fix(traffic): retain line identity across clipping

src/layers/traffic/flowSource.js

@@ -20,6 +20,19 @@ function tileFreshness(response) {
   };
 }
 
+/**
+ * Clip a flow line to a box. Every piece keeps its uncut line as `line`, so
+ * pieces of one line that doubles back are never paired with each other.
+ */
+function clipFlowSegment(segment, bbox) {
+  const line = segment.line || segment;
+  return clipTileLine(segment.coords, bbox).map((coords) => ({
+    ...segment,
+    coords,
+    line,
+  }));
+}
+
 /** Own one bounded decoded flow cache; every request still passes the server tile budget. */
 export function createFlowTileSource({
   fetchImpl = (...args) => globalThis.fetch(...args),
@@ -41,12 +54,7 @@ export function createFlowTileSource({
       entry?.release();
       const segments = decodeFlowTile(bytes, z, x, y, { strict: true })
         .filter(isDrivableFlowSegment)
-        .flatMap((segment) =>
-          clipTileLine(segment.coords, tileToBBox(z, x, y)).map((coords) => ({
-            ...segment,
-            coords,
-          })),
-        );
+        .flatMap((segment) => clipFlowSegment(segment, tileToBBox(z, x, y)));
       if (mark) freshness.set(segments, mark);
       return segments;
     },
@@ -87,12 +95,9 @@ export function createFlowTileSource({
     partial = result.partial;
     const segments = bounds.coverage
       ? result.tiles.flat()
-      : result.tiles.flat().flatMap((segment) =>
-          clipTileLine(segment.coords, bounds).map((coords) => ({
-            ...segment,
-            coords,
-          })),
-        );
+      : result.tiles
+          .flat()
+          .flatMap((segment) => clipFlowSegment(segment, bounds));
     const marks = result.tiles.map((tile) => freshness.get(tile) || {});
     const times = marks
       .map((mark) => mark.fetchedAt)

src/layers/traffic/roadModes.js

@@ -112,6 +112,7 @@ function pairedSampleShare(segment, index) {
   }
   const length = cum[cum.length - 1];
   if (!(length > 0)) return 0;
+  const line = segment.line || segment;
   let paired = 0;
   let i = 1;
   for (const share of PAIRED_SIDE_SAMPLES) {
@@ -126,8 +127,10 @@ function pairedSampleShare(segment, index) {
         xs[i - 1] + dx * t,
         ys[i - 1] + dy * t,
         reverse,
-        // A line that doubles back beside itself is not its own pair.
-        (piece) => (piece.flow === segment ? 0 : PAIRED_SIDE_RADIUS_M),
+        // A line that doubles back beside itself, even clipped into pieces,
+        // is not its own pair.
+        (piece) =>
+          (piece.flow.line || piece.flow) === line ? 0 : PAIRED_SIDE_RADIUS_M,
       ).best
     )
       paired++;

src/layers/traffic/roadModes.test.mjs

@@ -1,10 +1,12 @@
 import test from 'node:test';
 import assert from 'node:assert/strict';
+import { PbfWriter } from 'pbf';
 import {
   flowSegmentsToRoads,
   resolveRoadMode,
   selectTrafficRoads,
 } from './roadModes.js';
+import { createFlowTileSource } from './flowSource.js';
 import { createTrafficSource } from './source.js';
 import { createTrafficLayer } from './index.js';
 import {
@@ -14,6 +16,7 @@ import {
   serializeStoredLayerState,
   parseStoredLayerState,
 } from '../../data/layerState.js';
+import { tileToBBox } from '../../data/tomtomTiles.js';
 
 const coordinates = [
   [-97.744, 30.267],
@@ -171,6 +174,78 @@ test('a TomTom side that doubles back beside itself is not its own pair', () =>
   };
   assert.equal(flowSegmentsToRoads([hairpin])[0].densityWeight ?? 1, 1);
 });
+/** A flow tile of one-way Major roads; each line is [x, y, x, y, ...]. */
+function flowTile(lines) {
+  const writer = new PbfWriter();
+  writer.writeMessage(3, writeFlowLayer, lines);
+  return writer.finish();
+}
+function writeFlowLayer(lines, layer) {
+  layer.writeVarintField(15, 2);
+  layer.writeStringField(1, 'Traffic flow');
+  layer.writeVarintField(5, 4096);
+  layer.writeStringField(3, 'traffic_level');
+  layer.writeMessage(4, (_, value) => value.writeDoubleField(3, 0.6), null);
+  layer.writeStringField(3, 'road_type');
+  layer.writeMessage(
+    4,
+    (_, value) => value.writeStringField(1, 'Major road'),
+    null,
+  );
+  layer.writeStringField(3, 'traffic_road_coverage');
+  layer.writeMessage(
+    4,
+    (_, value) => value.writeStringField(1, 'one_side'),
+    null,
+  );
+  for (const xy of lines) {
+    // MoveTo one point, LineTo the rest, as zigzag deltas.
+    const geometry = [9];
+    for (let i = 0; i < xy.length; i++) {
+      if (i === 2) geometry.push(2 | ((xy.length / 2 - 1) << 3));
+      const delta = xy[i] - (i < 2 ? 0 : xy[i - 2]);
+      geometry.push((delta << 1) ^ (delta >> 31));
+    }
+    layer.writeMessage(
+      2,
+      (_, feature) => {
+        feature.writeVarintField(3, 2);
+        feature.writePackedVarint(2, [0, 0, 1, 1, 2, 2]);
+        feature.writePackedVarint(4, geometry);
+      },
+      null,
+    );
+  }
+}
+test('TomTom sides clipped by their tile and the view pair only with other lines', async () => {
+  // A one-way hairpin turning in the tile buffer, its arms one tile unit
+  // (about 2 m) apart, and a separate opposite pair. The tile clip cuts the
+  // hairpin into two arms; the view clip then cuts every line again.
+  const lines = [
+    [500, 1000, 4300, 1000, 4300, 1001, 500, 1001],
+    [-200, 3000, 4300, 3000],
+    [4300, 3001, -200, 3001],
+  ];
+  const source = createFlowTileSource({
+    fetchImpl: async () => new Response(flowTile(lines)),
+  });
+  const tile = tileToBBox(12, 935, 1686);
+  const west = tile.west + (tile.east - tile.west) * 0.2;
+  const { segments } = await source.fetchFlowDetail({
+    ...tile,
+    west,
+    east: tile.east - 1e-9,
+    south: tile.south + 1e-9,
+  });
+  assert.equal(segments.length, 4);
+  assert.ok(
+    segments.every((s) => s.coords.every(([lon]) => lon >= west - 1e-10)),
+  );
+  assert.deepEqual(
+    flowSegmentsToRoads(segments).map((road) => road.densityWeight ?? 1),
+    [1, 1, 0.5, 0.5],
+  );
+});
 test('bearing and distance reject nearby crossing and separate OSM roads', () => {
   const parallel = {
     ...main,