Added array_mode expression function

resources/function_help/json/array_mode

@@ -0,0 +1,11 @@
+{
+  "name": "array_mode",
+  "type": "function",
+  "groups": ["Arrays"],
+  "description": "Returns an array containing the mode value (most common value) of an array. The returned array may contain more than one value if more values occur equally often.",
+  "arguments": [ {"arg":"array","description":"an array"} ],
+  "examples": [ 
+    { "expression":"array_mode(array(0,1,42,42,43))", "returns":"[ 42 ]"},
+    { "expression":"array_mode(array(0,0,1,2,2,42))", "returns":"[ 0, 2 ]"}
+  ]
+}

src/core/expression/qgsexpressionfunction.cpp

@@ -5354,6 +5354,19 @@ static QVariant fcnArrayMaximum( const QVariantList &values, const QgsExpression
   return list.isEmpty() ? QVariant() : *std::max_element( list.constBegin(), list.constEnd(), []( QVariant a, QVariant b ) -> bool { return ( qgsVariantLessThan( a, b ) ); } );
 }
 
+static QVariant fcnArrayMode( const QVariantList &values, const QgsExpressionContext *, QgsExpression *parent, const QgsExpressionNodeFunction * )
+{
+  const QVariantList list = QgsExpressionUtils::getListValue( values.at( 0 ), parent );
+  QHash< QVariant, int > hash;
+  for ( const auto &item : list )
+  {
+    ++hash[item];
+  }
+  const QList< int > occurrences = hash.values();
+  const int maxValue = *std::max_element( occurrences.constBegin(), occurrences.constEnd() );
+  return list.isEmpty() ? QVariant() : hash.keys( maxValue );
+}
+
 static QVariant convertToSameType( const QVariant &value, QVariant::Type type )
 {
   QVariant result = value;
@@ -6961,6 +6974,7 @@ const QList<QgsExpressionFunction *> &QgsExpression::Functions()
         << new QgsStaticExpressionFunction( QStringLiteral( "array_last" ), QgsExpressionFunction::ParameterList() << QgsExpressionFunction::Parameter( QStringLiteral( "array" ) ), fcnArrayLast, QStringLiteral( "Arrays" ) )
         << new QgsStaticExpressionFunction( QStringLiteral( "array_min" ), QgsExpressionFunction::ParameterList() << QgsExpressionFunction::Parameter( QStringLiteral( "array" ) ), fcnArrayMinimum, QStringLiteral( "Arrays" ) )
         << new QgsStaticExpressionFunction( QStringLiteral( "array_max" ), QgsExpressionFunction::ParameterList() << QgsExpressionFunction::Parameter( QStringLiteral( "array" ) ), fcnArrayMaximum, QStringLiteral( "Arrays" ) )
+        << new QgsStaticExpressionFunction( QStringLiteral( "array_mode" ), QgsExpressionFunction::ParameterList() << QgsExpressionFunction::Parameter( QStringLiteral( "array" ) ), fcnArrayMode, QStringLiteral( "Arrays" ) )
         << new QgsStaticExpressionFunction( QStringLiteral( "array_append" ), QgsExpressionFunction::ParameterList() << QgsExpressionFunction::Parameter( QStringLiteral( "array" ) ) << QgsExpressionFunction::Parameter( QStringLiteral( "value" ) ), fcnArrayAppend, QStringLiteral( "Arrays" ) )
         << new QgsStaticExpressionFunction( QStringLiteral( "array_prepend" ), QgsExpressionFunction::ParameterList() << QgsExpressionFunction::Parameter( QStringLiteral( "array" ) ) << QgsExpressionFunction::Parameter( QStringLiteral( "value" ) ), fcnArrayPrepend, QStringLiteral( "Arrays" ) )
         << new QgsStaticExpressionFunction( QStringLiteral( "array_insert" ), QgsExpressionFunction::ParameterList() << QgsExpressionFunction::Parameter( QStringLiteral( "array" ) ) << QgsExpressionFunction::Parameter( QStringLiteral( "pos" ) ) << QgsExpressionFunction::Parameter( QStringLiteral( "value" ) ), fcnArrayInsert, QStringLiteral( "Arrays" ) )