Avoid losing precision when the user enters an exchange rate.

We were storing the rate as originAmount/convertedAmount, which was
limited to the precision of the currencies. Now we store directly the
entered rate.

Fixes #479

app/src/main/java/org/gnucash/android/model/Price.java

@@ -2,7 +2,11 @@
 
 import org.gnucash.android.util.TimestampHelper;
 
+
+import java.math.BigDecimal;
 import java.sql.Timestamp;
+import java.text.DecimalFormat;
+import java.text.NumberFormat;
 
 /**
  * Model for commodity prices
@@ -37,6 +41,19 @@ public Price(String commodityUID, String currencyUID){
         mDate = TimestampHelper.getTimestampFromNow();
     }
 
+    /**
+     * Create new instance with the GUIDs of the commodities and the specified exchange rate.
+     * @param commodity1UID GUID of the origin commodity
+     * @param commodity2UID GUID of the target commodity
+     * @param exchangeRate exchange rate between the commodities
+     */
+    public Price(String commodity1UID, String commodity2UID, BigDecimal exchangeRate) {
+        this(commodity1UID, commodity2UID);
+        // Store 0.1234 as 1234/10000
+        setValueNum(exchangeRate.unscaledValue().longValue());
+        setValueDenom(BigDecimal.ONE.scaleByPowerOfTen(exchangeRate.scale()).longValue());
+    }
+
     public String getCommodityUID() {
         return mCommodityUID;
     }
@@ -123,4 +140,20 @@ public void reduce() {
             mValueDenom /= commonDivisor;
         }
     }
+
+    /**
+     * Returns the exchange rate as a string formatted with the default locale.
+     *
+     * <p>It will have up to 6 decimal places.
+     *
+     * <p>Example: "0.123456"
+     */
+    @Override
+    public String toString() {
+        BigDecimal numerator = new BigDecimal(mValueNum);
+        BigDecimal denominator = new BigDecimal(mValueDenom);
+        DecimalFormat formatter = (DecimalFormat) NumberFormat.getNumberInstance();
+        formatter.setMaximumFractionDigits(6);
+        return formatter.format(numerator.divide(denominator));
+    }
 }

app/src/main/java/org/gnucash/android/ui/transaction/dialog/TransferFundsDialogFragment.java

@@ -116,17 +116,21 @@ public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle sa
         Commodity currencyCommodity = commoditiesDbAdapter.getCommodity(mTargetCurrency.getCurrencyCode());
         String currencyUID = currencyCommodity.getUID();
         PricesDbAdapter pricesDbAdapter = PricesDbAdapter.getInstance();
-        Pair<Long, Long> price = pricesDbAdapter.getPrice(commodityUID, currencyUID);
+        Pair<Long, Long> pricePair = pricesDbAdapter.getPrice(commodityUID, currencyUID);
 
-        if (price.first > 0 && price.second > 0) {
+        if (pricePair.first > 0 && pricePair.second > 0) {
             // a valid price exists
-            BigDecimal numerator = new BigDecimal(price.first);
-            BigDecimal denominator = new BigDecimal(price.second);
-            DecimalFormat formatter = (DecimalFormat) NumberFormat.getNumberInstance();
-            mExchangeRateInput.setText(formatter.format(numerator.divide(denominator, MathContext.DECIMAL32)));
+            Price price = new Price(commodityUID, currencyUID);
+            price.setValueNum(pricePair.first);
+            price.setValueDenom(pricePair.second);
+            mExchangeRateInput.setText(price.toString());
+
+            BigDecimal numerator = new BigDecimal(pricePair.first);
+            BigDecimal denominator = new BigDecimal(pricePair.second);
             // convertedAmount = mOriginAmount * numerator / denominator
             BigDecimal convertedAmount = mOriginAmount.asBigDecimal().multiply(numerator)
                 .divide(denominator, currencyCommodity.getSmallestFractionDigits(), BigDecimal.ROUND_HALF_EVEN);
+            DecimalFormat formatter = (DecimalFormat) NumberFormat.getNumberInstance();
             mConvertedAmountInput.setText(formatter.format(convertedAmount));
         }
 
@@ -194,17 +198,24 @@ public Dialog onCreateDialog(Bundle savedInstanceState) {
      * Converts the currency amount with the given exchange rate and saves the price to the db
      */
     private void transferFunds() {
+        Price price = null;
+
+        CommoditiesDbAdapter commoditiesDbAdapter = CommoditiesDbAdapter.getInstance();
+        String originCommodityUID = commoditiesDbAdapter.getCommodityUID(mOriginAmount.getCurrency().getCurrencyCode());
+        String targetCommodityUID = commoditiesDbAdapter.getCommodityUID(mTargetCurrency.getCurrencyCode());
+
         if (mExchangeRateRadioButton.isChecked()){
+            BigDecimal rate;
             String exchangeRateString = mExchangeRateInput.getText().toString();
             DecimalFormat formatter = (DecimalFormat) NumberFormat.getNumberInstance();
             formatter.setParseBigDecimal(true);
-            BigDecimal rate;
             try {
                 rate = (BigDecimal) formatter.parse(exchangeRateString);
             } catch (ParseException e) {
                 mExchangeRateInputLayout.setError(getString(R.string.error_invalid_exchange_rate));
                 return;
             }
+            price = new Price(originCommodityUID, targetCommodityUID, rate);
             mConvertedAmount = mOriginAmount.multiply(rate);
         }
 
@@ -217,22 +228,20 @@ private void transferFunds() {
 
             BigDecimal amount = TransactionFormFragment.parseInputToDecimal(convertedAmount);
             mConvertedAmount = new Money(amount, Commodity.getInstance(mTargetCurrency.getCurrencyCode()));
-        }
 
-        if (mOnTransferFundsListener != null) {
-            PricesDbAdapter pricesDbAdapter = PricesDbAdapter.getInstance();
-            CommoditiesDbAdapter commoditiesDbAdapter = CommoditiesDbAdapter.getInstance();
-            Price price = new Price(commoditiesDbAdapter.getCommodityUID(mOriginAmount.getCurrency().getCurrencyCode()),
-                    commoditiesDbAdapter.getCommodityUID(mTargetCurrency.getCurrencyCode()));
-            price.setSource(Price.SOURCE_USER);
-            // fractions cannot be exacted represented by BigDecimal.
+            price = new Price(originCommodityUID, targetCommodityUID);
+            // fractions cannot be exactly represented by BigDecimal.
             price.setValueNum(mConvertedAmount.getNumerator() * mOriginAmount.getDenominator());
             price.setValueDenom(mOriginAmount.getNumerator() * mConvertedAmount.getDenominator());
-            price.reduce();
-            pricesDbAdapter.addRecord(price);
+        }
 
+        price.setSource(Price.SOURCE_USER);
+        price.reduce();
+        PricesDbAdapter.getInstance().addRecord(price);
+
+        if (mOnTransferFundsListener != null)
             mOnTransferFundsListener.transferComplete(mConvertedAmount);
-        }
+
         dismiss();
     }
 

app/src/test/java/org/gnucash/android/test/unit/model/PriceTest.java

@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) 2016 Àlex Magaz Graça <rivaldi8@gmail.com>
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.gnucash.android.test.unit.model;
+
+import org.gnucash.android.model.Price;
+import org.junit.Test;
+
+import java.math.BigDecimal;
+import java.util.Locale;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+
+public class PriceTest {
+    @Test
+    public void creatingFromExchangeRate_ShouldGetPrecisionRight() {
+        Locale.setDefault(Locale.US);
+
+        String exchangeRateString = "0.123456";
+        BigDecimal exchangeRate = new BigDecimal(exchangeRateString);
+        Price price = new Price("commodity1UID", "commodity2UID", exchangeRate);
+        assertThat(price.toString()).isEqualTo(exchangeRateString);
+
+        // ensure we don't get more decimal places than needed (0.123000)
+        exchangeRateString = "0.123";
+        exchangeRate = new BigDecimal(exchangeRateString);
+        price = new Price("commodity1UID", "commodity2UID", exchangeRate);
+        assertThat(price.toString()).isEqualTo(exchangeRateString);
+    }
+
+    @Test
+    public void toString_shouldUseDefaultLocale() {
+        Locale.setDefault(Locale.GERMANY);
+
+        String exchangeRateString = "1.234";
+        BigDecimal exchangeRate = new BigDecimal(exchangeRateString);
+        Price price = new Price("commodity1UID", "commodity2UID", exchangeRate);
+        assertThat(price.toString()).isEqualTo("1,234");
+    }
+}