From cb6c8eed284a67272bee36dacc039506cc515dc7 Mon Sep 17 00:00:00 2001
From: Peter Johnson <insomnia@rcpt.at>
Date: Fri, 20 Sep 2024 12:52:48 +0200
Subject: [PATCH] fix(geojson): detect invalid polygon loops

---
 geojson/RegionCoverer_test.ts | 47 +++++++++++++++++++++++++++++++++++
 geojson/loop.ts               | 25 ++++++++++++++++++-
 2 files changed, 71 insertions(+), 1 deletion(-)

diff --git a/geojson/RegionCoverer_test.ts b/geojson/RegionCoverer_test.ts
index 73969cf..6be3c30 100644
--- a/geojson/RegionCoverer_test.ts
+++ b/geojson/RegionCoverer_test.ts
@@ -63,4 +63,51 @@ describe('RegionCoverer', () => {
       ['48795eb9', '48795ec4', '48795ed04', '48795ed0c', '48795ed74', '48795edc', '48795ee7c', '48795ee84']
     )
   })
+
+  test('polygon - twisted + contains duplicate vertices', (t) => {
+    const mpolygon: geojson.MultiPolygon = {
+      type: 'MultiPolygon',
+      coordinates: [
+        [
+          [
+            [-122.420357, 37.651333],
+            [-122.42047100000001, 37.652073],
+            [-122.421204, 37.651173],
+            [-122.42038700000001, 37.651276],
+            [-122.405418, 37.634312],
+            [-122.400352, 37.634029],
+            [-122.39450100000001, 37.632862],
+            [-122.388313, 37.633675],
+            [-122.37602200000001, 37.631088],
+            [-122.362549, 37.638908],
+            [-122.360283, 37.634068],
+            [-122.35611, 37.614025],
+            [-122.353706, 37.612396],
+            [-122.351601, 37.61134],
+            [-122.349411, 37.610287],
+            [-122.345123, 37.607704],
+            [-122.34137699999999, 37.590256],
+            [-122.359131, 37.585941],
+            [-122.36784400000001, 37.600216],
+            [-122.37653400000001, 37.605061],
+            [-122.380539, 37.607029],
+            [-122.383797, 37.607666],
+            [-122.395447, 37.60276],
+            [-122.401848, 37.605137],
+            [-122.404831, 37.611164],
+            [-122.405632, 37.613293],
+            [-122.40589900000001, 37.614941],
+            [-122.40582999999999, 37.615002],
+            [-122.40129899999999, 37.625465],
+            [-122.405418, 37.634312],
+            [-122.420357, 37.651333]
+          ]
+        ]
+      ]
+    }
+
+    const cov = new RegionCoverer()
+    const union = cov.covering(mpolygon)
+    deepEqual([...union.map(cellid.toToken)], []) // cannot be fixed, return []
+  })
 })
diff --git a/geojson/loop.ts b/geojson/loop.ts
index 82a5ea5..88b50b9 100644
--- a/geojson/loop.ts
+++ b/geojson/loop.ts
@@ -20,7 +20,30 @@ 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
+
+  // Loops are not allowed to have any duplicate vertices (whether adjacent or not)
+  if (containsDuplicateVertices(ring)) {
+    // adjacent duplicates are fixable
+    ring = removeAdjacentDuplicateVertices(ring)
+
+    // non-adjacent duplicates are not fixable
+    if (containsDuplicateVertices(ring)) return new Loop([])
+  }
+
   return new Loop(ring.map(position.unmarshal))
 }
+
+/**
+ * Removes *adjacent* duplicate (and near-duplicate) vertices from ring.
+ */
+export const removeAdjacentDuplicateVertices = (ring: geojson.Position[], epsilon = 1e-8): geojson.Position[] => {
+  return ring.filter((p, i) => !i || !position.equal(ring.at(i - 1)!, p, epsilon))
+}
+
+/**
+ * Returns true IFF ring contains duplicate vertices at any position.
+ */
+export const containsDuplicateVertices = (ring: geojson.Position[]): boolean => {
+  return ring.some((A, i) => ring.slice(i + 1).some((B) => position.equal(A, B)))
+}