fix(@turf/polygon-tangents): return distinct tangents when nearest vertex is below viewpoint inside bbox

packages/turf-polygon-tangents/index.ts

@@ -75,6 +75,26 @@ function polygonTangents<T extends Polygon | MultiPolygon>(
         rtan,
         ltan
       );
+      // When the nearest-vertex initialisation sets ltan === rtan (both below
+      // the viewpoint), processPolygon may fail to find a distinct left tangent
+      // because every above-viewpoint candidate is considered "above" the
+      // current ltan and therefore not chosen.  Detect this degenerate result
+      // and retry with ltan reset to vertex[0] so the traversal can pick the
+      // correct left tangent.  See https://github.com/Turfjs/turf/issues/2898
+      if (
+        nearestPtIndex !== 0 &&
+        rtan[0] === ltan[0] &&
+        rtan[1] === ltan[1]
+      ) {
+        ltan = polyCoords[0][0];
+        [rtan, ltan] = processPolygon(
+          polyCoords[0],
+          pointCoords,
+          eprev,
+          rtan,
+          ltan
+        );
+      }
       break;
     case "MultiPolygon":
       var closestFeature = 0;

packages/turf-polygon-tangents/test.ts

@@ -109,3 +109,24 @@ test("turf-polygon-tangents - Issue #1050", (t) => {
   }
   t.end();
 });
+
+test("turf-polygon-tangents - Issue #2898", (t) => {
+  // When the viewpoint is inside the polygon bbox and the nearest vertex is
+  // below the viewpoint, polygonTangents previously returned the same point
+  // for both tangents instead of two distinct tangent vertices.
+  const pt = point([-79.41285676230808, 43.627309605975235]);
+  const poly = polygon([
+    [
+      [-79.428776, 43.708224],
+      [-79.325734, 43.675502],
+      [-79.414155, 43.595383],
+      [-79.353931, 43.670969],
+      [-79.428776, 43.708224],
+    ],
+  ]);
+  const tangents = polygonTangents(pt, poly);
+  t.equal(tangents.features.length, 2, "returns two features");
+  const [t1, t2] = tangents.features.map((f) => f.geometry.coordinates);
+  t.notDeepEqual(t1, t2, "tangent points must be distinct vertices");
+  t.end();
+});

packages/turf-polygon-tangents/test/in/issue#2898.geojson

@@ -0,0 +1,29 @@
+{
+  "type": "FeatureCollection",
+  "features": [
+    {
+      "type": "Feature",
+      "properties": {},
+      "geometry": {
+        "type": "Polygon",
+        "coordinates": [
+          [
+            [-79.428776, 43.708224],
+            [-79.325734, 43.675502],
+            [-79.414155, 43.595383],
+            [-79.353931, 43.670969],
+            [-79.428776, 43.708224]
+          ]
+        ]
+      }
+    },
+    {
+      "type": "Feature",
+      "properties": {},
+      "geometry": {
+        "type": "Point",
+        "coordinates": [-79.41285676230808, 43.627309605975235]
+      }
+    }
+  ]
+}

packages/turf-polygon-tangents/test/out/issue#2898.geojson

@@ -0,0 +1,69 @@
+{
+  "type": "FeatureCollection",
+  "features": [
+    {
+      "type": "Feature",
+      "properties": {},
+      "geometry": {
+        "type": "Point",
+        "coordinates": [
+          -79.414155,
+          43.595383
+        ]
+      }
+    },
+    {
+      "type": "Feature",
+      "properties": {},
+      "geometry": {
+        "type": "Point",
+        "coordinates": [
+          -79.428776,
+          43.708224
+        ]
+      }
+    },
+    {
+      "type": "Feature",
+      "properties": {},
+      "geometry": {
+        "type": "Polygon",
+        "coordinates": [
+          [
+            [
+              -79.428776,
+              43.708224
+            ],
+            [
+              -79.325734,
+              43.675502
+            ],
+            [
+              -79.414155,
+              43.595383
+            ],
+            [
+              -79.353931,
+              43.670969
+            ],
+            [
+              -79.428776,
+              43.708224
+            ]
+          ]
+        ]
+      }
+    },
+    {
+      "type": "Feature",
+      "properties": {},
+      "geometry": {
+        "type": "Point",
+        "coordinates": [
+          -79.41285676230808,
+          43.627309605975235
+        ]
+      }
+    }
+  ]
+}