Date difference: display the difference in days (only) when we aren't able to calculate the difference in days/weeks/months.. (#555)

* Display date difference in days if can't display in days/weeks/months/...

* add comments

* remove not used variable totalDaysDiff

* improve UpdateDisplayResult

* Display error message when the calculator can't calculate the difference between 2 dates

src/CalcViewModel/Common/DateCalculator.cpp

@@ -119,7 +119,7 @@ bool DateCalculationEngine::SubtractDuration(_In_ DateTime startDate, _In_ const
 }
 
 // Calculate the difference between two dates
-void DateCalculationEngine::GetDateDifference(_In_ DateTime date1, _In_ DateTime date2, _In_ DateUnit outputFormat, _Out_ DateDifference* difference)
+bool DateCalculationEngine::TryGetDateDifference(_In_ DateTime date1, _In_ DateTime date2, _In_ DateUnit outputFormat, _Out_ DateDifference* difference)
 {
     DateTime startDate;
     DateTime endDate;
@@ -176,8 +176,9 @@ void DateCalculationEngine::GetDateDifference(_In_ DateTime date1, _In_ DateTime
                         catch (Platform::InvalidArgumentException ^)
                         {
                             // Operation failed due to out of bound result
-                            // Do nothing
-                            differenceInDates[unitIndex] = 0;
+                            // For example: 31st Dec, 9999 - last valid date
+                            *difference = DateDifferenceUnknown;
+                            return false;
                         }
                     }
 
@@ -190,6 +191,12 @@ void DateCalculationEngine::GetDateDifference(_In_ DateTime date1, _In_ DateTime
                         if (tempDaysDiff < 0)
                         {
                             // pivotDate has gone over the end date; start from the beginning of this unit
+                            if (differenceInDates[unitIndex] == 0)
+                            {
+                                // differenceInDates[unitIndex] is unsigned, the value can't be negative
+                                *difference = DateDifferenceUnknown;
+                                return false;
+                            }
                             differenceInDates[unitIndex] -= 1;
                             pivotDate = tempPivotDate;
                             pivotDate = AdjustCalendarDate(pivotDate, dateUnit, static_cast<int>(differenceInDates[unitIndex]));
@@ -211,16 +218,25 @@ void DateCalculationEngine::GetDateDifference(_In_ DateTime date1, _In_ DateTime
                             }
                             catch (Platform::InvalidArgumentException ^)
                             {
-                                // handling for 31st Dec, 9999 last valid date
-                                // Do nothing - break out
-                                break;
+                                // Operation failed due to out of bound result
+                                // For example: 31st Dec, 9999 - last valid date
+                                *difference = DateDifferenceUnknown;
+                                return false;
                             }
                         }
                     } while (tempDaysDiff != 0); // dates are the same - exit the loop
 
                     tempPivotDate = AdjustCalendarDate(tempPivotDate, dateUnit, static_cast<int>(differenceInDates[unitIndex]));
                     pivotDate = tempPivotDate;
-                    daysDiff = GetDifferenceInDays(pivotDate, endDate);
+                    int signedDaysDiff = GetDifferenceInDays(pivotDate, endDate);
+                    if (signedDaysDiff < 0)
+                    {
+                        // daysDiff is unsigned, the value can't be negative
+                        *difference = DateDifferenceUnknown;
+                        return false;
+                    }
+
+                    daysDiff = signedDaysDiff;
                 }
             }
         }
@@ -232,6 +248,7 @@ void DateCalculationEngine::GetDateDifference(_In_ DateTime date1, _In_ DateTime
     difference->month = differenceInDates[1];
     difference->week = differenceInDates[2];
     difference->day = differenceInDates[3];
+    return true;
 }
 
 // Private Methods

src/CalcViewModel/Common/DateCalculator.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
@@ -35,8 +35,15 @@ namespace CalculatorApp
                 int month = 0;
                 int week = 0;
                 int day = 0;
+
+                bool operator==(const DateDifference& dd) const
+                {
+                    return year == dd.year && month == dd.month && week == dd.week && day == day;
+                }
             };
 
+            const DateDifference DateDifferenceUnknown{ INT_MIN, INT_MIN, INT_MIN, INT_MIN };
+
             class DateCalculationEngine
             {
             public:
@@ -50,7 +57,7 @@ namespace CalculatorApp
                     _In_ Windows::Foundation::DateTime startDate,
                     _In_ const DateDifference& duration,
                     _Out_ Windows::Foundation::DateTime* endDate);
-                void __nothrow GetDateDifference(
+                bool __nothrow TryGetDateDifference(
                     _In_ Windows::Foundation::DateTime date1,
                     _In_ Windows::Foundation::DateTime date2,
                     _In_ DateUnit outputFormat,