Add a scale_linear function, for linear interpolation between and inp…

…ut domain and output range

resources/function_help/scale_linear-en_US

@@ -0,0 +1,19 @@
+<h3>scale_linear() function</h3>
+Transforms a given value from an input domain to an output range using linear interpolation. 
+
+<p><h4>Syntax</h4>
+     scale_linear(<i>val</i>,<i>domain_min</i>,<i>domain_max</i>,<i>range_min</i>,<i>range_max</i>)</p>
+
+<p><h4>Arguments</h4>
+<!-- List args for functions here-->
+<i>  val</i> &rarr; is a value in the input domain. The function will return a corresponding scaled value in the output range.<br>
+<i>  domain_min, domain_max</i> &rarr; specify the input domain, the smallest and largest values the input <i>val</i> should take.<br>
+<i>  range_min, range_max</i> &rarr; sepcify the output range, the smallest and largest values which should be output by the function.<br>
+
+<h4>Example</h4>
+<!-- Show example of function.-->
+     scale_linear(5,0,10,0,100) &rarr; 50<br>
+     scale_linear(0.2,0,1,0,360) &rarr; 72 <i>(eg, scaling a value between 0 and 1 to an angle between 0 and 360)</i><br>
+     scale_linear(1500,1000,10000,9,20) &rarr; 10.22 <i>(eg, scaling a population which varies between 1000 and 10000 to a font size between 9 and 20)</i><br>
+
+

src/core/qgsexpression.cpp

@@ -439,6 +439,32 @@ static QVariant fcnRnd( const QVariantList& values, QgsFeature* , QgsExpression*
   return QVariant( min + ( rand() % ( int )( max - min + 1 ) ) );
 }
 
+static QVariant fcnLinearScale( const QVariantList& values, QgsFeature* , QgsExpression* parent )
+{
+  double val = getDoubleValue( values.at( 0 ), parent );
+  double domain_min = getDoubleValue( values.at( 1 ), parent );
+  double domain_max = getDoubleValue( values.at( 2 ), parent );
+  double range_min = getDoubleValue( values.at( 3 ), parent );
+  double range_max = getDoubleValue( values.at( 4 ), parent );
+
+  // outside of domain?
+  if ( val >= domain_max )
+  {
+    return range_max;
+  }
+  else if ( val <= domain_min )
+  {
+    return range_min;
+  }
+
+  // calculate linear scale
+  double m = ( range_max - range_min ) / ( domain_max - domain_min );
+  double c = range_min - ( domain_min * m );
+
+  // Return linearly scaled value
+  return QVariant( m * val + c );
+}
+
 static QVariant fcnMax( const QVariantList& values, QgsFeature* , QgsExpression *parent )
 {
   //initially set max as first value
@@ -1317,7 +1343,7 @@ const QStringList &QgsExpression::BuiltinFunctions()
     << "asin" << "acos" << "atan" << "atan2"
     << "exp" << "ln" << "log10" << "log"
     << "round" << "rand" << "randf" << "max" << "min"
-    << "floor" << "ceil"
+    << "scale_linear" << "floor" << "ceil"
     << "toint" << "toreal" << "tostring"
     << "todatetime" << "todate" << "totime" << "tointerval"
     << "coalesce" << "regexp_match" << "$now" << "age" << "year"
@@ -1363,6 +1389,7 @@ const QList<QgsExpression::Function*> &QgsExpression::Functions()
     << new StaticFunction( "randf", 2, fcnRndF, QObject::tr( "Math" ) )
     << new StaticFunction( "max", -1, fcnMax, QObject::tr( "Math" ) )
     << new StaticFunction( "min", -1, fcnMin, QObject::tr( "Math" ) )
+    << new StaticFunction( "scale_linear", 5, fcnLinearScale, QObject::tr( "Math" ) )
     << new StaticFunction( "floor", 1, fcnFloor, QObject::tr( "Math" ) )
     << new StaticFunction( "ceil", 1, fcnCeil, QObject::tr( "Math" ) )
     << new StaticFunction( "$pi", 0, fcnPi, QObject::tr( "Math" ) )

tests/src/core/testqgsexpression.cpp

@@ -264,6 +264,12 @@ class TestQgsExpression: public QObject
       QTest::newRow( "floor(-4.9)" ) << "floor(-4.9)" << false << QVariant( -5. );
       QTest::newRow( "ceil(4.9)" ) << "ceil(4.9)" << false << QVariant( 5. );
       QTest::newRow( "ceil(-4.9)" ) << "ceil(-4.9)" << false << QVariant( -4. );
+      QTest::newRow( "scale_linear(0.5,0,1,0,1)" ) << "scale_linear(0.5,0,1,0,1)" << false << QVariant( 0.5 );
+      QTest::newRow( "scale_linear(0,0,10,100,200)" ) << "scale_linear(0,0,10,100,200)" << false << QVariant( 100. );
+      QTest::newRow( "scale_linear(5,0,10,100,200)" ) << "scale_linear(5,0,10,100,200)" << false << QVariant( 150. );
+      QTest::newRow( "scale_linear(10,0,10,100,200)" ) << "scale_linear(10,0,10,100,200)" << false << QVariant( 200. );
+      QTest::newRow( "scale_linear(-1,0,10,100,200)" ) << "scale_linear(-1,0,10,100,200)" << false << QVariant( 100. );
+      QTest::newRow( "scale_linear(11,0,10,100,200)" ) << "scale_linear(11,0,10,100,200)" << false << QVariant( 200. );
 
       // cast functions
       QTest::newRow( "double to int" ) << "toint(3.2)" << false << QVariant( 3 );