[GitHub] [incubator-sedona] jiayuasu commented on a diff in pull request #621: [Sedona-113] Add ST_PointN to Apache Sedona

[GitBox <gi...@apache.org>]

jiayuasu commented on code in PR #621:
URL: https://github.com/apache/incubator-sedona/pull/621#discussion_r856841378


##########
docs/api/flink/Function.md:
##########
@@ -152,6 +152,34 @@ Input: `POLYGON ((-0.5 -0.5, -0.5 0.5, 0.5 0.5, 0.5 -0.5, -0.5 -0.5))`
 
 Output: `POLYGON ((-0.5 -0.5, 0.5 -0.5, 0.5 0.5, -0.5 0.5, -0.5 -0.5))`
 
+## ST_PointN

Review Comment:
   Please add docs for ST_ExteriorRing too.



##########
core/src/main/java/org/apache/sedona/core/utils/GeomUtils.java:
##########
@@ -81,6 +84,38 @@ public static Geometry getInteriorPoint(Geometry geometry) {
         return geometry.getInteriorPoint();
     }
 
+    /**
+     * Return the nth point from the given geometry (which could be a linestring or a circular linestring)
+     * If the value of n is negative, return a point backwards
+     * E.g. if n = 1, return 1st point, if n = -1, return last point
+     *
+     * @param lineString from which the nth point is to be returned
+     * @param n is the position of the point in the geometry
+     * @return a point
+     */
+    public static Geometry getNthPoint(LineString lineString, int n) {
+        if (lineString == null || n == 0) {
+            return null;
+        }
+
+        int p = lineString.getNumPoints();
+        if (Math.abs(n) > p) {
+            return null;
+        }
+
+        Coordinate[] nthCoordinate = new Coordinate[1];
+        if (n > 0) {
+            nthCoordinate[0] = lineString.getCoordinates()[n - 1];
+        } else {
+            nthCoordinate[0] = lineString.getCoordinates()[p + n];
+        }
+        return new Point(new CoordinateArraySequence(nthCoordinate), lineString.getFactory());
+    }
+
+    public static Geometry getExteriorRing(Geometry geometry) {
+        return geometry.getFactory().createLinearRing(geometry.getCoordinates());

Review Comment:
   Use this function in JTS Polygon. Dont create LinearRing by coordinates as they might NOT be the rings.https://locationtech.github.io/jts/javadoc/org/locationtech/jts/geom/Polygon.html#getExteriorRing--



##########
sql/src/main/scala/org/apache/sedona/sql/UDF/Catalog.scala:
##########
@@ -105,6 +105,7 @@ object Catalog {
     ST_Multi,
     ST_PointOnSurface,
     ST_Reverse,
+    ST_PointN,

Review Comment:
   Register ST_ExteriorRing as well



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