[FEATURE] API + expression function for merging linestrings

Adds a new method to QgsGeometry for merging linestrings.
By passing a multilinestring, any connected lines will
be joined into single linestrings. Behind the scenes this
uses GEOS' line merge.

A corresponding expression function "line_merge" has also
been added.

python/core/geometry/qgsgeometry.sip

@@ -485,6 +485,15 @@ class QgsGeometry
      */
     QgsGeometry* combine( const QgsGeometry* geometry ) const /Factory/;
 
+    /** Merges any connected lines in a LineString/MultiLineString geometry and
+     * converts them to single line strings.
+     * @returns a LineString or MultiLineString geometry, with any connected lines
+     * joined. An empty geometry will be returned if the input geometry was not a
+     * MultiLineString geometry.
+     * @note added in QGIS 3.0
+     */
+    QgsGeometry mergeLines() const;
+
     /** Returns a geometry representing the points making up this geometry that do not make up other. */
     QgsGeometry* difference( const QgsGeometry* geometry ) const /Factory/;
 

resources/function_help/json/line_merge

@@ -0,0 +1,9 @@
+{
+  "name": "line_merge",
+  "type": "function",
+  "description":"Returns a LineString or MultiLineString geometry, where any connected LineStrings from the input geometry have been merged into a single linestring. This function will return null if passed a geometry which is not a LineString/MultiLineString.",
+  "arguments": [ {"arg":"geometry","description":"a LineString/MultiLineString geometry"} ],
+  "examples": [ { "expression":"geom_to_wkt(line_merge(geom_from_wkt('MULTILINESTRING((0 0, 1 1),(1 1, 2 2))')))", "returns":"'LineString(0 0,1 1,2 2)'"},
+  { "expression":"geom_to_wkt(line_merge(geom_from_wkt('MULTILINESTRING((0 0, 1 1),(11 1, 21 2))')))", "returns":"'MultiLineString((0 0, 1 1),(11 1, 21 2)'"}]
+}
+

src/core/geometry/qgsgeometry.cpp

@@ -1422,6 +1422,23 @@ QgsGeometry* QgsGeometry::combine( const QgsGeometry* geometry ) const
   return new QgsGeometry( resultGeom );
 }
 
+QgsGeometry QgsGeometry::mergeLines() const
+{
+  if ( !d->geometry )
+  {
+    return QgsGeometry();
+  }
+
+  if ( QgsWKBTypes::flatType( d->geometry->wkbType() ) == QgsWKBTypes::LineString )
+  {
+    // special case - a single linestring was passed
+    return QgsGeometry( *this );
+  }
+
+  QgsGeos geos( d->geometry );
+  return geos.mergeLines();
+}
+
 QgsGeometry* QgsGeometry::difference( const QgsGeometry* geometry ) const
 {
   if ( !d->geometry || !geometry->d->geometry )

src/core/geometry/qgsgeometry.h

@@ -528,6 +528,15 @@ class CORE_EXPORT QgsGeometry
      */
     QgsGeometry* combine( const QgsGeometry* geometry ) const;
 
+    /** Merges any connected lines in a LineString/MultiLineString geometry and
+     * converts them to single line strings.
+     * @returns a LineString or MultiLineString geometry, with any connected lines
+     * joined. An empty geometry will be returned if the input geometry was not a
+     * MultiLineString geometry.
+     * @note added in QGIS 3.0
+     */
+    QgsGeometry mergeLines() const;
+
     /** Returns a geometry representing the points making up this geometry that do not make up other. */
     QgsGeometry* difference( const QgsGeometry* geometry ) const;
 

src/core/geometry/qgsgeos.cpp

@@ -1793,6 +1793,25 @@ QgsAbstractGeometryV2* QgsGeos::reshapeGeometry( const QgsLineStringV2& reshapeW
   }
 }
 
+QgsGeometry QgsGeos::mergeLines( QString* errorMsg ) const
+{
+  if ( !mGeos )
+  {
+    return QgsGeometry();
+  }
+
+  if ( GEOSGeomTypeId_r( geosinit.ctxt, mGeos ) != GEOS_MULTILINESTRING )
+    return QgsGeometry();
+
+  GEOSGeomScopedPtr geos;
+  try
+  {
+    geos.reset( GEOSLineMerge_r( geosinit.ctxt, mGeos ) );
+  }
+  CATCH_GEOS_WITH_ERRMSG( QgsGeometry() );
+  return QgsGeometry( fromGeos( geos.get() ) );
+}
+
 QgsGeometry QgsGeos::closestPoint( const QgsGeometry& other, QString* errorMsg ) const
 {
   if ( !mGeos || other.isEmpty() )

src/core/geometry/qgsgeos.h

@@ -87,6 +87,16 @@ class CORE_EXPORT QgsGeos: public QgsGeometryEngine
     QgsAbstractGeometryV2* offsetCurve( double distance, int segments, int joinStyle, double mitreLimit, QString* errorMsg = nullptr ) const override;
     QgsAbstractGeometryV2* reshapeGeometry( const QgsLineStringV2& reshapeWithLine, int* errorCode, QString* errorMsg = nullptr ) const;
 
+    /** Merges any connected lines in a LineString/MultiLineString geometry and
+     * converts them to single line strings.
+     * @param errorMsg if specified, will be set to any reported GEOS errors
+     * @returns a LineString or MultiLineString geometry, with any connected lines
+     * joined. An empty geometry will be returned if the input geometry was not a
+     * LineString/MultiLineString geometry.
+     * @note added in QGIS 3.0
+     */
+    QgsGeometry mergeLines( QString* errorMsg = nullptr ) const;
+
     /** Returns the closest point on the geometry to the other geometry.
      * @note added in QGIS 2.14
      * @see shortestLine()

src/core/qgsexpression.cpp

@@ -1839,6 +1839,20 @@ static QVariant fcnBoundary( const QVariantList& values, const QgsExpressionCont
   return QVariant::fromValue( QgsGeometry( boundary ) );
 }
 
+static QVariant fcnLineMerge( const QVariantList& values, const QgsExpressionContext*, QgsExpression* parent )
+{
+  QgsGeometry geom = getGeometry( values.at( 0 ), parent );
+
+  if ( geom.isEmpty() )
+    return QVariant();
+
+  QgsGeometry merged = geom.mergeLines();
+  if ( merged.isEmpty() )
+    return QVariant();
+
+  return QVariant::fromValue( merged );
+}
+
 static QVariant fcnMakePoint( const QVariantList& values, const QgsExpressionContext*, QgsExpression* parent )
 {
   if ( values.count() < 2 || values.count() > 4 )
@@ -3187,7 +3201,7 @@ const QStringList& QgsExpression::BuiltinFunctions()
     << "disjoint" << "intersects" << "touches" << "crosses" << "contains"
     << "relate"
     << "overlaps" << "within" << "buffer" << "centroid" << "bounds" << "reverse" << "exterior_ring"
-    << "boundary"
+    << "boundary" << "line_merge"
     << "bounds_width" << "bounds_height" << "is_closed" << "convex_hull" << "difference"
     << "distance" << "intersection" << "sym_difference" << "combine"
     << "extrude" << "azimuth" <<  "project" << "closest_point" << "shortest_line"
@@ -3380,6 +3394,7 @@ const QList<QgsExpression::Function*>& QgsExpression::Functions()
     << new StaticFunction( "interior_ring_n", 2, fcnInteriorRingN, "GeometryGroup" )
     << new StaticFunction( "geometry_n", 2, fcnGeometryN, "GeometryGroup" )
     << new StaticFunction( "boundary", ParameterList() << Parameter( "geometry" ), fcnBoundary, "GeometryGroup" )
+    << new StaticFunction( "line_merge", ParameterList() << Parameter( "geometry" ), fcnLineMerge, "GeometryGroup" )
     << new StaticFunction( "bounds", 1, fcnBounds, "GeometryGroup" )
     << new StaticFunction( "num_points", 1, fcnGeomNumPoints, "GeometryGroup" )
     << new StaticFunction( "num_interior_rings", 1, fcnGeomNumInteriorRings, "GeometryGroup" )

tests/src/core/testqgsexpression.cpp

@@ -672,6 +672,11 @@ class TestQgsExpression: public QObject
       QTest::newRow( "boundary point" ) << "boundary(geom_from_wkt('POINT(1 2)'))" << false << QVariant();
       QTest::newRow( "boundary polygon" ) << "geom_to_wkt(boundary(geometry:=geom_from_wkt('POLYGON((-1 -1, 4 0, 4 2, 0 2, -1 -1))')))" << false << QVariant( "LineString (-1 -1, 4 0, 4 2, 0 2, -1 -1)" );
       QTest::newRow( "boundary line" ) << "geom_to_wkt(boundary(geom_from_wkt('LINESTRING(0 0, 1 1, 2 2)')))" << false << QVariant( "MultiPoint ((0 0),(2 2))" );
+      QTest::newRow( "line_merge not geom" ) << "line_merge('g')" << true << QVariant();
+      QTest::newRow( "line_merge null" ) << "line_merge(NULL)" << false << QVariant();
+      QTest::newRow( "line_merge point" ) << "line_merge(geom_from_wkt('POINT(1 2)'))" << false << QVariant();
+      QTest::newRow( "line_merge line" ) << "geom_to_wkt(line_merge(geometry:=geom_from_wkt('LineString(0 0, 10 10)')))" << false << QVariant( "LineString (0 0, 10 10)" );
+      QTest::newRow( "line_merge multiline" ) << "geom_to_wkt(line_merge(geom_from_wkt('MultiLineString((0 0, 10 10),(10 10, 20 20))')))" << false << QVariant( "LineString (0 0, 10 10, 20 20)" );
       QTest::newRow( "start_point point" ) << "geom_to_wkt(start_point(geom_from_wkt('POINT(2 0)')))" << false << QVariant( "Point (2 0)" );
       QTest::newRow( "start_point multipoint" ) << "geom_to_wkt(start_point(geom_from_wkt('MULTIPOINT((3 3), (1 1), (2 2))')))" << false << QVariant( "Point (3 3)" );
       QTest::newRow( "start_point line" ) << "geom_to_wkt(start_point(geom_from_wkt('LINESTRING(4 1, 1 1, 2 2)')))" << false << QVariant( "Point (4 1)" );

tests/src/python/test_qgsgeometry.py

@@ -3360,5 +3360,35 @@ def testMisc(self):
         wkb2 = geom.asWkb()
         self.assertEqual(wkb1, wkb2)
 
+    def testMergeLines(self):
+        """ test merging linestrings """
+
+        # not a (multi)linestring
+        geom = QgsGeometry.fromWkt('Point(1 2)')
+        result = geom.mergeLines()
+        self.assertTrue(result.isEmpty())
+
+        # linestring should be returned intact
+        geom = QgsGeometry.fromWkt('LineString(0 0, 10 10)')
+        result = geom.mergeLines().exportToWkt()
+        exp = 'LineString(0 0, 10 10)'
+        self.assertTrue(compareWkt(result, exp, 0.00001), "Merge lines: mismatch Expected:\n{}\nGot:\n{}\n".format(exp, result))
+
+        # multilinestring
+        geom = QgsGeometry.fromWkt('MultiLineString((0 0, 10 10),(10 10, 20 20))')
+        result = geom.mergeLines().exportToWkt()
+        exp = 'LineString(0 0, 10 10, 20 20)'
+        self.assertTrue(compareWkt(result, exp, 0.00001), "Merge lines: mismatch Expected:\n{}\nGot:\n{}\n".format(exp, result))
+
+        geom = QgsGeometry.fromWkt('MultiLineString((0 0, 10 10),(12 2, 14 4),(10 10, 20 20))')
+        result = geom.mergeLines().exportToWkt()
+        exp = 'MultiLineString((0 0, 10 10, 20 20),(12 2, 14 4))'
+        self.assertTrue(compareWkt(result, exp, 0.00001), "Merge lines: mismatch Expected:\n{}\nGot:\n{}\n".format(exp, result))
+
+        geom = QgsGeometry.fromWkt('MultiLineString((0 0, 10 10),(12 2, 14 4))')
+        result = geom.mergeLines().exportToWkt()
+        exp = 'MultiLineString((0 0, 10 10),(12 2, 14 4))'
+        self.assertTrue(compareWkt(result, exp, 0.00001), "Merge lines: mismatch Expected:\n{}\nGot:\n{}\n".format(exp, result))
+
 if __name__ == '__main__':
     unittest.main()