Draw offline hiking trails sharing a path as parallel lanes

Several marked routes running down one path drew as a single line, only
the topmost color visible. maplibre-gl-lanes puts each route in its own
lane; OSM relations already share exact vertices, which is all it needs.
One LaneLayer across every picked country; click hit-testing and the
highlight move to queryLane/setHighlight.

public/js/hiking_trails.js

@@ -2,6 +2,7 @@ import { WALKING_ICON } from "/js/icons.js";
 import { firstSymbolLayerId } from "/js/radar.js";
 import { escapeHtml } from "/js/gpx.js";
 import { lineLength, formatDistance } from "/js/measure.js";
+import { buildLineGraph, orderLanesAsync, LaneLayer } from "/vendor/maplibre-gl-lanes/maplibre-gl-lanes.mjs";
 
 // Overpass has permissive CORS (verified: Access-Control-Allow-Origin: *),
 // so this queries it directly from the browser — no server-side proxy
@@ -40,22 +41,28 @@ const ATTRIBUTION =
   'Trails &copy; <a href="https://waymarkedtrails.org" target="_blank">Waymarked Trails</a> (CC-BY-SA), map data &copy; OpenStreetMap contributors';
 const OPACITY = 0.85;
 
-// Offline: one real vector line layer per country the viewer actually
-// picks, GeoJSON generated at build time by Maptool::HikingTrails straight
+// Offline: real vector trails for each country the viewer actually picks,
+// GeoJSON generated at build time by Maptool::HikingTrails straight
 // from the same Geofabrik .osm.pbf offline.pmtiles itself is built from
 // (see lib/maptool/hiking_trails.rb, PmtilesBuilder.extract_hiking_trails)
 // — no network needed, and no waiting on Overpass for click details either
-// (queried locally via queryRenderedFeatures instead, see _handleClick
+// (hit-tested locally via LaneLayer.queryLane instead, see _handleClick
 // below). One file per country rather than one combined file: a viewer
 // covering a dozen countries' worth of offline map data rarely wants all
 // of their hiking trails loaded at once, so this is opt-in per country
 // instead.
-function offlineSourceId(code) {
-  return `hiking-trails-offline-${code}`;
-}
-function offlineLayerId(code) {
-  return `hiking-trails-offline-${code}-line`;
-}
+//
+// Drawn with maplibre-gl-lanes rather than a plain line layer: a stretch
+// of path several marked routes share (very common with KST blazes) is
+// otherwise one line drawn over and over, only the topmost color visible.
+// Lanes puts each route in its own parallel lane instead. It needs routes
+// sharing a path to share exact vertices, which OSM relations over shared
+// ways already do. One LaneLayer over every picked country, not one per
+// country, so lanes at a border stay ordered together. Doesn't drape over
+// 3D terrain (sinks under raised ground, warns on the console) — a known
+// limitation of the library, not worth working around for an overlay.
+const LANES_LAYER_ID = "hiking-trails-lanes";
+const LANE_SIZES = () => ({ spacing: 3.5, width: 3, casingWidth: 0.5 });
 function offlineUrl(code) {
   return `/tiles/hiking_trails/${code}.geojson`;
 }
@@ -76,11 +83,10 @@ function normalizedTrailColor(raw) {
   return /^#[0-9a-fA-F]{3,8}$/.test(trimmed) ? trimmed : DEFAULT_TRAIL_COLOR;
 }
 
-// A highlight outline for whichever route(s) the last click matched — drawn
-// as its own separate source/layer (real geometry, from either the offline
-// vector feature or the online Overpass response) rather than a paint-
-// property change on the base layer, since the online raster tiles have no
-// paintable features to highlight at all. Same "yellow outline on a picked
+// Online mode's highlight outline for whichever route(s) the last click
+// matched — drawn as its own separate source/layer (real geometry, from the
+// Overpass response), since the raster tiles have no paintable features to
+// highlight at all. Offline lanes use LaneLayer.setHighlight instead. Same "yellow outline on a picked
 // route" treatment the earlier SVG prototype visualization used.
 const HIGHLIGHT_SOURCE_ID = "hiking-trails-highlight";
 const HIGHLIGHT_LAYER_ID = "hiking-trails-highlight-line";
@@ -97,6 +103,9 @@ export class HikingTrailsControl {
     this._active = false; // the online-mode checkbox's own state
     this._offlineCountries = []; // codes with an extract available on the server
     this._selectedCountries = new Set(); // codes currently shown, offline mode
+    this._trailData = new Map(); // code -> fetched GeoJSON, kept across toggles/style swaps
+    this._routeInfo = new Map(); // String(relation id) -> feature properties, for the panel
+    this._lanes = null; // the current LaneLayer, null when no country is shown
 
     map.on("click", (event) => this._handleClick(event));
 
@@ -111,8 +120,8 @@ export class HikingTrailsControl {
     // here than try to carry it across a style swap.
     map.on("style.load", () => {
       if (this._active) this._addOnlineLayer();
-      for (const code of this._selectedCountries) this._addOfflineLayer(code);
       this._clearHighlight();
+      if (this._selectedCountries.size > 0) this._rebuildLanes();
     });
   }
 
@@ -177,14 +186,56 @@ export class HikingTrailsControl {
     else this._removeOnlineLayer();
   }
 
-  _toggleCountry(code, active) {
-    if (active) {
-      this._selectedCountries.add(code);
-      this._addOfflineLayer(code);
-    } else {
+  async _toggleCountry(code, active) {
+    if (!active) {
       this._selectedCountries.delete(code);
-      this._removeOfflineLayer(code);
+      this._rebuildLanes();
+      return;
     }
+    this._selectedCountries.add(code);
+    if (!this._trailData.has(code)) {
+      let geojson;
+      try {
+        geojson = await (await fetch(offlineUrl(code))).json();
+      } catch {
+        return; // degrade quietly, same story as every other optional layer here
+      }
+      // Normalized once here so an untrustworthy `colour` tag value never
+      // reaches the layer's color directly — same reasoning gpx.js's own
+      // feature.properties.color ||= TRACK_COLOR normalization gives.
+      for (const feature of geojson.features) {
+        feature.properties.color = normalizedTrailColor(feature.properties.colour);
+      }
+      this._trailData.set(code, geojson);
+      if (!this._selectedCountries.has(code)) return; // toggled back off while fetching
+    }
+    this._rebuildLanes();
+  }
+
+  // Rebuilt from scratch on every country toggle and style swap (setStyle
+  // drops custom layers too) — lane order depends on every route drawn, so
+  // there's no cheaper incremental path. Ordering runs in a worker; a newer
+  // rebuild started meanwhile wins, same token guard as _handleOnlineClick.
+  async _rebuildLanes() {
+    const build = (this._lanesBuild = {});
+    const features = [...this._selectedCountries].flatMap((code) => this._trailData.get(code)?.features || []);
+    let graph = null;
+    if (features.length > 0) {
+      graph = buildLineGraph(features, { routeProperty: "id" });
+      await orderLanesAsync(graph);
+    }
+    if (this._lanesBuild !== build) return;
+
+    this._hidePanel();
+    if (this._map.getLayer(LANES_LAYER_ID)) this._map.removeLayer(LANES_LAYER_ID);
+    this._lanes = null;
+    this._routeInfo = new Map(features.map((feature) => [String(feature.properties.id), feature.properties]));
+    if (!graph) return;
+
+    this._lanes = new LaneLayer({ id: LANES_LAYER_ID, graph, sizes: LANE_SIZES, casingColor: "rgba(255, 255, 255, 0.8)" });
+    // Below labels (same firstSymbolLayerId spot contour.js/radar.js use),
+    // so place names/POI icons stay readable on top of the trail lines.
+    this._map.addLayer(this._lanes, firstSymbolLayerId(this._map));
   }
 
   _handleClick(event) {
@@ -192,24 +243,20 @@ export class HikingTrailsControl {
     else if (this._active) this._handleOnlineClick(event);
   }
 
-  // The local vector layers have real features to hit-test directly — no
-  // network round-trip, so this resolves instantly. A genuine miss (empty
-  // result) just closes an already-open panel rather than showing a "not
+  // The lanes have real geometry to hit-test directly — no network
+  // round-trip, so this resolves instantly. Each route has its own lane, so
+  // a click picks exactly one, highlighted by the layer itself. A genuine
+  // miss just closes an already-open panel rather than showing a "not
   // found" message, same as clicking empty space anywhere else on the map.
   _handleOfflineClick(event) {
-    const layers = [...this._selectedCountries].map(offlineLayerId).filter((id) => this._map.getLayer(id));
-    if (layers.length === 0) return;
-
-    const features = this._map.queryRenderedFeatures(event.point, { layers });
-    if (features.length === 0) {
+    const hit = this._lanes?.queryLane(event.point, 8);
+    const info = hit && this._routeInfo.get(String(hit.route));
+    if (!info) {
       this._hidePanel();
       return;
     }
-
-    const matched = features.filter((feature, i) => features.findIndex((f) => f.properties.id === feature.properties.id) === i);
-    const html = matched.map((feature) => routeHtml(feature.properties)).join("<hr>");
-    this._setHighlight(matched.map((feature) => ({ geometry: feature.geometry, color: feature.properties.color })));
-    this._showPanel(html);
+    this._lanes.setHighlight(hit.route);
+    this._showPanel(routeHtml(info));
   }
 
   _showPanel(html) {
@@ -277,44 +324,6 @@ export class HikingTrailsControl {
     if (this._map.getSource(ONLINE_SOURCE_ID)) this._map.removeSource(ONLINE_SOURCE_ID);
   }
 
-  async _addOfflineLayer(code) {
-    if (this._map.getSource(offlineSourceId(code))) return;
-
-    let geojson;
-    try {
-      geojson = await (await fetch(offlineUrl(code))).json();
-    } catch {
-      return; // degrade quietly, same story as every other optional layer here
-    }
-    // Toggled back off, or the base map switched, while this was fetching.
-    if (!this._selectedCountries.has(code) || this._map.getSource(offlineSourceId(code))) return;
-
-    // Normalized once here (rather than a MapLibre expression in the paint
-    // property below) so an untrustworthy `colour` tag value never reaches
-    // a style property directly — same reasoning gpx.js's own
-    // feature.properties.color ||= TRACK_COLOR normalization gives.
-    for (const feature of geojson.features) {
-      feature.properties.color = normalizedTrailColor(feature.properties.colour);
-    }
-
-    this._map.addSource(offlineSourceId(code), { type: "geojson", data: geojson });
-    this._map.addLayer(
-      {
-        id: offlineLayerId(code),
-        type: "line",
-        source: offlineSourceId(code),
-        layout: { "line-cap": "round", "line-join": "round" },
-        paint: { "line-color": ["get", "color"], "line-width": 2 },
-      },
-      firstSymbolLayerId(this._map),
-    );
-  }
-
-  _removeOfflineLayer(code) {
-    if (this._map.getLayer(offlineLayerId(code))) this._map.removeLayer(offlineLayerId(code));
-    if (this._map.getSource(offlineSourceId(code))) this._map.removeSource(offlineSourceId(code));
-  }
-
   // features: an array of { geometry, color } — geometry a GeoJSON
   // LineString or MultiLineString (mixed is fine), color the trail's own
   // (already-normalized) colour — everything the last click matched, drawn
@@ -342,6 +351,7 @@ export class HikingTrailsControl {
   }
 
   _clearHighlight() {
+    this._lanes?.setHighlight(null);
     if (this._map.getLayer(HIGHLIGHT_LAYER_ID)) this._map.removeLayer(HIGHLIGHT_LAYER_ID);
     if (this._map.getSource(HIGHLIGHT_SOURCE_ID)) this._map.removeSource(HIGHLIGHT_SOURCE_ID);
   }