[GitHub] [lucene-solr] nknize commented on a change in pull request #709: LUCENE-8850: Calculate the area of a polygon and throw error when values are invalid

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nknize commented on a change in pull request #709: LUCENE-8850: Calculate the area of a polygon and throw error when values are invalid
URL: https://github.com/apache/lucene-solr/pull/709#discussion_r299071383
 
 

 ##########
 File path: lucene/core/src/java/org/apache/lucene/geo/Polygon.java
 ##########
 @@ -96,34 +99,50 @@ public Polygon(double[] polyLats, double[] polyLons, Polygon... holes) {
     this.holes = holes.clone();
 
     // compute bounding box
-    double minLat = polyLats[0];
-    double maxLat = polyLats[0];
-    double minLon = polyLons[0];
-    double maxLon = polyLons[0];
+    double minLat = Double.POSITIVE_INFINITY;
+    double maxLat = Double.NEGATIVE_INFINITY;
+    double minLon = Double.POSITIVE_INFINITY;
+    double maxLon = Double.NEGATIVE_INFINITY;
 
     double windingSum = 0d;
     final int numPts = polyLats.length - 1;
-    for (int i = 1, j = 0; i < numPts; j = i++) {
+    for (int i = 0; i < numPts; i++) {
       minLat = Math.min(polyLats[i], minLat);
       maxLat = Math.max(polyLats[i], maxLat);
       minLon = Math.min(polyLons[i], minLon);
       maxLon = Math.max(polyLons[i], maxLon);
       // compute signed area
-      windingSum += (polyLons[j] - polyLons[numPts])*(polyLats[i] - polyLats[numPts])
-          - (polyLats[j] - polyLats[numPts])*(polyLons[i] - polyLons[numPts]);
+      windingSum += polyLons[i] * polyLats[i + 1] - polyLats[i] * polyLons[i + 1];
+    }
+    if (windingSum == 0) {
+      throw new IllegalArgumentException("Cannot compute the polygon / hole orientation.");
     }
     this.minLat = minLat;
     this.maxLat = maxLat;
     this.minLon = minLon;
     this.maxLon = maxLon;
     this.windingOrder = (windingSum < 0) ? GeoUtils.WindingOrder.CCW : GeoUtils.WindingOrder.CW;
+    double area = Math.abs(windingSum / 2d);
+    for (Polygon hole : holes) {
+      area -= hole.area();
+    }
+    if (area <= 0) {
+      throw new IllegalArgumentException("Polygon has an invalid area (area = " + area + ").");
+    }
+    this.area = area;
+
   }
 
   /** returns the number of vertex points */
   public int numPoints() {
     return polyLats.length;
   }
 
+  /** returns the area of the polygon */
+  public double area() {
 
 Review comment:
   > * Currently we compute the signed area to check the orientation of a polygon, if the area is 0 the polygon is considered CW, should we fail instead?
   
   +1 to fail in this situation since it is not a valid polygon. 
   
   
   > * I have seen polygons with one hole where the hole and polygon are the same, should we capture that situation and fail?
   
   +1 to fail in this situation as well. 
   

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