fix(geojson): strip duplicate adjacent polygon ring vertices

geojson/RegionCoverer_test.ts

@@ -0,0 +1,66 @@
+import type * as geojson from 'geojson'
+import { test, describe } from 'node:test'
+import { deepEqual } from 'node:assert/strict'
+import { RegionCoverer } from './RegionCoverer'
+import * as cellid from '../s2/cellid'
+
+describe('RegionCoverer', () => {
+  test('polygon - incorrect winding + duplicate adjacent vertices', (t) => {
+    const polygon: geojson.Polygon = {
+      type: 'Polygon',
+      coordinates: [
+        [
+          [-1.599437, 53.803895],
+          [-1.598511, 53.803895],
+          [-1.595764, 53.803895],
+          [-1.593018, 53.803895],
+          [-1.593018, 53.802273],
+          [-1.590271, 53.802273],
+          [-1.587524, 53.802273],
+          [-1.585241, 53.802273],
+          [-1.584778, 53.802273],
+          [-1.582031, 53.802273],
+          [-1.582031, 53.801097],
+          [-1.582031, 53.800651],
+          [-1.579285, 53.800651],
+          [-1.576538, 53.800651],
+          [-1.576538, 53.799029],
+          [-1.576538, 53.797406],
+          [-1.577464, 53.797406],
+          [-1.577464, 53.797406],
+          [-1.581105, 53.797406],
+          [-1.581424, 53.795784],
+          [-1.584778, 53.795784],
+          [-1.584778, 53.794162],
+          [-1.584778, 53.794144],
+          [-1.587524, 53.792594],
+          [-1.587524, 53.790917],
+          [-1.587524, 53.790917],
+          [-1.592091, 53.790917],
+          [-1.592091, 53.790917],
+          [-1.593018, 53.790917],
+          [-1.593018, 53.792539],
+          [-1.595764, 53.792539],
+          [-1.596722, 53.794162],
+          [-1.596722, 53.794162],
+          [-1.595764, 53.795784],
+          [-1.595764, 53.797406],
+          [-1.595764, 53.799029],
+          [-1.595764, 53.800651],
+          [-1.598511, 53.800651],
+          [-1.599205, 53.801827],
+          [-1.599118, 53.802273],
+          [-1.599437, 53.803895],
+          [-1.599437, 53.803895]
+        ]
+      ]
+    }
+
+    const cov = new RegionCoverer()
+    const union = cov.covering(polygon)
+    deepEqual(
+      [...union.map(cellid.toToken)],
+      ['48795eb9', '48795ec4', '48795ed04', '48795ed0c', '48795ed74', '48795edc', '48795ee7c', '48795ee84']
+    )
+  })
+})

geojson/loop.ts

@@ -20,6 +20,7 @@ export const marshal = (loop: Loop, ordinal: number): geojson.Position[] => {
 export const unmarshal = (ring: geojson.Position[], ordinal: number): Loop => {
   ring = ring.slice() // make a copy to avoid mutating input
   ring.length -= 1 // remove matching start/end points
+  ring = ring.filter((p, i) => !i || !position.equal(ring.at(i - 1)!, p, 0)) // remove equal+adjacent vertices
   if (ordinal > 0) ring.reverse() // ensure all rings are CCW
   return new Loop(ring.map(position.unmarshal))
 }

geojson/position.ts

@@ -19,3 +19,10 @@ export const marshal = (point: Point): geojson.Position => {
 export const unmarshal = (position: geojson.Position): Point => {
   return Point.fromLatLng(LatLng.fromDegrees(position[1], position[0]))
 }
+
+/**
+ * Returns true IFF the two positions are equal.
+ */
+export const equal = (a: geojson.Position, b: geojson.Position, epsilon = 0) => {
+  return Math.abs(a[0] - b[0]) <= epsilon && Math.abs(a[1] - b[1]) <= epsilon
+}

geojson/testing.ts

@@ -1,26 +1,23 @@
 import type * as geojson from 'geojson'
+import * as position from './position'
 
 // default distance threshold for approx equality
 const EPSILON = 1e-13
 
-export const approxEqualPosition = (a: geojson.Position, b: geojson.Position, epsilon = EPSILON) => {
-  return Math.abs(a[0] - b[0]) <= epsilon && Math.abs(a[1] - b[1]) <= epsilon
-}
-
 export const approxEqual = (a: geojson.Geometry, b: geojson.Geometry, epsilon = EPSILON) => {
   if (a?.type !== b?.type) return false
   switch (a.type) {
     case 'Point': {
       const aa = a as geojson.Point
       const bb = b as geojson.Point
-      return approxEqualPosition(aa.coordinates, bb.coordinates, epsilon)
+      return position.equal(aa.coordinates, bb.coordinates, epsilon)
     }
 
     case 'LineString': {
       const aa = a as geojson.LineString
       const bb = b as geojson.LineString
       if (aa.coordinates.length !== bb.coordinates.length) return false
-      return aa.coordinates.every((c, i) => approxEqualPosition(c, bb.coordinates[i], epsilon))
+      return aa.coordinates.every((c, i) => position.equal(c, bb.coordinates[i], epsilon))
     }
 
     case 'Polygon': {
@@ -29,7 +26,7 @@ export const approxEqual = (a: geojson.Geometry, b: geojson.Geometry, epsilon =
       if (aa.coordinates.length !== bb.coordinates.length) return false
       return aa.coordinates.every((r, ri) => {
         if (r.length !== bb.coordinates[ri].length) return false
-        return r.every((c, ci) => approxEqualPosition(c, bb.coordinates[ri][ci], epsilon))
+        return r.every((c, ci) => position.equal(c, bb.coordinates[ri][ci], epsilon))
       })
     }