fix(geojson): simplify loop rewinding

geojson/loop.ts

@@ -16,13 +16,24 @@ export const marshal = (loop: Loop, ordinal: number): geojson.Position[] => {
 /**
  * Constructs an s2 Loop given a geojson Polygon ring & ordinal.
  * @category Constructors
+ *
+ * Handles differences between GeoJSON and S2:
+ * - GeoJSON rings are oriented CCW for the exterior and CW for holes, in S2 all loops are oriented CCW.
+ * - GeoJSON rings duplicate the start/end points, in S2 they do not.
+ *
+ * S2 Loops require the following properties be met:
+ * - Loops are not allowed to have any duplicate vertices (whether adjacent or not).
+ * - Non-adjacent edges are not allowed to intersect, and furthermore edges of length 180 degrees are not allowed (i.e., adjacent vertices cannot be antipodal).
+ * - Loops must have at least 3 vertices.
  */
-export const unmarshal = (ring: geojson.Position[], ordinal: number): Loop => {
+export const unmarshal = (ring: geojson.Position[]): Loop => {
+  if (ring.length < 3) return new Loop([])
+
   ring = ring.slice() // make a copy to avoid mutating input
-  ring.length -= 1 // remove matching start/end points
-  if (ordinal > 0) ring.reverse() // ensure all rings are CCW
+  if (clockwise(ring)) ring.reverse() // all rings must be CCW
+  if (position.equal(ring.at(0)!, ring.at(-1)!)) ring.length -= 1 // remove matching start/end points
 
-  // Loops are not allowed to have any duplicate vertices (whether adjacent or not)
+  // Loops are not allowed to have duplicate vertices (whether adjacent or not)
   if (containsDuplicateVertices(ring)) {
     // adjacent duplicates are fixable
     ring = removeAdjacentDuplicateVertices(ring)
@@ -47,3 +58,14 @@ export const removeAdjacentDuplicateVertices = (ring: geojson.Position[], epsilo
 export const containsDuplicateVertices = (ring: geojson.Position[]): boolean => {
   return new Set(ring.map((c) => `${c[0]}|${c[1]}`)).size !== ring.length
 }
+
+/**
+ * Returns true IFF ring is oriented Clockwise.
+ */
+export const clockwise = (ring: geojson.Position[]): boolean => {
+  let sum = 0
+  for (let i = 1; i < ring.length; i++) {
+    sum += (ring[i][0] - ring[i - 1][0]) * (ring[i][1] + ring[i - 1][1])
+  }
+  return sum > 0
+}

geojson/polygon.ts

@@ -1,7 +1,6 @@
 import type * as geojson from 'geojson'
 import * as loop from './loop'
 import { Polygon } from '../s2/Polygon'
-import { rewindPolygon } from './turf'
 
 /**
  * Returns a geojson Polygon geometry given an s2 Polygon.
@@ -18,8 +17,7 @@ export const marshal = (polygon: Polygon): geojson.Polygon => {
  * Constructs an s2 Polygon given a geojson Polygon geometry.
  * @category Constructors
  */
-export const unmarshal = (geometry: geojson.Polygon, rewind = true): Polygon => {
-  if (rewind) rewindPolygon(geometry.coordinates, false)
+export const unmarshal = (geometry: geojson.Polygon): Polygon => {
   return new Polygon(geometry.coordinates.map(loop.unmarshal))
 }
 
@@ -38,9 +36,6 @@ export const marshalMulti = (polygons: Polygon[]): geojson.MultiPolygon => {
  * Constructs s2 Polygons given a geojson MultiPolygon geometry.
  * @category Constructors
  */
-export const unmarshalMulti = (geometry: geojson.MultiPolygon, rewind = true): Polygon[] => {
-  return geometry.coordinates.map((coords) => {
-    if (rewind) rewindPolygon(coords, false)
-    return new Polygon(coords.map(loop.unmarshal))
-  })
+export const unmarshalMulti = (geometry: geojson.MultiPolygon): Polygon[] => {
+  return geometry.coordinates.map((coords) => new Polygon(coords.map(loop.unmarshal)))
 }