Currency rate: always display minimum 4 significant decimals or 2 trailing zeros if the number is integer (#256)

* Currency rate: Compute how many decimals we need to display two meaningful digits at minimum

* formatting

* nit

* Increase the number of meaningfull digits (2->4)

* Revert "Increase the number of meaningfull digits (2->4)"

This reverts commit 9ad93e0d59.

* Rename constants

* modify FORMATTER_RATE_* values

* format CurrencyConverterUnitTests.cpp

src/CalcViewModel/DataLoaders/CurrencyDataLoader.cpp

@@ -39,7 +39,9 @@ static constexpr auto CURRENCY_UNIT_TO_KEY = L"CURRENCY_UNIT_TO_KEY";
 static constexpr long long DAY_DURATION = 1LL * 60 * 60 * 24 * 10000000;
 static constexpr long long WEEK_DURATION = DAY_DURATION * 7;
 
-static constexpr int FORMATTER_DIGIT_COUNT = 4;
+static constexpr int FORMATTER_RATE_FRACTION_PADDING = 2;
+static constexpr int FORMATTER_RATE_MIN_DECIMALS = 4;
+static constexpr int FORMATTER_RATE_MIN_SIGNIFICANT_DECIMALS = 4;
 
 static constexpr auto CACHE_TIMESTAMP_KEY = L"CURRENCY_CONVERTER_TIMESTAMP";
 static constexpr auto CACHE_LANGCODE_KEY = L"CURRENCY_CONVERTER_LANGCODE";
@@ -128,7 +130,7 @@ CurrencyDataLoader::CurrencyDataLoader(_In_ unique_ptr<ICurrencyHttpClient> clie
     m_ratioFormatter = localizationService->GetRegionalSettingsAwareDecimalFormatter();
     m_ratioFormatter->IsGrouped = true;
     m_ratioFormatter->IsDecimalPointAlwaysDisplayed = true;
-    m_ratioFormatter->FractionDigits = FORMATTER_DIGIT_COUNT;
+    m_ratioFormatter->FractionDigits = FORMATTER_RATE_FRACTION_PADDING;
 
     m_ratioFormat = AppResourceProvider::GetInstance().GetResourceString(L"CurrencyFromToRatioFormat")->Data();
     m_timestampFormat = AppResourceProvider::GetInstance().GetResourceString(L"CurrencyTimestampFormat")->Data();
@@ -267,6 +269,23 @@ pair<wstring, wstring> CurrencyDataLoader::GetCurrencySymbols(const UCM::Unit& u
     return make_pair(symbol1, symbol2);
 }
 
+double CurrencyDataLoader::RoundCurrencyRatio(double ratio)
+{
+    // Compute how many decimals we need to display two meaningful digits at minimum
+    // For example: 0.00000000342334 -> 0.000000003423, 0.000212 -> 0.000212
+    int numberDecimals = FORMATTER_RATE_MIN_DECIMALS;
+    if (ratio < 1)
+    {
+        numberDecimals = max(
+            FORMATTER_RATE_MIN_DECIMALS,
+            (int)(-log10(ratio)) + FORMATTER_RATE_MIN_SIGNIFICANT_DECIMALS);
+    }
+
+    unsigned long long scale = (unsigned long long)powl(10l, numberDecimals);
+
+    return (double)(round(ratio * scale) / scale);
+}
+
 pair<wstring, wstring> CurrencyDataLoader::GetCurrencyRatioEquality(_In_ const UCM::Unit& unit1, _In_ const UCM::Unit& unit2)
 {
     try
@@ -279,12 +298,7 @@ pair<wstring, wstring> CurrencyDataLoader::GetCurrencyRatioEquality(_In_ const U
             if (iter2 != ratioMap.end())
             {
                 double ratio = (iter2->second).ratio;
-
+                double rounded = RoundCurrencyRatio(ratio);
-                // Round the ratio to FORMATTER_DIGIT_COUNT digits using int math.
-                // Ex: to round 1.23456 to three digits, use
-                //     ((int) 1.23456 * (10^3)) / (10^3)
-                double scale = pow(10, FORMATTER_DIGIT_COUNT);
-                double rounded = static_cast<int>(ratio * static_cast<int>(scale)) / scale;
 
                 wstring digitSymbol = wstring{ LocalizationSettings::GetInstance().GetDigitSymbolFromEnUsDigit(L'1') };
                 wstring roundedFormat = m_ratioFormatter->Format(rounded)->Data();

src/CalcViewModel/DataLoaders/CurrencyDataLoader.h

@@ -1,4 +1,4 @@
-// Copyright (c) Microsoft Corporation. All rights reserved.
+// Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT License.
 
 #pragma once
@@ -75,6 +75,7 @@ namespace CalculatorApp
             std::pair<std::wstring, std::wstring>
             GetCurrencyRatioEquality(_In_ const UnitConversionManager::Unit& unit1, _In_ const UnitConversionManager::Unit& unit2) override;
             std::wstring GetCurrencyTimestamp() override;
+            static double RoundCurrencyRatio(double ratio);
 
             std::future<bool> TryLoadDataFromCacheAsync() override;
             std::future<bool> TryLoadDataFromWebAsync() override;

src/CalculatorUnitTests/CurrencyConverterUnitTests.cpp

@@ -590,6 +590,38 @@ TEST_METHOD(Loaded_GetCurrencyRatioEquality_Invalid)
     VERIFY_ARE_EQUAL(wstring(L""), ratio.first);
     VERIFY_ARE_EQUAL(wstring(L""), ratio.second);
 }
+
+TEST_METHOD(Test_RoundCurrencyRatio)
+{
+    CurrencyDataLoader loader{ nullptr };
+    VERIFY_ARE_EQUAL(CurrencyDataLoader::RoundCurrencyRatio(1234567), 1234567);
+    VERIFY_ARE_EQUAL(CurrencyDataLoader::RoundCurrencyRatio(0), 0);
+    VERIFY_ARE_EQUAL(CurrencyDataLoader::RoundCurrencyRatio(9999.999), 9999.999);
+    VERIFY_ARE_EQUAL(CurrencyDataLoader::RoundCurrencyRatio(8765.4321), 8765.4321);
+    VERIFY_ARE_EQUAL(CurrencyDataLoader::RoundCurrencyRatio(4815.162342), 4815.1623);
+    VERIFY_ARE_EQUAL(CurrencyDataLoader::RoundCurrencyRatio(4815.162358), 4815.1624);
+    VERIFY_ARE_EQUAL(CurrencyDataLoader::RoundCurrencyRatio(4815.162388934723), 4815.1624);
+    VERIFY_ARE_EQUAL(CurrencyDataLoader::RoundCurrencyRatio(0.12), 0.12);
+    VERIFY_ARE_EQUAL(CurrencyDataLoader::RoundCurrencyRatio(0.123), 0.123);
+    VERIFY_ARE_EQUAL(CurrencyDataLoader::RoundCurrencyRatio(0.1234), 0.1234);
+    VERIFY_ARE_EQUAL(CurrencyDataLoader::RoundCurrencyRatio(0.12343), 0.1234);
+    VERIFY_ARE_EQUAL(CurrencyDataLoader::RoundCurrencyRatio(0.0321), 0.0321);
+    VERIFY_ARE_EQUAL(CurrencyDataLoader::RoundCurrencyRatio(0.03211), 0.03211);
+    VERIFY_ARE_EQUAL(CurrencyDataLoader::RoundCurrencyRatio(0.032119), 0.03212);
+    VERIFY_ARE_EQUAL(CurrencyDataLoader::RoundCurrencyRatio(0.00322119), 0.003221);
+    VERIFY_ARE_EQUAL(CurrencyDataLoader::RoundCurrencyRatio(0.00123269), 0.001233);
+    VERIFY_ARE_EQUAL(CurrencyDataLoader::RoundCurrencyRatio(0.00076269), 0.0007627);
+    VERIFY_ARE_EQUAL(CurrencyDataLoader::RoundCurrencyRatio(0.000069), 0.000069);
+    VERIFY_ARE_EQUAL(CurrencyDataLoader::RoundCurrencyRatio(0.000061), 0.000061);
+    VERIFY_ARE_EQUAL(CurrencyDataLoader::RoundCurrencyRatio(0.000054612), 0.00005461);
+    VERIFY_ARE_EQUAL(CurrencyDataLoader::RoundCurrencyRatio(0.000054616), 0.00005462);
+    VERIFY_ARE_EQUAL(CurrencyDataLoader::RoundCurrencyRatio(0.000005416), 0.000005416);
+    VERIFY_ARE_EQUAL(CurrencyDataLoader::RoundCurrencyRatio(0.0000016134324), 0.000001613);
+    VERIFY_ARE_EQUAL(CurrencyDataLoader::RoundCurrencyRatio(0.0000096134324), 0.000009613);
+    VERIFY_ARE_EQUAL(CurrencyDataLoader::RoundCurrencyRatio(0.0000032169348392), 0.000003217);
+    VERIFY_ARE_EQUAL(CurrencyDataLoader::RoundCurrencyRatio(0.000000002134987218), 0.000000002135);
+    VERIFY_ARE_EQUAL(CurrencyDataLoader::RoundCurrencyRatio(0.000000000000087231445), 0.00000000000008723);
+}
 }
 ;
 }