From 6475353f3d81dc57cca5c6b40654098e0635bbda Mon Sep 17 00:00:00 2001
From: Jia Yu <jiayu@apache.org>
Date: Wed, 29 Jul 2026 00:17:31 -0700
Subject: [PATCH] [GH-3200] Serialize empty Geography LineStrings as valid WKB

---
 .../sedona/common/S2Geography/WKBWriter.java       |  4 ++++
 .../common/S2Geography/WKBGeographyTest.java       | 14 ++++++++++++++
 .../sedona/common/S2Geography/WKBWriterTest.java   | 12 ++++++++++++
 3 files changed, 30 insertions(+)

diff --git a/common/src/main/java/org/apache/sedona/common/S2Geography/WKBWriter.java b/common/src/main/java/org/apache/sedona/common/S2Geography/WKBWriter.java
index 32a8651270b..1559a8aa6d5 100644
--- a/common/src/main/java/org/apache/sedona/common/S2Geography/WKBWriter.java
+++ b/common/src/main/java/org/apache/sedona/common/S2Geography/WKBWriter.java
@@ -250,6 +250,10 @@ private void writePolyline(int geometryType, SinglePolylineGeography polyline, O
     writeByteOrder(os);
     writeGeometryType(geometryType, polyline, os);
     List<S2Polyline> s2line = polyline.getPolylines();
+    if (s2line.isEmpty()) {
+      writeInt(0, os);
+      return;
+    }
     for (S2Polyline s2 : s2line) {
       List<S2Point> verts = s2.vertices();
       writeInt(verts.size(), os);
diff --git a/common/src/test/java/org/apache/sedona/common/S2Geography/WKBGeographyTest.java b/common/src/test/java/org/apache/sedona/common/S2Geography/WKBGeographyTest.java
index 165cdecd9bb..dae3aae79a6 100644
--- a/common/src/test/java/org/apache/sedona/common/S2Geography/WKBGeographyTest.java
+++ b/common/src/test/java/org/apache/sedona/common/S2Geography/WKBGeographyTest.java
@@ -28,6 +28,7 @@
 import org.locationtech.jts.geom.Coordinate;
 import org.locationtech.jts.geom.Geometry;
 import org.locationtech.jts.geom.GeometryFactory;
+import org.locationtech.jts.geom.LineString;
 import org.locationtech.jts.geom.Point;
 import org.locationtech.jts.io.ByteOrderValues;
 import org.locationtech.jts.io.ParseException;
@@ -137,6 +138,19 @@ public void fromS2Geography_point() {
     assertSame(s2Geog, roundTrip);
   }
 
+  @Test
+  public void fromS2Geography_emptyLineString_roundTripsToJTS() {
+    Geography s2Geog = new SinglePolylineGeography();
+    s2Geog.setSRID(4326);
+
+    WKBGeography geog = WKBGeography.fromS2Geography(s2Geog);
+    Geometry jts = geog.getJTSGeometry();
+
+    assertTrue(jts instanceof LineString);
+    assertTrue(jts.isEmpty());
+    assertEquals(4326, jts.getSRID());
+  }
+
   // ─── Lazy S2 delegation ──────────────────────────────────────────────────
 
   @Test
diff --git a/common/src/test/java/org/apache/sedona/common/S2Geography/WKBWriterTest.java b/common/src/test/java/org/apache/sedona/common/S2Geography/WKBWriterTest.java
index d905635ca45..3b4c2ac3c45 100644
--- a/common/src/test/java/org/apache/sedona/common/S2Geography/WKBWriterTest.java
+++ b/common/src/test/java/org/apache/sedona/common/S2Geography/WKBWriterTest.java
@@ -90,6 +90,18 @@ public void PolylineGeographyToHexTest() throws IOException, ParseException {
     }
   }
 
+  @Test
+  public void EmptySinglePolylineWritesValidWKB() throws IOException, ParseException {
+    Geography inputLine = new SinglePolylineGeography();
+
+    byte[] wkb = new WKBWriter(2, ByteOrderValues.LITTLE_ENDIAN, false).write(inputLine);
+
+    assertEquals("010200000000000000", WKBWriter.toHex(wkb));
+    Geography outputLine = new WKBReader().read(wkb);
+    assertTrue(outputLine instanceof SinglePolylineGeography);
+    assertEquals(0, outputLine.numShapes());
+  }
+
   @Test
   public void MultiPointTest() throws ParseException, IOException {
     String wkt = "MULTIPOINT ((10 40), (40 30))";