refactor(geojson): loop edge cases

Author: missinglink

geojson/loop.ts

@@ -9,12 +9,12 @@ import { Loop } from '../s2/Loop'
 export const marshal = (loop: Loop, ordinal: number): geojson.Position[] => {
   const ring = loop.vertices.map(position.marshal)
   if (ordinal > 0) ring.reverse() // outer ring remains CCW, inner rings become CW
-  ring.push(ring[0]) // add matching start/end points
+  if (ring.length) ring.push(ring[0]) // add matching start/end points
   return ring
 }
 
 /**
- * Constructs an s2 Loop given a geojson Polygon ring & ordinal.
+ * Constructs an s2 Loop given a geojson Polygon ring.
  * @category Constructors
  *
  * Handles differences between GeoJSON and S2:
@@ -36,7 +36,8 @@ export const unmarshal = (ring: geojson.Position[]): Loop => {
   // Loops are not allowed to have duplicate vertices (whether adjacent or not)
   if (containsDuplicateVertices(ring)) {
     // adjacent duplicates are fixable
-    ring = removeAdjacentDuplicateVertices(ring)
+    ring = removeAdjacentDuplicateVertices(ring, 0)
+    if (ring.length < 3) return new Loop([])
 
     // non-adjacent duplicates are not fixable
     if (containsDuplicateVertices(ring)) return new Loop([])