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       1                 :            : /***************************************************************************
       2                 :            :   qgsvectortilemvtutils.cpp
       3                 :            :   --------------------------------------
       4                 :            :   Date                 : April 2020
       5                 :            :   Copyright            : (C) 2020 by Martin Dobias
       6                 :            :   Email                : wonder dot sk at gmail dot com
       7                 :            :  ***************************************************************************
       8                 :            :  *                                                                         *
       9                 :            :  *   This program is free software; you can redistribute it and/or modify  *
      10                 :            :  *   it under the terms of the GNU General Public License as published by  *
      11                 :            :  *   the Free Software Foundation; either version 2 of the License, or     *
      12                 :            :  *   (at your option) any later version.                                   *
      13                 :            :  *                                                                         *
      14                 :            :  ***************************************************************************/
      15                 :            : 
      16                 :            : #include "qgsvectortilemvtutils.h"
      17                 :            : 
      18                 :            : #include "qgslinestring.h"
      19                 :            : 
      20                 :            : 
      21                 :          0 : bool QgsVectorTileMVTUtils::isExteriorRing( const QgsLineString *lineString )
      22                 :            : {
      23                 :            :   // Exterior rings have POSITIVE area while interior rings have NEGATIVE area
      24                 :            :   // when calculated with https://en.wikipedia.org/wiki/Shoelace_formula
      25                 :            :   // The orientation of axes is that X grows to the right and Y grows to the bottom.
      26                 :            :   // the input data are expected to form a closed ring, i.e. first pt == last pt.
      27                 :            : 
      28                 :          0 :   double total = 0.0;
      29                 :          0 :   int count = lineString->numPoints();
      30                 :          0 :   const double *xData = lineString->xData();
      31                 :          0 :   const double *yData = lineString->yData();
      32                 :            : 
      33                 :          0 :   for ( int i = 0; i < count - 1; i++ )
      34                 :            :   {
      35                 :          0 :     double val = ( xData[i + 1] - xData[i] ) * ( yData[i + 1] + yData[i] );
      36                 :            :     //double val = xData[i] * (-yData[i+1]) - xData[i+1] * (-yData[i]);  // gives the same result
      37                 :          0 :     total += val;
      38                 :          0 :   }
      39                 :          0 :   return total >= 0;
      40                 :            : }