Add QgsRectangle::snappedToGrid

Snaps a rectangle to a grid.

Author: m-kuhn

python/core/auto_generated/geometry/qgsrectangle.sip.in

@@ -329,6 +329,14 @@ Converts the rectangle to a 3D box, with the specified
 
     operator QVariant() const;
 
+    QgsRectangle snappedToGrid( double spacing ) const;
+%Docstring
+Returns a copy of this rectangle that is snapped to a grid with
+the specified ``spacing`` between the grid lines.
+
+.. versionadded:: 3.4
+%End
+
     SIP_PYOBJECT __repr__();
 %MethodCode
     QString str = QStringLiteral( "<QgsRectangle: %1>" ).arg( sipCpp->asWktCoordinates() );

src/core/geometry/qgsrectangle.cpp

@@ -182,6 +182,25 @@ QgsBox3d QgsRectangle::toBox3d( double zMin, double zMax ) const
   return QgsBox3d( mXmin, mYmin, zMin, mXmax, mYmax, zMax );
 }
 
+QgsRectangle QgsRectangle::snappedToGrid( double spacing ) const
+{
+  // helper function
+  auto gridifyValue = []( double value, double spacing ) -> double
+  {
+    if ( spacing > 0 )
+      return  std::round( value / spacing ) * spacing;
+    else
+      return value;
+  };
+
+  return QgsRectangle(
+           gridifyValue( mXmin, spacing ),
+           gridifyValue( mYmin, spacing ),
+           gridifyValue( mXmax, spacing ),
+           gridifyValue( mYmax, spacing )
+         );
+}
+
 QDataStream &operator<<( QDataStream &out, const QgsRectangle &rectangle )
 {
   out << rectangle.xMinimum() << rectangle.yMinimum() << rectangle.xMaximum() << rectangle.yMaximum();

src/core/geometry/qgsrectangle.h

@@ -547,6 +547,14 @@ class CORE_EXPORT QgsRectangle
       return QVariant::fromValue( *this );
     }
 
+    /**
+     * Returns a copy of this rectangle that is snapped to a grid with
+     * the specified \a spacing between the grid lines.
+     *
+     * \since QGIS 3.4
+     */
+    QgsRectangle snappedToGrid( double spacing ) const;
+
 #ifdef SIP_RUN
     SIP_PYOBJECT __repr__();
     % MethodCode

tests/src/core/testqgsrectangle.cpp

@@ -41,6 +41,7 @@ class TestQgsRectangle: public QObject
     void combine();
     void dataStream();
     void scale();
+    void snappedToGrid();
 };
 
 void TestQgsRectangle::isEmpty()
@@ -366,5 +367,20 @@ void TestQgsRectangle::scale()
   QCOMPARE( rect, QgsRectangle( 10, 20, 30, 60 ).scaled( 2, &center ) );
 }
 
+void TestQgsRectangle::snappedToGrid()
+{
+  QgsRectangle original( 10.123, 20.333, 10.788, 20.788 );
+  QgsRectangle snapped = original.snappedToGrid( 0.1 );
+
+  QgsRectangle control( 10.1, 20.3, 10.8, 20.8 );
+
+  QVERIFY( qgsDoubleNear( snapped.xMinimum(), control.xMinimum(), 0.000001 ) );
+  QVERIFY( qgsDoubleNear( snapped.xMaximum(), control.xMaximum(), 0.000001 ) );
+  QVERIFY( qgsDoubleNear( snapped.yMinimum(), control.yMinimum(), 0.000001 ) );
+  QVERIFY( qgsDoubleNear( snapped.yMaximum(), control.yMaximum(), 0.000001 ) );
+
+  QCOMPARE( QgsRectangle().snappedToGrid( 0.1 ), QgsRectangle() );
+}
+
 QGSTEST_MAIN( TestQgsRectangle )
 #include "testqgsrectangle.moc"