Add class for comparing distance from two line segments to an origin

Author: nyalldawson

src/core/geometry/qgsinternalgeometryengine.cpp

@@ -711,6 +711,64 @@ QgsGeometry QgsInternalGeometryEngine::densifyByDistance( double distance ) cons
   }
 }
 
+//
+// QgsLineSegmentDistanceComparer
+//
+
+// adapted for QGIS geometry classes from original work at https://github.com/trylock/visibility by trylock
+bool QgsLineSegmentDistanceComparer::operator()( QgsLineSegment2D ab, QgsLineSegment2D cd ) const
+{
+  Q_ASSERT_X( ab.pointLeftOfLine( mOrigin ) != 0,
+              "line_segment_dist_comparer",
+              "AB must not be collinear with the origin." );
+  Q_ASSERT_X( cd.pointLeftOfLine( mOrigin ) != 0,
+              "line_segment_dist_comparer",
+              "CD must not be collinear with the origin." );
+
+  // flip the segments so that if there are common endpoints,
+  // they will be the segment's start points
+  if ( ab.end() == cd.start() || ab.end() == cd.end() )
+    ab.reverse();
+  if ( ab.start() == cd.end() )
+    cd.reverse();
+
+  // cases with common endpoints
+  if ( ab.start() == cd.start() )
+  {
+    const int oad = QgsGeometryUtils::leftOfLine( cd.endX(), cd.endY(), mOrigin.x(), mOrigin.y(), ab.startX(), ab.startY() );
+    const int oab = ab.pointLeftOfLine( mOrigin );
+    if ( ab.end() == cd.end() || oad != oab )
+      return false;
+    else
+      return ab.pointLeftOfLine( cd.end() ) != oab;
+  }
+  else
+  {
+    // cases without common endpoints
+    const int cda = cd.pointLeftOfLine( ab.start() );
+    const int cdb = cd.pointLeftOfLine( ab.end() );
+    if ( cdb == 0 && cda == 0 )
+    {
+      return mOrigin.sqrDist( ab.start() ) < mOrigin.sqrDist( cd.start() );
+    }
+    else if ( cda == cdb || cda == 0 || cdb == 0 )
+    {
+      const int cdo = cd.pointLeftOfLine( mOrigin );
+      return cdo == cda || cdo == cdb;
+    }
+    else
+    {
+      const int abo = ab.pointLeftOfLine( mOrigin );
+      return abo != ab.pointLeftOfLine( cd.start() );
+    }
+  }
+}
+
+
+//
+// QgsRay2D
+//
+
 bool QgsRay2D::intersects( const QgsLineSegment2D &segment, QgsPointXY &intersectPoint ) const
 {
   const QgsVector ao = origin - segment.start();

src/core/geometry/qgsinternalgeometryengine.h

@@ -110,6 +110,7 @@ class QgsInternalGeometryEngine
  * A 2D ray which extends from an origin point to an infinite distance in a given direction.
  * \ingroup core
  * \since QGIS 3.2
+ * \note not available in Python bindings
  */
 class CORE_EXPORT QgsRay2D
 {
@@ -139,4 +140,43 @@ class CORE_EXPORT QgsRay2D
     QgsVector direction;
 };
 
+///@cond PRIVATE
+
+// adapted for QGIS geometry classes from original work at https://github.com/trylock/visibility by trylock
+
+/**
+ * Compares two line segments based on their distance from a given point
+ * Assumes: (1) the line segments are intersected by some ray from the origin
+ *          (2) the line segments do not intersect except at their endpoints
+ *          (3) no line segment is collinear with the origin
+ */
+class CORE_EXPORT QgsLineSegmentDistanceComparer
+{
+  public:
+
+    /**
+     * Constructor for QgsLineSegmentDistanceComparer, comparing points
+     * to the specified \a origin point.
+     */
+    explicit QgsLineSegmentDistanceComparer( const QgsPointXY &origin )
+      : mOrigin( origin )
+    {}
+
+    /**
+     * Checks whether the line segment \a ab is closer to the origin than the
+     * line segment \a cd.
+     * \param ab line segment: left hand side of the comparison operator
+     * \param cd line segment: right hand side of the comparison operator
+     * \returns true if ab < cd (ab is closer than cd) to origin
+     */
+    bool operator()( QgsLineSegment2D ab, QgsLineSegment2D cd ) const;
+
+  private:
+
+    QgsPointXY mOrigin;
+
+};
+
+///@endcond PRIVATE
+
 #endif // QGSINTERNALGEOMETRYENGINE_H

tests/src/core/testqgsinternalgeometryengine.cpp

@@ -30,6 +30,7 @@ class TestQgsInternalGeometryEngine : public QObject
     void init();// will be called before each testfunction is executed.
     void cleanup();// will be called after every testfunction.
     void ray();
+    void lineSegmentDistanceComparer();
 
 };
 
@@ -95,5 +96,33 @@ void TestQgsInternalGeometryEngine::ray()
 
 }
 
+void TestQgsInternalGeometryEngine::lineSegmentDistanceComparer()
+{
+  QgsLineSegmentDistanceComparer comp( QgsPointXY( 3, 5 ) );
+
+  QVERIFY( comp( QgsLineSegment2D( QgsPointXY( 1, 2 ), QgsPointXY( 3, 4 ) ),
+                 QgsLineSegment2D( QgsPointXY( 11, 2 ), QgsPointXY( 13, 4 ) ) ) );
+  QVERIFY( comp( QgsLineSegment2D( QgsPointXY( 1, 2 ), QgsPointXY( 3, 4 ) ),
+                 QgsLineSegment2D( QgsPointXY( 13, 4 ), QgsPointXY( 11, 2 ) ) ) );
+  QVERIFY( comp( QgsLineSegment2D( QgsPointXY( 1, 2 ), QgsPointXY( 3, 4 ) ),
+                 QgsLineSegment2D( QgsPointXY( -13, 4 ), QgsPointXY( -11, 2 ) ) ) );
+  QVERIFY( comp( QgsLineSegment2D( QgsPointXY( 1, 2 ), QgsPointXY( 3, 4 ) ),
+                 QgsLineSegment2D( QgsPointXY( -13, -4 ), QgsPointXY( -11, 2 ) ) ) );
+  QVERIFY( comp( QgsLineSegment2D( QgsPointXY( 1, 2 ), QgsPointXY( 3, 4 ) ),
+                 QgsLineSegment2D( QgsPointXY( 11, 2 ), QgsPointXY( 13, 4 ) ) ) );
+  QVERIFY( !comp( QgsLineSegment2D( QgsPointXY( 11, 2 ), QgsPointXY( 13, 4 ) ),
+                  QgsLineSegment2D( QgsPointXY( 1, 2 ), QgsPointXY( 3, 4 ) ) ) );
+  QVERIFY( !comp( QgsLineSegment2D( QgsPointXY( 1, 2 ), QgsPointXY( 3, 4 ) ),
+                  QgsLineSegment2D( QgsPointXY( 5, 6 ), QgsPointXY( 8, 9 ) ) ) );
+  QVERIFY( comp( QgsLineSegment2D( QgsPointXY( 5, 6 ), QgsPointXY( 8, 9 ) ),
+                 QgsLineSegment2D( QgsPointXY( 1, 2 ), QgsPointXY( 3, 4 ) ) ) );
+  QVERIFY( !comp( QgsLineSegment2D( QgsPointXY( 5, 6 ), QgsPointXY( 8, 9 ) ),
+                  QgsLineSegment2D( QgsPointXY( 8, 9 ), QgsPointXY( 13, 14 ) ) ) );
+  QVERIFY( !comp( QgsLineSegment2D( QgsPointXY( 8, 9 ), QgsPointXY( 13, 14 ) ),
+                  QgsLineSegment2D( QgsPointXY( 5, 6 ), QgsPointXY( 8, 9 ) ) ) );
+  QVERIFY( comp( QgsLineSegment2D( QgsPointXY( 1, 4 ), QgsPointXY( 8, 9 ) ),
+                 QgsLineSegment2D( QgsPointXY( 8, 9 ), QgsPointXY( 15, 16 ) ) ) );
+}
+
 QGSTEST_MAIN( TestQgsInternalGeometryEngine )
 #include "testqgsinternalgeometryengine.moc"