(usd_gbx.lookupRate(LocalDate.now()).get().getValue().doubleValue(),
+                        closeTo(calculatedRate, 0.00000001));
+
     }
 }

name.abuchen.portfolio/src/name/abuchen/portfolio/money/ExchangeRateProviderFactory.java

@@ -18,7 +18,7 @@ public class ExchangeRateProviderFactory
 
     public ExchangeRateProviderFactory()
     {
-        providers = new ArrayList<ExchangeRateProvider>();
+        providers = new ArrayList<>();
         Iterator<ExchangeRateProvider> registeredProvider = ServiceRegistry.lookupProviders(ExchangeRateProvider.class);
         while (registeredProvider.hasNext())
         {
@@ -35,7 +35,7 @@ public List<ExchangeRateProvider> getProviders()
 
     public List<ExchangeRateTimeSeries> getAvailableTimeSeries()
     {
-        List<ExchangeRateTimeSeries> series = new ArrayList<ExchangeRateTimeSeries>();
+        List<ExchangeRateTimeSeries> series = new ArrayList<>();
         for (ExchangeRateProvider p : providers)
             series.addAll(p.getAvailableTimeSeries());
         return series;

name.abuchen.portfolio/src/name/abuchen/portfolio/money/impl/GBXExchangeRateProvider.java

@@ -68,15 +68,46 @@ else if (GBP.equals(baseCurrency) && GBX.equals(termCurrency))
         {
             return gbp2gbx;
         }
-        else if (GBX.equals(baseCurrency) && CurrencyUnit.EUR.equals(termCurrency))
-        {
-            ExchangeRateTimeSeries series = factory.getTimeSeries(GBP, CurrencyUnit.EUR);
-            return new ChainedExchangeRateTimeSeries(gbx2gbp, series);
-        }
-        else if (CurrencyUnit.EUR.equals(baseCurrency) && GBX.equals(termCurrency))
-        {
-            ExchangeRateTimeSeries series = factory.getTimeSeries(CurrencyUnit.EUR, GBP);
-            return new ChainedExchangeRateTimeSeries(series, gbp2gbx);
+        else if (GBX.equals(baseCurrency))
+        {
+            if (CurrencyUnit.EUR.equals(termCurrency))
+            {
+                ExchangeRateTimeSeries series = factory.getTimeSeries(GBP, CurrencyUnit.EUR);
+                return new ChainedExchangeRateTimeSeries(gbx2gbp, series);
+            }
+            else
+            {
+                // We continue to hack here b/c we know that ECB provides
+                // everything we know to and from EUR. However, when adding more
+                // providers, this will not work anymore. We need something like
+                // Dijkstra's shortest path algorithm implemented outside of
+                // rate provider.
+
+                ExchangeRateTimeSeries eur2term = factory.getTimeSeries(CurrencyUnit.EUR, termCurrency);
+                if (eur2term == null)
+                    return null;
+
+                ExchangeRateTimeSeries gbp2eur = factory.getTimeSeries(GBP, CurrencyUnit.EUR);
+                return new ChainedExchangeRateTimeSeries(gbx2gbp, gbp2eur, eur2term);
+            }
+
+        }
+        else if (GBX.equals(termCurrency))
+        {
+            if (CurrencyUnit.EUR.equals(baseCurrency))
+            {
+                ExchangeRateTimeSeries series = factory.getTimeSeries(CurrencyUnit.EUR, GBP);
+                return new ChainedExchangeRateTimeSeries(series, gbp2gbx);
+            }
+            else
+            {
+                ExchangeRateTimeSeries base2eur = factory.getTimeSeries(baseCurrency, CurrencyUnit.EUR);
+                if (base2eur == null)
+                    return null;
+
+                ExchangeRateTimeSeries eur2gbp = factory.getTimeSeries(CurrencyUnit.EUR, GBP);
+                return new ChainedExchangeRateTimeSeries(base2eur, eur2gbp, gbp2gbx);
+            }
         }
         else
         {