Allow QgsStatisticalSummary to calculate first/last value

Trivial, but simplifies other code which desires to expose
a choice between first/last/min/max/mean/etc...

python/core/auto_generated/qgsstatisticalsummary.sip.in

@@ -46,6 +46,8 @@ specifying the statistic in the constructor or via setStatistics().
       FirstQuartile,
       ThirdQuartile,
       InterQuartileRange,
+      First,
+      Last,
       All
     };
     typedef QFlags<QgsStatisticalSummary::Statistic> Statistics;
@@ -214,6 +216,24 @@ be calculated.
 %Docstring
 Returns calculated range (difference between maximum and minimum values). A NaN value may be returned if the range cannot
 be calculated.
+%End
+
+    double first() const;
+%Docstring
+Returns the first value obtained. A NaN value may be returned if no values were encountered.
+
+.. seealso:: :py:func:`last`
+
+.. versionadded:: 3.6
+%End
+
+    double last() const;
+%Docstring
+Returns the last value obtained. A NaN value may be returned if no values were encountered.
+
+.. seealso:: :py:func:`first`
+
+.. versionadded:: 3.6
 %End
 
     double stDev() const;

src/core/qgsstatisticalsummary.cpp

@@ -32,6 +32,8 @@ QgsStatisticalSummary::QgsStatisticalSummary( Statistics stats )
 
 void QgsStatisticalSummary::reset()
 {
+  mFirst = std::numeric_limits<double>::quiet_NaN();
+  mLast = std::numeric_limits<double>::quiet_NaN();
   mCount = 0;
   mMissing = 0;
   mSum = 0;
@@ -59,7 +61,7 @@ void QgsStatisticalSummary::calculate( const QList<double> &values )
 {
   reset();
 
-  Q_FOREACH ( double value, values )
+  for ( double value : values )
   {
     addValue( value );
   }
@@ -69,10 +71,13 @@ void QgsStatisticalSummary::calculate( const QList<double> &values )
 
 void QgsStatisticalSummary::addValue( double value )
 {
+  if ( mCount == 0 )
+    mFirst = value;
   mCount++;
   mSum += value;
   mMin = std::min( mMin, value );
   mMax = std::max( mMax, value );
+  mLast = value;
 
   if ( mStatistics & QgsStatisticalSummary::Majority || mStatistics & QgsStatisticalSummary::Minority || mStatistics & QgsStatisticalSummary::Variety )
     mValueCount.insert( value, mValueCount.value( value, 0 ) + 1 );
@@ -102,6 +107,8 @@ void QgsStatisticalSummary::finalize()
 {
   if ( mCount == 0 )
   {
+    mFirst = std::numeric_limits<double>::quiet_NaN();
+    mLast = std::numeric_limits<double>::quiet_NaN();
     mMin = std::numeric_limits<double>::quiet_NaN();
     mMax = std::numeric_limits<double>::quiet_NaN();
     mMean = std::numeric_limits<double>::quiet_NaN();
@@ -266,6 +273,10 @@ double QgsStatisticalSummary::statistic( QgsStatisticalSummary::Statistic stat )
       return mThirdQuartile;
     case InterQuartileRange:
       return mThirdQuartile - mFirstQuartile;
+    case First:
+      return mFirst;
+    case Last:
+      return mLast;
     case All:
       return 0;
   }
@@ -308,6 +319,10 @@ QString QgsStatisticalSummary::displayName( QgsStatisticalSummary::Statistic sta
       return QObject::tr( "Q3" );
     case InterQuartileRange:
       return QObject::tr( "IQR" );
+    case First:
+      return QObject::tr( "First" );
+    case Last:
+      return QObject::tr( "Last" );
     case All:
       return QString();
   }

src/core/qgsstatisticalsummary.h

@@ -47,23 +47,25 @@ class CORE_EXPORT QgsStatisticalSummary
     //! Enumeration of flags that specify statistics to be calculated
     enum Statistic
     {
-      Count = 1,  //!< Count
-      CountMissing = 32770, //!< Number of missing (null) values
-      Sum = 2,  //!< Sum of values
-      Mean = 4,  //!< Mean of values
-      Median = 8, //!< Median of values
-      StDev = 16, //!< Standard deviation of values
-      StDevSample = 32, //!< Sample standard deviation of values
-      Min = 64,  //!< Min of values
-      Max = 128,  //!< Max of values
-      Range = 256, //!< Range of values (max - min)
-      Minority = 512, //!< Minority of values
-      Majority = 1024, //!< Majority of values
-      Variety = 2048, //!< Variety (count of distinct) values
-      FirstQuartile = 4096, //!< First quartile
-      ThirdQuartile = 8192, //!< Third quartile
-      InterQuartileRange = 16384, //!< Inter quartile range (IQR)
-      All = Count | CountMissing | Sum | Mean | Median | StDev | Max | Min | Range | Minority | Majority | Variety | FirstQuartile | ThirdQuartile | InterQuartileRange
+      Count = 1 << 0,  //!< Count
+      CountMissing = 1 << 15, //!< Number of missing (null) values
+      Sum = 1 << 1,  //!< Sum of values
+      Mean = 1 << 2,  //!< Mean of values
+      Median = 1 << 3, //!< Median of values
+      StDev = 1 << 4, //!< Standard deviation of values
+      StDevSample = 1 << 5, //!< Sample standard deviation of values
+      Min = 1 << 6,  //!< Min of values
+      Max = 1 << 7,  //!< Max of values
+      Range = 1 << 8, //!< Range of values (max - min)
+      Minority = 1 << 9, //!< Minority of values
+      Majority = 1 << 10, //!< Majority of values
+      Variety = 1 << 11, //!< Variety (count of distinct) values
+      FirstQuartile = 1 << 12, //!< First quartile
+      ThirdQuartile = 1 << 13, //!< Third quartile
+      InterQuartileRange = 1 << 14, //!< Inter quartile range (IQR)
+      First = 1 << 16, //!< First value (since QGIS 3.6)
+      Last = 1 << 17, //!< Last value (since QGIS 3.6)
+      All = Count | CountMissing | Sum | Mean | Median | StDev | Max | Min | Range | Minority | Majority | Variety | FirstQuartile | ThirdQuartile | InterQuartileRange | First | Last
     };
     Q_DECLARE_FLAGS( Statistics, Statistic )
 
@@ -199,6 +201,22 @@ class CORE_EXPORT QgsStatisticalSummary
      */
     double range() const { return std::isnan( mMax ) || std::isnan( mMin ) ? std::numeric_limits<double>::quiet_NaN() : mMax - mMin; }
 
+    /**
+     * Returns the first value obtained. A NaN value may be returned if no values were encountered.
+     *
+     * \see last()
+     * \since QGIS 3.6
+     */
+    double first() const { return mFirst; }
+
+    /**
+     * Returns the last value obtained. A NaN value may be returned if no values were encountered.
+     *
+     * \see first()
+     * \since QGIS 3.6
+     */
+    double last() const { return mLast; }
+
     /**
      * Returns population standard deviation. This is only calculated if Statistic::StDev has
      * been specified in the constructor or via setStatistics. A NaN value may be returned if the standard deviation cannot
@@ -290,6 +308,8 @@ class CORE_EXPORT QgsStatisticalSummary
     double mMajority;
     double mFirstQuartile;
     double mThirdQuartile;
+    double mFirst;
+    double mLast;
     QMap< double, int > mValueCount;
     QList< double > mValues;
 };

tests/src/core/testqgsstatisticalsummary.cpp

@@ -85,6 +85,10 @@ void TestQgsStatisticSummary::stats()
   QCOMPARE( s2.sum(), 24.0 );
   QCOMPARE( s.mean(), 4.0 );
   QCOMPARE( s2.mean(), 4.0 );
+  QCOMPARE( s.first(), 4.0 );
+  QCOMPARE( s2.first(), 4.0 );
+  QCOMPARE( s.last(), 8.0 );
+  QCOMPARE( s2.last(), 8.0 );
   QGSCOMPARENEAR( s.stDev(), 2.0816, 0.0001 );
   QGSCOMPARENEAR( s2.stDev(), 2.0816, 0.0001 );
   QGSCOMPARENEAR( s.sampleStDev(), 2.2803, 0.0001 );
@@ -227,6 +231,8 @@ void TestQgsStatisticSummary::individualStatCalculations_data()
   QTest::newRow( "third_quartile" ) << ( int )QgsStatisticalSummary::ThirdQuartile << 5.0;
   QTest::newRow( "iqr" ) << ( int )QgsStatisticalSummary::InterQuartileRange << 2.0;
   QTest::newRow( "missing" ) << ( int )QgsStatisticalSummary::CountMissing << 0.0;
+  QTest::newRow( "first" ) << static_cast< int >( QgsStatisticalSummary::First ) << 4.0;
+  QTest::newRow( "last" ) << static_cast< int >( QgsStatisticalSummary::Last ) << 8.0;
 }
 
 void TestQgsStatisticSummary::individualStatCalculations()
@@ -311,6 +317,10 @@ void TestQgsStatisticSummary::noValues()
   QCOMPARE( s.statistic( QgsStatisticalSummary::CountMissing ), 0.0 );
   QCOMPARE( s.sum(), 0.0 );
   QCOMPARE( s.statistic( QgsStatisticalSummary::Sum ), 0.0 );
+  QVERIFY( std::isnan( s.first() ) );
+  QVERIFY( std::isnan( s.statistic( QgsStatisticalSummary::First ) ) );
+  QVERIFY( std::isnan( s.last() ) );
+  QVERIFY( std::isnan( s.statistic( QgsStatisticalSummary::Last ) ) );
   QVERIFY( std::isnan( s.mean() ) );
   QVERIFY( std::isnan( s.statistic( QgsStatisticalSummary::Mean ) ) );
   QVERIFY( std::isnan( s.median() ) );