Compute multiFeatureLocation in addition to location property in @turf/nearest-point-on-line

Author: EmilJunker

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

@@ -18,7 +18,7 @@ import { getCoord, getCoords } from "@turf/invariant";
  * @param {Geometry|Feature<Point>|number[]} pt point to snap from
  * @param {Object} [options={}] Optional parameters
  * @param {string} [options.units='kilometers'] can be degrees, radians, miles, or kilometers
- * @returns {Feature<Point>} closest point on the `line` to `point`. The properties object will contain four values: `index`: closest point was found on nth line part, `multiFeatureIndex`: closest point was found on the nth line of the `MultiLineString`, `dist`: distance between pt and the closest point, `location`: distance along the line between start and the closest point.
+ * @returns {Feature<Point>} closest point on the `line` to `point`. The properties object will contain four values: `index`: closest point was found on nth line part, `multiFeatureIndex`: closest point was found on the nth line of the `MultiLineString`, `dist`: distance between pt and the closest point, `location`: distance along the line between start and the closest point, `multiFeatureLocation`: distance along the line between start of the `MultiLineString` where closest point was found and the closest point.
  * @example
  * var line = turf.lineString([
  *     [-77.031669, 38.878605],
@@ -47,6 +47,7 @@ function nearestPointOnLine<G extends LineString | MultiLineString>(
     index: number;
     multiFeatureIndex: number;
     location: number;
+    multiFeatureLocation: number;
     [key: string]: any;
   }
 > {
@@ -58,18 +59,33 @@ function nearestPointOnLine<G extends LineString | MultiLineString>(
 
   let closestPt: Feature<
     Point,
-    { dist: number; index: number; multiFeatureIndex: number; location: number }
+    {
+      dist: number;
+      index: number;
+      multiFeatureIndex: number;
+      location: number;
+      multiFeatureLocation: number;
+    }
   > = point([Infinity, Infinity], {
     dist: Infinity,
     index: -1,
     multiFeatureIndex: -1,
     location: -1,
+    multiFeatureLocation: -1,
   });
 
   let length = 0.0;
+  let multiFeatureLength = 0.0;
+  let currentMultiFeatureIndex = -1;
   flattenEach(
     lines,
     function (line: any, _featureIndex: number, multiFeatureIndex: number) {
+      //reset multiFeatureLength at each changed multiFeatureIndex
+      if (currentMultiFeatureIndex !== multiFeatureIndex) {
+        currentMultiFeatureIndex = multiFeatureIndex;
+        multiFeatureLength = 0.0;
+      }
+
       const coords: any = getCoords(line);
 
       for (let i = 0; i < coords.length - 1; i++) {
@@ -107,15 +123,22 @@ function nearestPointOnLine<G extends LineString | MultiLineString>(
         let intersectPt:
           | Feature<
               Point,
-              { dist: number; multiFeatureIndex: number; location: number }
+              {
+                dist: number;
+                multiFeatureIndex: number;
+                location: number;
+                multiFeatureLocation: number;
+              }
             >
           | undefined;
 
         if (intersectPos) {
+          const lineLocationDist = distance(start, intersectPos, options);
           intersectPt = point(intersectPos, {
             dist: distance(pt, intersectPos, options),
             multiFeatureIndex: multiFeatureIndex,
-            location: length + distance(start, intersectPos, options),
+            location: length + lineLocationDist,
+            multiFeatureLocation: multiFeatureLength + lineLocationDist,
           });
         }
 
@@ -134,8 +157,9 @@ function nearestPointOnLine<G extends LineString | MultiLineString>(
           };
         }
 
-        // update length
+        // update length and multiFeatureLength
         length += sectionLength;
+        multiFeatureLength += sectionLength;
       }
     }
   );

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

@@ -398,6 +398,37 @@ test("turf-nearest-point-on-line -- multifeature index", (t) => {
   t.end();
 });
 
+test("turf-nearest-point-on-line -- issue 2753 multifeature location", (t) => {
+  const multiLine = multiLineString([
+    [
+      [-122.3125, 47.6632],
+      [-122.3102, 47.6646],
+    ],
+    [
+      [-122.3116, 47.6623],
+      [-122.3091, 47.6636],
+    ],
+  ]);
+
+  const ptA = point([-122.3106, 47.6638], { name: "A" });
+  const ptB = point([-122.3102, 47.6634], { name: "B" });
+
+  const nearestToA = nearestPointOnLine(multiLine, ptA, { units: "meters" });
+  const nearestToB = nearestPointOnLine(multiLine, ptB, { units: "meters" });
+
+  t.equal(
+    Number(nearestToA.properties.multiFeatureLocation.toFixed(6)),
+    150.293316,
+    "nearestToA multiFeatureLocation"
+  );
+  t.equal(
+    Number(nearestToB.properties.multiFeatureLocation.toFixed(6)),
+    157.736676,
+    "nearestToB multiFeatureLocation"
+  );
+  t.end();
+});
+
 test("turf-nearest-point-on-line -- issue 1514", (t) => {
   const pt = point([-40.01, 56]);
   const line = lineString([