Handle input geom with infinitely thin peninsula

Handle the situation where one of the input polygons has an infinitely
thin 'peninsula' just before its last coordinate.

Fixes #85

src/clean-input.js

@@ -116,22 +116,23 @@ export const cleanMultiPoly = multipoly => {
  * WARN: input modified directly */
 export const cleanRing = ring => {
   if (ring.length === 0) return
-  const firstPt = ring[0]
-  const lastPt = ring[ring.length - 1]
-  if (firstPt.x === lastPt.x && firstPt.y === lastPt.y) ring.pop()
-
-  const isPointUncessary = (prevPt, pt, nextPt) =>
-    (prevPt.x === pt.x && prevPt.y === pt.y) ||
-    (nextPt.x === pt.x && nextPt.y === pt.y) ||
-    compareVectorAngles(pt, prevPt, nextPt) === 0
 
   let i = 0
-  let prevPt, nextPt
+  let pt, prevPt, nextPt
   while (i < ring.length) {
+    pt = ring[i]
     prevPt = (i === 0 ? ring[ring.length - 1] : ring[i - 1])
+    if (pt.x === prevPt.x && pt.y === prevPt.y) {
+      ring.splice(i - 1, 1)
+      continue
+    }
     nextPt = (i === ring.length - 1 ? ring[0] : ring[i + 1])
-    if (isPointUncessary(prevPt, ring[i], nextPt)) ring.splice(i, 1)
-    else i++
+    if (compareVectorAngles(pt, prevPt, nextPt) === 0) {
+      ring.splice(i, 1)
+      if (i > 0) i--
+      continue
+    }
+    i++
   }
 
   // if our ring has less than 3 distinct points now (so is degenerate)

test/clean-input.test.js

@@ -257,6 +257,41 @@ describe('cleanRing()', () => {
     cleanRing(ringBad)
     expect(ringBad).toEqual([])
   })
+
+  test('removes last two when colinear', () => {
+    const ringBad = [
+      { x: 0, y: 0 },
+      { x: 1, y: 0 },
+      { x: 1, y: 1 },
+      { x: 0, y: 0 },
+      { x: 0, y: 1 },
+      { x: 0, y: 0 },
+    ]
+    const ringGood = [
+      { x: 0, y: 0 },
+      { x: 1, y: 0 },
+      { x: 1, y: 1 },
+    ]
+    cleanRing(ringBad)
+    expect(ringBad).toEqual(ringGood)
+  })
+
+  test('removes last two when repeats', () => {
+    const ringBad = [
+      { x: 0, y: 0 },
+      { x: 1, y: 0 },
+      { x: 1, y: 1 },
+      { x: 0, y: 0 },
+      { x: 0, y: 0 },
+    ]
+    const ringGood = [
+      { x: 0, y: 0 },
+      { x: 1, y: 0 },
+      { x: 1, y: 1 },
+    ]
+    cleanRing(ringBad)
+    expect(ringBad).toEqual(ringGood)
+  })
 })
 
 describe('pointsAsObjects()', () => {

test/end-to-end/issue-85/args.geojson

@@ -0,0 +1,24 @@
+{
+  "type": "FeatureCollection",
+  "features": [
+    {
+      "type": "Feature",
+      "properties": null,
+      "geometry": {
+        "type": "MultiPolygon",
+        "coordinates": [
+          [
+            [
+              [0.11978, 18.32302],
+              [0.14685, 17.90202],
+              [0.15809, 18.11877],
+              [0.11978, 18.32302],
+              [0.11978, 18.32306],
+              [0.11978, 18.32302]
+            ]
+          ]
+        ]
+      }
+    }
+  ]
+}

test/end-to-end/issue-85/union.geojson

@@ -0,0 +1,17 @@
+{
+  "type": "Feature",
+  "properties": {},
+  "geometry": {
+    "type": "MultiPolygon",
+    "coordinates": [
+      [
+        [
+          [0.11978, 18.32302],
+          [0.14685, 17.90202],
+          [0.15809, 18.11877],
+          [0.11978, 18.32302]
+        ]
+      ]
+    ]
+  }
+}