Add `firstCoordinate` vector tile feature helper (#814)

planetiler-core/src/main/java/com/onthegomap/planetiler/VectorTile.java

@@ -43,6 +43,7 @@ import javax.annotation.concurrent.NotThreadSafe;
 import org.locationtech.jts.algorithm.Orientation;
 import org.locationtech.jts.geom.Coordinate;
 import org.locationtech.jts.geom.CoordinateSequence;
+import org.locationtech.jts.geom.CoordinateXY;
 import org.locationtech.jts.geom.Geometry;
 import org.locationtech.jts.geom.GeometryFactory;
 import org.locationtech.jts.geom.LineString;
@@ -987,6 +988,20 @@ public class VectorTile {
       return Hilbert.hilbertXYToIndex(15, x >> scale, y >> scale);
     }
 
+
+    /**
+     * Returns the coordinate of the first point in this geometry in tile pixel coordinates from (0,0) at the top left
+     * to (256,256) at the bottom right.
+     */
+    public CoordinateXY firstCoordinate() {
+      if (commands.length < 3) {
+        return null;
+      }
+      double factor = 1 << scale;
+      double x = zigZagDecode(commands[1]) * SIZE / EXTENT / factor;
+      double y = zigZagDecode(commands[2]) * SIZE / EXTENT / factor;
+      return new CoordinateXY(x, y);
+    }
   }
 
   /**

planetiler-core/src/test/java/com/onthegomap/planetiler/VectorTileTest.java

@@ -633,6 +633,65 @@ class VectorTileTest {
       }));
   }
 
+  @ParameterizedTest
+  @CsvSource({
+    "0,0",
+    "1,1",
+    "-10,-1",
+    "300.25,200.75",
+  })
+  void firstCoordinateOfPoint(double x, double y) {
+    for (int scale = 0; scale < 10; scale++) {
+      assertEquals(new CoordinateXY(x, y), VectorTile.encodeGeometry(newPoint(x, y), scale).firstCoordinate(),
+        "scale=" + scale);
+    }
+  }
+
+  @ParameterizedTest
+  @CsvSource({
+    "0,0",
+    "1,1",
+    "-10,-1",
+    "300.25,200.75",
+  })
+  void firstCoordinateOfMultiPoint(double x, double y) {
+    for (int scale = 0; scale < 10; scale++) {
+      assertEquals(new CoordinateXY(x, y),
+        VectorTile.encodeGeometry(newMultiPoint(newPoint(x, y), newPoint(0, 0)), scale).firstCoordinate(),
+        "scale=" + scale);
+    }
+  }
+
+  @ParameterizedTest
+  @CsvSource({
+    "0,0",
+    "1,1",
+    "-10,-1",
+    "300.25,200.75",
+  })
+  void firstCoordinateOfLine(double x, double y) {
+    for (int scale = 0; scale < 10; scale++) {
+      assertEquals(new CoordinateXY(x, y),
+        VectorTile.encodeGeometry(newLineString(x, y, x + 1, y + 1), scale).firstCoordinate(),
+        "scale=" + scale);
+    }
+  }
+
+  @ParameterizedTest
+  @CsvSource({
+    "0,0",
+    "1,1",
+    "-10,-1",
+    "300.25,200.75",
+  })
+  void firstCoordinateOfPolygon(double x, double y) {
+    for (int scale = 0; scale < 10; scale++) {
+      assertEquals(new CoordinateXY(x, y),
+        VectorTile.encodeGeometry(newPolygon(x, y, x + 1, y, x + 1, y + 1, x, y + 1, x, y), scale).firstCoordinate(),
+        "scale=" + scale);
+    }
+  }
+
   private static void assertArrayEquals(int[] a, int[] b) {
     assertEquals(
       IntStream.of(a).boxed().toList(),