[pal] Fix crash in labeling polygons in some rare circumstance which …

…I can

only reproduce on a single project

But this code is so totally unreadable that it's not suprising that
there's all kinds of nasty here. In this case the crash is caused
when the polygon part calculated by "splitPolygons" (which does...
something... to a polygon) results in a zero area polygon (because...
reasons). So add a safeguard in to avoid this situation.

Also rename class to something saner

src/core/pal/feature.cpp

@@ -1654,14 +1654,17 @@ std::size_t FeaturePart::createCandidatesForPolygon( std::vector< std::unique_pt
     double beta;
     double diago = std::sqrt( labelWidth * labelWidth / 4.0 + labelHeight * labelHeight / 4 );
     double rx, ry;
-    std::vector< CHullBox > boxes;
+    std::vector< OrientedConvexHullBoundingBox > boxes;
     boxes.reserve( shapes_final.size() );
 
     // Compute bounding box foreach finalShape
     while ( !shapes_final.isEmpty() )
     {
       PointSet *shape = shapes_final.takeFirst();
-      boxes.emplace_back( shape->compute_chull_bbox() );
+      bool ok = false;
+      OrientedConvexHullBoundingBox box = shape->computeConvexHullOrientedBoundingBox( ok );
+      if ( ok )
+        boxes.emplace_back( box );
 
       if ( shape->parent )
         delete shape;
@@ -1682,7 +1685,7 @@ std::size_t FeaturePart::createCandidatesForPolygon( std::vector< std::unique_pt
 
     do
     {
-      for ( CHullBox &box : boxes )
+      for ( OrientedConvexHullBoundingBox &box : boxes )
       {
         // there is two possibilities here:
         // 1. no maximum candidates for polygon setting is in effect (i.e. maxPolygonCandidates == 0). In that case,

src/core/pal/pointset.cpp

@@ -638,31 +638,20 @@ void PointSet::extendLineByDistance( double startDistance, double endDistance, d
   invalidateGeos();
 }
 
-CHullBox PointSet::compute_chull_bbox()
+OrientedConvexHullBoundingBox PointSet::computeConvexHullOrientedBoundingBox( bool &ok )
 {
-  int i;
-  int j;
-
+  ok = false;
   double bbox[4]; // xmin, ymin, xmax, ymax
 
   double alpha;
   int alpha_d;
 
   double alpha_seg;
 
-  double dref;
   double d1, d2;
 
   double bb[16];   // {ax, ay, bx, by, cx, cy, dx, dy, ex, ey, fx, fy, gx, gy, hx, hy}}
 
-  double cp;
-  double best_cp;
-  double distNearestPoint;
-
-  double area;
-  double width;
-  double length;
-
   double best_area = std::numeric_limits<double>::max();
   double best_alpha = -1;
   double best_bb[16];
@@ -675,7 +664,7 @@ CHullBox PointSet::compute_chull_bbox()
   bbox[2] = std::numeric_limits<double>::lowest();
   bbox[3] = std::numeric_limits<double>::lowest();
 
-  for ( i = 0; i < cHullSize; i++ )
+  for ( int i = 0; i < cHullSize; i++ )
   {
     if ( x[cHull[i]] < bbox[0] )
       bbox[0] = x[cHull[i]];
@@ -690,8 +679,14 @@ CHullBox PointSet::compute_chull_bbox()
       bbox[3] = y[cHull[i]];
   }
 
+  OrientedConvexHullBoundingBox finalBb;
 
-  dref = bbox[2] - bbox[0];
+  const double dref = bbox[2] - bbox[0];
+  if ( qgsDoubleNear( dref, 0 ) )
+  {
+    ok = false;
+    return finalBb;
+  }
 
   for ( alpha_d = 0; alpha_d < 90; alpha_d++ )
   {
@@ -722,22 +717,22 @@ CHullBox PointSet::compute_chull_bbox()
     bb[15] = bb[13] - d1; // hx, hy
 
     // adjust all points
-    for ( i = 0; i < 16; i += 4 )
+    for ( int i = 0; i < 16; i += 4 )
     {
 
       alpha_seg = ( ( i / 4 > 0 ? ( i / 4 ) - 1 : 3 ) ) * M_PI_2 + alpha;
 
-      best_cp = std::numeric_limits<double>::max();
-      for ( j = 0; j < nbPoints; j++ )
+      double best_cp = std::numeric_limits<double>::max();
+      for ( int j = 0; j < nbPoints; j++ )
       {
-        cp = GeomFunction::cross_product( bb[i + 2], bb[i + 3], bb[i], bb[i + 1], x[cHull[j]], y[cHull[j]] );
+        const double cp = GeomFunction::cross_product( bb[i + 2], bb[i + 3], bb[i], bb[i + 1], x[cHull[j]], y[cHull[j]] );
         if ( cp < best_cp )
         {
           best_cp = cp;
         }
       }
 
-      distNearestPoint = best_cp / dref;
+      const double distNearestPoint = best_cp / dref;
 
       d1 = std::cos( alpha_seg ) * distNearestPoint;
       d2 = std::sin( alpha_seg ) * distNearestPoint;
@@ -749,10 +744,10 @@ CHullBox PointSet::compute_chull_bbox()
     }
 
     // compute and compare AREA
-    width = GeomFunction::cross_product( bb[6], bb[7], bb[4], bb[5], bb[12], bb[13] ) / dref;
-    length = GeomFunction::cross_product( bb[2], bb[3], bb[0], bb[1], bb[8], bb[9] ) / dref;
+    const double width = GeomFunction::cross_product( bb[6], bb[7], bb[4], bb[5], bb[12], bb[13] ) / dref;
+    const double length = GeomFunction::cross_product( bb[2], bb[3], bb[0], bb[1], bb[8], bb[9] ) / dref;
 
-    area = width * length;
+    double area = width * length;
 
     if ( area < 0 )
       area *= -1;
@@ -769,10 +764,7 @@ CHullBox PointSet::compute_chull_bbox()
   }
 
   // best bbox is defined
-
-  CHullBox finalBb;
-
-  for ( i = 0; i < 16; i = i + 4 )
+  for ( int i = 0; i < 16; i = i + 4 )
   {
     GeomFunction::computeLineIntersection( best_bb[i], best_bb[i + 1], best_bb[i + 2], best_bb[i + 3],
                                            best_bb[( i + 4 ) % 16], best_bb[( i + 5 ) % 16], best_bb[( i + 6 ) % 16], best_bb[( i + 7 ) % 16],
@@ -783,6 +775,7 @@ CHullBox PointSet::compute_chull_bbox()
   finalBb.width = best_width;
   finalBb.length = best_length;
 
+  ok = true;
   return finalBb;
 }
 

src/core/pal/pointset.h

@@ -52,7 +52,10 @@ namespace pal
 
   class PointSet;
 
-  struct CHullBox
+  /**
+   * Represents the minimum area, oriented bounding box surrounding a convex hull.
+   */
+  struct OrientedConvexHullBoundingBox
   {
     double x[4];
     double y[4];
@@ -112,10 +115,9 @@ namespace pal
       bool containsLabelCandidate( double x, double y, double width, double height, double alpha = 0 ) const;
 
       /**
-       * Computes a con???? hull. Maybe convex, maybe concave. The person who wrote this
-       * had no care for anyone else ever reading their code.
+       * Computes an oriented bounding box for the shape's convex hull.
        */
-      CHullBox compute_chull_bbox();
+      OrientedConvexHullBoundingBox computeConvexHullOrientedBoundingBox( bool &ok );
 
       /**
        * Split a concave shape into several convex shapes.