Reverted backward incompatible fix to nearestPointOnLine introduced by PR #2951, while still keeping lineSlice issue #2946 resolved with a temporary workaround.

Author: smallsaucepan

packages/turf-line-slice/index.ts

@@ -1,7 +1,7 @@
 import { getCoords, getType } from "@turf/invariant";
 import { Coord, lineString as linestring } from "@turf/helpers";
 import { nearestPointOnLine } from "@turf/nearest-point-on-line";
-import { Feature, LineString } from "geojson";
+import type { Feature, LineString, Point } from "geojson";
 
 /**
  * Takes a {@link LineString|line}, a start {@link Point}, and a stop point
@@ -44,14 +44,20 @@ function lineSlice(
 
   const startVertex = nearestPointOnLine(line, startPt);
   const stopVertex = nearestPointOnLine(line, stopPt);
+
+  // Workaround until we can fix backwards incompatible nearestPointOnLine bug
+  // #3016
+  fixSegmentIndexBounds(line, startVertex);
+  fixSegmentIndexBounds(line, stopVertex);
+
   const ends =
-    startVertex.properties.index <= stopVertex.properties.index
+    startVertex.properties.segmentIndex <= stopVertex.properties.segmentIndex
       ? [startVertex, stopVertex]
       : [stopVertex, startVertex];
   const clipCoords = [ends[0].geometry.coordinates];
   for (
-    let i = ends[0].properties.index + 1;
-    i < ends[1].properties.index + 1;
+    let i = ends[0].properties.segmentIndex + 1;
+    i < ends[1].properties.segmentIndex + 1;
     i++
   ) {
     clipCoords.push(coords[i]);
@@ -60,5 +66,23 @@ function lineSlice(
   return linestring(clipCoords, line.type === "Feature" ? line.properties : {});
 }
 
+function fixSegmentIndexBounds(
+  line: Feature<LineString> | LineString,
+  vertex: Feature<Point, { segmentIndex: number }>
+) {
+  // There is a bug in nearestPointOnLine where the returned segmentIndex can
+  // refer to a non-existent segment (overflows). Until we can fix that bug
+  // (see https://github.com/Turfjs/turf/issues/3016) we adjust the
+  // segmentIndex here to make sure it's in range.
+
+  let geometry: LineString = line.type === "Feature" ? line.geometry : line;
+
+  if (vertex.properties.segmentIndex >= geometry.coordinates.length - 1) {
+    // segmentIndex refers to a non-existent segment beyond the end of the
+    // lineString. Override it.
+    vertex.properties.segmentIndex = geometry.coordinates.length - 2;
+  }
+}
+
 export { lineSlice };
 export default lineSlice;

packages/turf-line-slice/test/in/avoid-duplicated-end-points.geojson

@@ -0,0 +1,33 @@
+{
+  "type": "FeatureCollection",
+  "features": [
+    {
+      "type": "Feature",
+      "properties": {},
+      "geometry": {
+        "type": "LineString",
+        "coordinates": [
+          [3, 0],
+          [2, -1],
+          [2, 2]
+        ]
+      }
+    },
+    {
+      "type": "Feature",
+      "properties": {},
+      "geometry": {
+        "type": "Point",
+        "coordinates": [2, 0]
+      }
+    },
+    {
+      "type": "Feature",
+      "properties": {},
+      "geometry": {
+        "type": "Point",
+        "coordinates": [2, 2]
+      }
+    }
+  ]
+}

packages/turf-line-slice/test/out/avoid-duplicated-end-points.geojson

@@ -0,0 +1,47 @@
+{
+  "type": "FeatureCollection",
+  "features": [
+    {
+      "type": "Feature",
+      "properties": {},
+      "geometry": {
+        "type": "LineString",
+        "coordinates": [
+          [3, 0],
+          [2, -1],
+          [2, 2]
+        ]
+      }
+    },
+    {
+      "type": "Feature",
+      "properties": {},
+      "geometry": {
+        "type": "Point",
+        "coordinates": [2, 0]
+      }
+    },
+    {
+      "type": "Feature",
+      "properties": {},
+      "geometry": {
+        "type": "Point",
+        "coordinates": [2, 2]
+      }
+    },
+    {
+      "type": "Feature",
+      "properties": {
+        "stroke": "#f0f",
+        "stroke-width": 6
+      },
+      "geometry": {
+        "type": "LineString",
+        "coordinates": [
+          [2, 0],
+          [2, 2]
+        ]
+      }
+    }
+  ]
+}

packages/turf-nearest-point-on-line/index.ts

@@ -106,7 +106,6 @@ function nearestPointOnLine<G extends LineString | MultiLineString>(
       }
 
       const coords: any = getCoords(line);
-      const maxSegmentIndex = coords.length - 2;
 
       for (let i = 0; i < coords.length - 1; i++) {
         //start - start of current line section
@@ -144,11 +143,9 @@ function nearestPointOnLine<G extends LineString | MultiLineString>(
           const segmentDistance = distance(start, intersectPos, options);
           closestPt = point(intersectPos, {
             lineStringIndex: lineStringIndex,
-            // Legacy behaviour where index progresses to next segment if we
-            // went with the end point this iteration. Though make sure we
-            // only progress to the beginning of the next segment if one
-            // actually exists.
-            segmentIndex: wasEnd && i + 1 <= maxSegmentIndex ? i + 1 : i,
+            // Legacy behaviour where index progresses to next segment # if we
+            // went with the end point this iteration.
+            segmentIndex: wasEnd ? i + 1 : i,
             totalDistance: totalDistance + segmentDistance,
             lineDistance: lineDistance + segmentDistance,
             segmentDistance: segmentDistance,

packages/turf-nearest-point-on-line/test.ts

@@ -333,7 +333,7 @@ test("turf-nearest-point-on-line - segmentIndex and pointDistance", (t) => {
   const pt = point([-92.110576, 41.040649]);
   const snapped = truncate(nearestPointOnLine(line, pt));
 
-  t.equal(snapped.properties.segmentIndex, 7, "properties.segmentIndex");
+  t.equal(snapped.properties.segmentIndex, 8, "properties.segmentIndex");
   t.equal(
     Number(snapped.properties.pointDistance.toFixed(6)),
     0.823802,

packages/turf-nearest-point-on-line/test/in/end-point-1.geojson

@@ -68,15 +68,15 @@
       "type": "Feature",
       "properties": {
         "lineStringIndex": 1,
-        "segmentIndex": 20,
+        "segmentIndex": 21,
         "totalDistance": 9479.011715,
         "lineDistance": 4800.716022,
         "segmentDistance": 173.221741,
         "pointDistance": 114.725451,
         "dist": 114.725451,
         "multiFeatureIndex": 1,
         "location": 9479.011715,
-        "index": 20,
+        "index": 21,
         "marker-color": "#F0F"
       },
       "geometry": {