fix(traffic): keep a TomTom side from pairing with its own pieces

A one_side line that doubles back within 5 m of itself, such as a
turnaround drawn as one line, matched its own return leg and took a
two-way half share with no partner. Flow index pieces now carry their
source line, and a radiusFor of 0 leaves a piece out of nearest(), so
pairing skips the line's own pieces.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/data/flowMatch.js

@@ -139,6 +139,7 @@ export function createFlowSegmentIndex(flowSegments) {
           roadType,
           level,
           closure,
+          flow,
         };
         const key = cellKey(
           Math.floor((seg.ax + seg.bx) / 2 / CELL_SIZE_M),
@@ -158,7 +159,8 @@ export function createFlowSegmentIndex(flowSegments) {
   const radius2 = MATCH_RADIUS_M * MATCH_RADIUS_M;
   /**
    * Nearest direction-compatible segment within 35 m of a projected point.
-   * `radiusFor(segment)` may tighten the radius per segment (never widen it).
+   * `radiusFor(segment)` may tighten the radius per segment (never widen it);
+   * 0 leaves the segment out.
    */
   function nearest(px, py, bearing, radiusFor = null) {
     const cx = Math.floor(px / CELL_SIZE_M);
@@ -177,7 +179,7 @@ export function createFlowSegmentIndex(flowSegments) {
           candidate = true; // within radius, bearing not yet checked
           if (radiusFor) {
             const radius = Math.min(MATCH_RADIUS_M, radiusFor(seg));
-            if (d2 > radius * radius) continue;
+            if (!(radius > 0) || d2 > radius * radius) continue;
           }
           const diff = bearingDiffDeg(seg.bearing, bearing);
           if (

src/layers/traffic/roadModes.js

@@ -126,7 +126,8 @@ function pairedSampleShare(segment, index) {
         xs[i - 1] + dx * t,
         ys[i - 1] + dy * t,
         reverse,
-        () => PAIRED_SIDE_RADIUS_M,
+        // A line that doubles back beside itself is not its own pair.
+        (piece) => (piece.flow === segment ? 0 : PAIRED_SIDE_RADIUS_M),
       ).best
     )
       paired++;

src/layers/traffic/roadModes.test.mjs

@@ -163,6 +163,14 @@ test('a TomTom side paired along only part of its length keeps its share', () =>
   assert.equal(weightOfFlow(partner(-97.7434)), 1);
   assert.equal(weightOfFlow(partner(-97.7428)), 0.5);
 });
+test('a TomTom side that doubles back beside itself is not its own pair', () => {
+  // East along `coordinates`, then back west about 1 m north.
+  const hairpin = {
+    ...flow,
+    coords: [...coordinates, [-97.742, 30.26701], [-97.744, 30.26701]],
+  };
+  assert.equal(flowSegmentsToRoads([hairpin])[0].densityWeight ?? 1, 1);
+});
 test('bearing and distance reject nearby crossing and separate OSM roads', () => {
   const parallel = {
     ...main,