dsmiley 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_r298616974
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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:
Please add a comment to the code to to link to the "shoelace method" that
Nick points at. I think it's important for non-trivial algorithms to
incorporate references where the readers of this code can learn more. Who
knows... maybe some "velcro method" will come along and be superior and then
some student looking at this code will point this out to us :-)
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