feat(workshop): add reference implementation for DKP currency conversion

Provide reference implementation for custom DukePoints (DKP) currency
including local ExchangeRateProvider and bidirectional conversion
between CHF and DKP.

Signed-off-by: Marcus Fihlon <marcus@fihlon.swiss>

src/main/java/swiss/fihlon/workshop/money/part4/CsvExchangeRateProvider.java

@@ -1,18 +1,30 @@
 package swiss.fihlon.workshop.money.part4;
 
+import java.io.BufferedReader;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.math.BigDecimal;
+import java.nio.charset.StandardCharsets;
+import java.util.HashMap;
+import java.util.Map;
 import javax.money.convert.ConversionQuery;
+import javax.money.convert.ConversionContext;
+import javax.money.convert.ConversionContextBuilder;
 import javax.money.convert.CurrencyConversion;
 import javax.money.convert.ExchangeRate;
 import javax.money.convert.ExchangeRateProvider;
 import javax.money.convert.ProviderContext;
+import javax.money.convert.ProviderContextBuilder;
+import javax.money.convert.RateType;
+import org.javamoney.moneta.convert.ExchangeRateBuilder;
+import org.javamoney.moneta.spi.DefaultNumberValue;
+import org.javamoney.moneta.spi.LazyBoundCurrencyConversion;
 
 /**
  * <p>Bonus exercise 1 for implementing a local CSV-based exchange rate provider with JSR-354.</p>
  *
  * <p>The provider is intended for workshop usage and should read rates from the classpath resource
  * {@code exchange-rates.csv}.</p>
- *
- * <p>The implementation is intentionally incomplete and should be finished by making tests pass.</p>
  */
 public class CsvExchangeRateProvider implements ExchangeRateProvider {
 
@@ -20,6 +32,10 @@ public class CsvExchangeRateProvider implements ExchangeRateProvider {
      * <p>Name of the classpath CSV resource containing exchange rates.</p>
      */
     public static final String RESOURCE_NAME = "exchange-rates.csv";
+    private static final ProviderContext CONTEXT =
+            ProviderContextBuilder.of("csv-exchange-rate-provider", RateType.HISTORIC).build();
+
+    private final Map<CurrencyPair, BigDecimal> ratesByPair;
 
     /**
      * <p>Create a new provider instance.</p>
@@ -27,6 +43,7 @@ public class CsvExchangeRateProvider implements ExchangeRateProvider {
      * <p>The provider should load exchange rates from {@link #RESOURCE_NAME}.</p>
      */
     public CsvExchangeRateProvider() {
+        this.ratesByPair = loadRates();
     }
 
     /**
@@ -36,7 +53,7 @@ public class CsvExchangeRateProvider implements ExchangeRateProvider {
      */
     @Override
     public ProviderContext getContext() {
-        throw new UnsupportedOperationException("TODO");
+        return CONTEXT;
     }
 
     /**
@@ -47,7 +64,16 @@ public class CsvExchangeRateProvider implements ExchangeRateProvider {
      */
     @Override
     public ExchangeRate getExchangeRate(ConversionQuery conversionQuery) {
-        throw new UnsupportedOperationException("TODO");
+        CurrencyPair currencyPair = CurrencyPair.from(conversionQuery);
+        BigDecimal factor = ratesByPair.get(currencyPair);
+        if (factor == null) {
+            return null;
+        }
+        return new ExchangeRateBuilder(getContext().getProviderName(), RateType.DEFERRED)
+                .setBase(conversionQuery.getBaseCurrency())
+                .setTerm(conversionQuery.getCurrency())
+                .setFactor(DefaultNumberValue.of(factor))
+                .build();
     }
 
     /**
@@ -60,6 +86,60 @@ public class CsvExchangeRateProvider implements ExchangeRateProvider {
      */
     @Override
     public CurrencyConversion getCurrencyConversion(ConversionQuery conversionQuery) {
-        throw new UnsupportedOperationException("TODO");
+        if (getExchangeRate(conversionQuery) == null) {
+            return null;
+        }
+        ConversionContext conversionContext = ConversionContextBuilder.create(getContext(), RateType.DEFERRED).build();
+        return new LazyBoundCurrencyConversion(conversionQuery, this, conversionContext);
+    }
+
+    /**
+     * <p>Load exchange rates from the classpath CSV resource into a lookup map.</p>
+     *
+     * <p>The CSV is expected to contain a header and records in the format
+     * {@code base,term,rate}.</p>
+     *
+     * @return map keyed by currency pair containing the configured exchange factor
+     */
+    private static Map<CurrencyPair, BigDecimal> loadRates() {
+        InputStream inputStream = CsvExchangeRateProvider.class.getClassLoader().getResourceAsStream(RESOURCE_NAME);
+        if (inputStream == null) {
+            throw new IllegalStateException("Could not load classpath resource: " + RESOURCE_NAME);
+        }
+
+        try (BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream, StandardCharsets.UTF_8))) {
+            Map<CurrencyPair, BigDecimal> rates = new HashMap<>();
+            reader.readLine();
+            String line;
+            while ((line = reader.readLine()) != null) {
+                String[] values = line.split(",");
+                CurrencyPair currencyPair = new CurrencyPair(values[0], values[1]);
+                BigDecimal factor = new BigDecimal(values[2]);
+                rates.put(currencyPair, factor);
+            }
+            return rates;
+        } catch (Exception exception) {
+            throw new IllegalStateException("Failed to parse exchange rates from: " + RESOURCE_NAME, exception);
+        }
+    }
+
+    /**
+     * <p>Simple key for identifying an exchange rate by base and term currency code.</p>
+     *
+     * @param baseCurrencyCode ISO currency code of the base currency
+     * @param termCurrencyCode ISO currency code of the term currency
+     */
+    private record CurrencyPair(String baseCurrencyCode, String termCurrencyCode) {
+        /**
+         * <p>Create a currency-pair key from a conversion query.</p>
+         *
+         * @param conversionQuery query containing base and term currency
+         * @return key for map-based rate lookup
+         */
+        private static CurrencyPair from(ConversionQuery conversionQuery) {
+            return new CurrencyPair(
+                    conversionQuery.getBaseCurrency().getCurrencyCode(),
+                    conversionQuery.getCurrency().getCurrencyCode());
+        }
     }
 }

src/main/java/swiss/fihlon/workshop/money/part4/CsvHistoricalExchangeRateProvider.java

@@ -1,11 +1,27 @@
 package swiss.fihlon.workshop.money.part4;
 
+import java.io.BufferedReader;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.math.BigDecimal;
 import java.time.LocalDate;
+import java.nio.charset.StandardCharsets;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.NavigableMap;
+import java.util.TreeMap;
 import javax.money.convert.ConversionQuery;
+import javax.money.convert.ConversionContext;
+import javax.money.convert.ConversionContextBuilder;
 import javax.money.convert.CurrencyConversion;
 import javax.money.convert.ExchangeRate;
 import javax.money.convert.ExchangeRateProvider;
 import javax.money.convert.ProviderContext;
+import javax.money.convert.ProviderContextBuilder;
+import javax.money.convert.RateType;
+import org.javamoney.moneta.convert.ExchangeRateBuilder;
+import org.javamoney.moneta.spi.DefaultNumberValue;
+import org.javamoney.moneta.spi.LazyBoundCurrencyConversion;
 
 /**
  * <p>Bonus exercise 2 for implementing a local CSV-based historical exchange rate provider with JSR-354.</p>
@@ -14,8 +30,6 @@ import javax.money.convert.ProviderContext;
  * {@code exchange-rates-hist.csv}.</p>
  *
  * <p>The lookup should use base currency, term currency, and a date passed in the query context.</p>
- *
- * <p>The implementation is intentionally incomplete and should be finished by making tests pass.</p>
  */
 public class CsvHistoricalExchangeRateProvider implements ExchangeRateProvider {
 
@@ -28,6 +42,10 @@ public class CsvHistoricalExchangeRateProvider implements ExchangeRateProvider {
      * <p>Query key type for historical lookups.</p>
      */
     public static final Class<LocalDate> DATE_QUERY_KEY = LocalDate.class;
+    private static final ProviderContext CONTEXT =
+            ProviderContextBuilder.of("csv-historical-exchange-rate-provider", RateType.HISTORIC).build();
+
+    private final Map<CurrencyPair, NavigableMap<LocalDate, BigDecimal>> ratesByPairAndDate;
 
     /**
      * <p>Create a new provider instance.</p>
@@ -35,6 +53,7 @@ public class CsvHistoricalExchangeRateProvider implements ExchangeRateProvider {
      * <p>The provider should load exchange rates from {@link #RESOURCE_NAME}.</p>
      */
     public CsvHistoricalExchangeRateProvider() {
+        this.ratesByPairAndDate = loadRates();
     }
 
     /**
@@ -44,7 +63,7 @@ public class CsvHistoricalExchangeRateProvider implements ExchangeRateProvider {
      */
     @Override
     public ProviderContext getContext() {
-        throw new UnsupportedOperationException("TODO");
+        return CONTEXT;
     }
 
     /**
@@ -62,7 +81,27 @@ public class CsvHistoricalExchangeRateProvider implements ExchangeRateProvider {
      */
     @Override
     public ExchangeRate getExchangeRate(ConversionQuery conversionQuery) {
-        throw new UnsupportedOperationException("TODO");
+        LocalDate requestedDate = conversionQuery.get(DATE_QUERY_KEY);
+        if (requestedDate == null) {
+            return null;
+        }
+
+        CurrencyPair currencyPair = CurrencyPair.from(conversionQuery);
+        NavigableMap<LocalDate, BigDecimal> ratesByDate = ratesByPairAndDate.get(currencyPair);
+        if (ratesByDate == null) {
+            return null;
+        }
+
+        Map.Entry<LocalDate, BigDecimal> bestMatch = ratesByDate.floorEntry(requestedDate);
+        if (bestMatch == null) {
+            return null;
+        }
+
+        return new ExchangeRateBuilder(getContext().getProviderName(), RateType.HISTORIC)
+                .setBase(conversionQuery.getBaseCurrency())
+                .setTerm(conversionQuery.getCurrency())
+                .setFactor(DefaultNumberValue.of(bestMatch.getValue()))
+                .build();
     }
 
     /**
@@ -78,6 +117,61 @@ public class CsvHistoricalExchangeRateProvider implements ExchangeRateProvider {
      */
     @Override
     public CurrencyConversion getCurrencyConversion(ConversionQuery conversionQuery) {
-        throw new UnsupportedOperationException("TODO");
+        if (getExchangeRate(conversionQuery) == null) {
+            return null;
+        }
+        ConversionContext conversionContext = ConversionContextBuilder.create(getContext(), RateType.HISTORIC).build();
+        return new LazyBoundCurrencyConversion(conversionQuery, this, conversionContext);
+    }
+
+    /**
+     * <p>Load historical exchange rates from the classpath CSV resource into a date-indexed lookup map.</p>
+     *
+     * <p>The CSV is expected to contain a header and records in the format
+     * {@code date,base,term,rate}.</p>
+     *
+     * @return map keyed by currency pair with a navigable date-to-rate mapping
+     */
+    private static Map<CurrencyPair, NavigableMap<LocalDate, BigDecimal>> loadRates() {
+        InputStream inputStream = CsvHistoricalExchangeRateProvider.class.getClassLoader().getResourceAsStream(RESOURCE_NAME);
+        if (inputStream == null) {
+            throw new IllegalStateException("Could not load classpath resource: " + RESOURCE_NAME);
+        }
+
+        try (BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream, StandardCharsets.UTF_8))) {
+            Map<CurrencyPair, NavigableMap<LocalDate, BigDecimal>> rates = new HashMap<>();
+            reader.readLine();
+            String line;
+            while ((line = reader.readLine()) != null) {
+                String[] values = line.split(",");
+                LocalDate date = LocalDate.parse(values[0]);
+                CurrencyPair currencyPair = new CurrencyPair(values[1], values[2]);
+                BigDecimal factor = new BigDecimal(values[3]);
+                rates.computeIfAbsent(currencyPair, key -> new TreeMap<>()).put(date, factor);
+            }
+            return rates;
+        } catch (Exception exception) {
+            throw new IllegalStateException("Failed to parse historical exchange rates from: " + RESOURCE_NAME, exception);
+        }
+    }
+
+    /**
+     * <p>Simple key for identifying a historical exchange-rate series by base and term currency code.</p>
+     *
+     * @param baseCurrencyCode ISO currency code of the base currency
+     * @param termCurrencyCode ISO currency code of the term currency
+     */
+    private record CurrencyPair(String baseCurrencyCode, String termCurrencyCode) {
+        /**
+         * <p>Create a currency-pair key from a conversion query.</p>
+         *
+         * @param conversionQuery query containing base and term currency
+         * @return key for map-based historical rate lookup
+         */
+        private static CurrencyPair from(ConversionQuery conversionQuery) {
+            return new CurrencyPair(
+                    conversionQuery.getBaseCurrency().getCurrencyCode(),
+                    conversionQuery.getCurrency().getCurrencyCode());
+        }
     }
 }

src/main/java/swiss/fihlon/workshop/money/part4/DukePointsExchangeRateProvider.java

@@ -1,20 +1,32 @@
 package swiss.fihlon.workshop.money.part4;
 
+import java.math.BigDecimal;
 import javax.money.convert.ConversionQuery;
+import javax.money.convert.ConversionContext;
+import javax.money.convert.ConversionContextBuilder;
 import javax.money.convert.CurrencyConversion;
 import javax.money.convert.ExchangeRate;
 import javax.money.convert.ExchangeRateProvider;
 import javax.money.convert.ProviderContext;
+import javax.money.convert.ProviderContextBuilder;
+import javax.money.convert.RateType;
+import org.javamoney.moneta.convert.ExchangeRateBuilder;
+import org.javamoney.moneta.spi.DefaultNumberValue;
+import org.javamoney.moneta.spi.LazyBoundCurrencyConversion;
 
 /**
  * <p>Bonus exercise 3 for implementing local exchange rates between CHF and DukePoints.</p>
  *
  * <p>The provider should support deterministic conversion for {@code CHF -> DKP} and {@code DKP -> CHF}.</p>
- *
- * <p>The implementation is intentionally incomplete and should be finished by making tests pass.</p>
  */
 public class DukePointsExchangeRateProvider implements ExchangeRateProvider {
 
+    private static final BigDecimal CHF_TO_DKP_FACTOR = new BigDecimal("0.1");
+    private static final BigDecimal DKP_TO_CHF_FACTOR = new BigDecimal("10");
+    private static final ProviderContext CONTEXT =
+            ProviderContextBuilder.of("dkp-exchange-rate-provider", RateType.HISTORIC).build();
+    private static final String CHF = "CHF";
+
     /**
      * <p>Create a new exchange rate provider for DukePoints.</p>
      */
@@ -28,7 +40,7 @@ public class DukePointsExchangeRateProvider implements ExchangeRateProvider {
      */
     @Override
     public ProviderContext getContext() {
-        throw new UnsupportedOperationException("TODO");
+        return CONTEXT;
     }
 
     /**
@@ -41,7 +53,19 @@ public class DukePointsExchangeRateProvider implements ExchangeRateProvider {
      */
     @Override
     public ExchangeRate getExchangeRate(ConversionQuery conversionQuery) {
-        throw new UnsupportedOperationException("TODO");
+        String baseCurrencyCode = conversionQuery.getBaseCurrency().getCurrencyCode();
+        String termCurrencyCode = conversionQuery.getCurrency().getCurrencyCode();
+
+        BigDecimal factor = resolveFactor(baseCurrencyCode, termCurrencyCode);
+        if (factor == null) {
+            return null;
+        }
+
+        return new ExchangeRateBuilder(getContext().getProviderName(), RateType.HISTORIC)
+                .setBase(conversionQuery.getBaseCurrency())
+                .setTerm(conversionQuery.getCurrency())
+                .setFactor(DefaultNumberValue.of(factor))
+                .build();
     }
 
     /**
@@ -54,6 +78,27 @@ public class DukePointsExchangeRateProvider implements ExchangeRateProvider {
      */
     @Override
     public CurrencyConversion getCurrencyConversion(ConversionQuery conversionQuery) {
-        throw new UnsupportedOperationException("TODO");
+        if (getExchangeRate(conversionQuery) == null) {
+            return null;
+        }
+        ConversionContext conversionContext = ConversionContextBuilder.create(getContext(), RateType.HISTORIC).build();
+        return new LazyBoundCurrencyConversion(conversionQuery, this, conversionContext);
+    }
+
+    /**
+     * <p>Resolve the conversion factor for a supported DukePoints currency pair.</p>
+     *
+     * @param baseCurrencyCode currency code of the base currency
+     * @param termCurrencyCode currency code of the target currency
+     * @return conversion factor for supported pairs, otherwise {@code null}
+     */
+    private BigDecimal resolveFactor(String baseCurrencyCode, String termCurrencyCode) {
+        if (CHF.equals(baseCurrencyCode) && DukePointsCurrencyFactory.DKP.equals(termCurrencyCode)) {
+            return CHF_TO_DKP_FACTOR;
+        }
+        if (DukePointsCurrencyFactory.DKP.equals(baseCurrencyCode) && CHF.equals(termCurrencyCode)) {
+            return DKP_TO_CHF_FACTOR;
+        }
+        return null;
     }
 }