Add web frontend and refactor core to use OsmError

- Refactor core.py: replace sys.exit() calls with OsmError exceptions
  so the library is safe to use from Flask and other callers
- Add fetch_sibling_routes and fetch_route_master_routes to core.py
- Add Flask web frontend (web/app.py, templates, static assets):
  - Map view with Leaflet; full route drawn in grey on load
  - Sidebar stop list; active-slot UX for From/To selection
  - Segment preview and download (full route or selected segment)
  - Include-stops toggle applied client-side
  - Bookmarkable URLs: GET /<relation_id>
  - Clear selection button
  - Other directions panel (sibling routes from same route_master)
  - route_master handling: draws all member routes in colour on map
    with links to each individual direction
- Add SVG favicon
- Add py.typed marker; add .gitignore

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,8 @@
+__pycache__/
+*.pyc
+*.pyo
+*.egg-info/
+build/
+dist/
+.venv/
+*.geojson

src/osm_geojson/pt/cli.py

@@ -6,12 +6,13 @@ import click
 
 from osm_geojson.pt.core import (
     GeoJson,
+    OsmError,
     build_route_coords,
     fetch_relation_full,
     make_geojson,
+    nearest_coord_index,
     node_name,
     parse_elements,
-    nearest_coord_index,
 )
 
 
@@ -35,8 +36,12 @@ def cli() -> None:
 @click.argument("relation_id", type=int)
 def list_stations(relation_id: int) -> None:
     """List all stations in an OSM public transport route relation."""
-    data = fetch_relation_full(relation_id)
+    try:
-    nodes, ways, stop_ids, way_ids, tags = parse_elements(data, relation_id)
+        data = fetch_relation_full(relation_id)
+        nodes, ways, stop_ids, way_ids, tags = parse_elements(data, relation_id)
+    except OsmError as e:
+        click.echo(f"Error: {e}", err=True)
+        sys.exit(1)
     click.echo(f"Route: {tags.get('name', relation_id)}")
     click.echo(f"Stops ({len(stop_ids)}):")
     for i, sid in enumerate(stop_ids, 1):
@@ -60,8 +65,13 @@ def route_between(
     no_stops: bool,
 ) -> None:
     """Output GeoJSON for the route segment between two named stations."""
-    data = fetch_relation_full(relation_id)
+    try:
-    nodes, ways, stop_ids, way_ids, tags = parse_elements(data, relation_id)
+        data = fetch_relation_full(relation_id)
+        nodes, ways, stop_ids, way_ids, tags = parse_elements(data, relation_id)
+    except OsmError as e:
+        click.echo(f"Error: {e}", err=True)
+        sys.exit(1)
+
     route_coords = build_route_coords(way_ids, ways, nodes)
 
     def find_stop(name: str) -> int | None:
@@ -80,8 +90,8 @@ def route_between(
     if sid_to is None:
         errors.append(f"Station not found: {to_station!r}")
     if errors:
-        for e in errors:
+        for msg in errors:
-            click.echo(f"Error: {e}", err=True)
+            click.echo(f"Error: {msg}", err=True)
         click.echo("Available stations:", err=True)
         for sid in stop_ids:
             if sid in nodes:
@@ -104,8 +114,12 @@ def route_between(
 @click.option("--no-stops", is_flag=True, default=False, help="Omit stop points from output.")
 def full_route(relation_id: int, output: str | None, no_stops: bool) -> None:
     """Output GeoJSON for the entire route, end to end."""
-    data = fetch_relation_full(relation_id)
+    try:
-    nodes, ways, stop_ids, way_ids, tags = parse_elements(data, relation_id)
+        data = fetch_relation_full(relation_id)
+        nodes, ways, stop_ids, way_ids, tags = parse_elements(data, relation_id)
+    except OsmError as e:
+        click.echo(f"Error: {e}", err=True)
+        sys.exit(1)
     route_coords = build_route_coords(way_ids, ways, nodes)
     geojson = make_geojson(route_coords, stop_ids, nodes, tags, no_stops=no_stops)
     output_geojson(geojson, output)

src/osm_geojson/pt/core.py

@@ -1,8 +1,7 @@
 """Core data-fetching and processing functions for osm-pt-geojson."""
-import sys
+import warnings
 from typing import Any
 
-import click
 import requests
 
 OSM_API = "https://www.openstreetmap.org/api/0.6"
@@ -14,17 +13,28 @@ OsmElement = dict[str, Any]
 GeoJson = dict[str, Any]
 
 
+class OsmError(Exception):
+    """Raised when an OSM API request fails or returns unexpected data."""
+
+    def __init__(self, message: str, status_code: int = 500) -> None:
+        """Initialise with a human-readable message and an HTTP-style status code."""
+        super().__init__(message)
+        self.status_code = status_code
+
+
 def fetch_relation_full(relation_id: int) -> dict[str, Any]:
     """Fetch the full OSM API response for a relation, including all member ways and nodes."""
     url = f"{OSM_API}/relation/{relation_id}/full.json"
     try:
         resp = requests.get(url, headers={"User-Agent": "osm-pt-geojson/1.0"}, timeout=30)
     except requests.RequestException as e:
-        click.echo(f"Error: {e}", err=True)
+        raise OsmError(f"Network error fetching relation {relation_id}: {e}", 502) from e
-        sys.exit(1)
+    if resp.status_code == 404:
+        raise OsmError(f"Relation {relation_id} not found on OpenStreetMap.", 404)
     if resp.status_code != 200:
-        click.echo(f"Error: HTTP {resp.status_code} fetching relation {relation_id}", err=True)
+        raise OsmError(
-        sys.exit(1)
+            f"Unexpected HTTP {resp.status_code} fetching relation {relation_id}.", 502
+        )
     result: dict[str, Any] = resp.json()
     return result
 
@@ -48,8 +58,7 @@ def parse_elements(
             relation = elem
 
     if relation is None:
-        click.echo(f"Error: relation {relation_id} not found in API response", err=True)
+        raise OsmError(f"Relation {relation_id} not found in API response.", 404)
-        sys.exit(1)
 
     stop_ids: list[int] = []
     way_ids: list[int] = []
@@ -92,7 +101,7 @@ def build_route_coords(
         elif chain[0] == wn[0]:
             chain = list(reversed(wn)) + chain[1:]
         else:
-            click.echo(f"Warning: gap before way {way_id}", err=True)
+            warnings.warn(f"Gap in route geometry before way {way_id}", stacklevel=2)
             chain.extend(wn)
 
     return [[nodes[nid]["lon"], nodes[nid]["lat"]] for nid in chain if nid in nodes]
@@ -110,6 +119,56 @@ def nearest_coord_index(lon: float, lat: float, route_coords: list[Coord]) -> in
     return best_i
 
 
+def fetch_sibling_routes(relation_id: int) -> list[dict[str, Any]]:
+    """Return sibling route relations from the same route_master, excluding self.
+
+    Fetches parent relations of relation_id, finds any route_master parents,
+    and returns the other member relations with their names. Returns an empty
+    list if there is no route_master parent or if any request fails.
+    """
+    url = f"{OSM_API}/relation/{relation_id}/relations.json"
+    try:
+        resp = requests.get(url, headers={"User-Agent": "osm-pt-geojson/1.0"}, timeout=30)
+    except requests.RequestException:
+        return []
+    if resp.status_code != 200:
+        return []
+
+    data: dict[str, Any] = resp.json()
+    sibling_ids: list[int] = []
+    for elem in data.get("elements", []):
+        if elem.get("tags", {}).get("type") != "route_master":
+            continue
+        for member in elem.get("members", []):
+            if member["type"] == "relation" and member["ref"] != relation_id:
+                sibling_ids.append(member["ref"])
+
+    result: list[dict[str, Any]] = []
+    for sid in sibling_ids:
+        surl = f"{OSM_API}/relation/{sid}.json"
+        try:
+            sresp = requests.get(
+                surl, headers={"User-Agent": "osm-pt-geojson/1.0"}, timeout=30
+            )
+        except requests.RequestException:
+            continue
+        if sresp.status_code != 200:
+            continue
+        sdata: dict[str, Any] = sresp.json()
+        for elem in sdata.get("elements", []):
+            if elem["type"] == "relation" and elem["id"] == sid:
+                stags: OsmTags = elem.get("tags", {})
+                result.append(
+                    {
+                        "id": sid,
+                        "name": stags.get("name", str(sid)),
+                        "ref": stags.get("ref"),
+                    }
+                )
+                break
+    return result
+
+
 def node_name(node: OsmElement) -> str:
     """Return a human-readable name for a node: name tag, ref tag, or node ID."""
     tags: OsmTags = node.get("tags", {})
@@ -172,3 +231,73 @@ def make_geojson(
             )
 
     return {"type": "FeatureCollection", "features": features}
+
+
+def fetch_route_master_routes(
+    relation_id: int,
+) -> tuple[OsmTags, list[dict[str, Any]]]:
+    """Fetch a route_master relation and return (rm_tags, routes).
+
+    Raises OsmError if the relation is not found or is not a route_master.
+    Each entry in routes has: id, name, ref, from, to, geojson (lines only).
+    Members that cannot be fetched are included with null geojson.
+    """
+    url = f"{OSM_API}/relation/{relation_id}.json"
+    try:
+        resp = requests.get(url, headers={"User-Agent": "osm-pt-geojson/1.0"}, timeout=30)
+    except requests.RequestException as e:
+        raise OsmError(f"Network error fetching relation {relation_id}: {e}", 502) from e
+    if resp.status_code == 404:
+        raise OsmError(f"Relation {relation_id} not found on OpenStreetMap.", 404)
+    if resp.status_code != 200:
+        raise OsmError(
+            f"Unexpected HTTP {resp.status_code} fetching relation {relation_id}.", 502
+        )
+
+    data: dict[str, Any] = resp.json()
+    rm_tags: OsmTags = {}
+    member_ids: list[int] = []
+    for elem in data.get("elements", []):
+        if elem["type"] == "relation" and elem["id"] == relation_id:
+            rm_tags = elem.get("tags", {})
+            for m in elem.get("members", []):
+                if m["type"] == "relation":
+                    member_ids.append(m["ref"])
+            break
+
+    if not rm_tags:
+        raise OsmError(f"Relation {relation_id} not found in API response.", 404)
+    if rm_tags.get("type") != "route_master":
+        raise OsmError(
+            f"Relation {relation_id} is not a route_master "
+            f"(type={rm_tags.get('type')!r}).",
+            422,
+        )
+
+    routes: list[dict[str, Any]] = []
+    for mid in member_ids:
+        entry: dict[str, Any]
+        try:
+            full_data = fetch_relation_full(mid)
+            nodes, ways, stop_ids, way_ids, tags = parse_elements(full_data, mid)
+            route_coords = build_route_coords(way_ids, ways, nodes)
+            entry = {
+                "id": mid,
+                "name": tags.get("name", str(mid)),
+                "ref": tags.get("ref"),
+                "from": tags.get("from"),
+                "to": tags.get("to"),
+                "geojson": make_geojson(route_coords, stop_ids, nodes, tags, no_stops=True),
+            }
+        except OsmError:
+            entry = {
+                "id": mid,
+                "name": str(mid),
+                "ref": None,
+                "from": None,
+                "to": None,
+                "geojson": None,
+            }
+        routes.append(entry)
+
+    return rm_tags, routes

tests/test_osm_pt_geojson.py

@@ -8,6 +8,7 @@ from click.testing import CliRunner
 
 from osm_geojson.pt import core
 from osm_geojson.pt.cli import cli, output_geojson
+from osm_geojson.pt.core import OsmError
 
 FIXTURES = Path(__file__).parent / "fixtures"
 FULL_URL = "https://www.openstreetmap.org/api/0.6/relation/15083963/full.json"
@@ -57,8 +58,8 @@ def test_parse_elements_tags(parsed: tuple) -> None:
 
 
 def test_parse_elements_unknown_relation(full_data: dict) -> None:
-    """Requesting a relation ID not present in the response exits with an error."""
+    """Requesting a relation ID not present in the response raises OsmError."""
-    with pytest.raises(SystemExit):
+    with pytest.raises(OsmError):
         core.parse_elements(full_data, 9999999)
 
 

web/app.py

@@ -0,0 +1,210 @@
+"""Flask web frontend for osm-pt-geojson."""
+import re
+from typing import Any
+
+from flask import Flask, jsonify, redirect, render_template, request, url_for
+from flask.typing import ResponseReturnValue
+
+from osm_geojson.pt.core import (
+    OsmError,
+    build_route_coords,
+    fetch_relation_full,
+    fetch_route_master_routes,
+    fetch_sibling_routes,
+    make_geojson,
+    nearest_coord_index,
+    node_name,
+    parse_elements,
+)
+
+app = Flask(__name__)
+app.secret_key = "dev"
+
+PUBLIC_TRANSPORT_ROUTE_TYPES = {
+    "bus", "trolleybus", "tram", "subway", "train",
+    "light_rail", "monorail", "ferry", "funicular",
+}
+
+
+def parse_relation_id(text: str) -> int | None:
+    """Extract a relation ID from a bare integer, partial path, or full OSM URL."""
+    text = text.strip()
+    m = re.search(r"relation/(\d+)", text)
+    if m:
+        return int(str(m.group(1)))
+    if re.fullmatch(r"\d+", text):
+        return int(text)
+    return None
+
+
+def check_route_tags(
+    tags: dict[str, str], relation_id: int
+) -> tuple[dict[str, Any], int] | None:
+    """Return a (error_dict, http_status) if the relation is not a supported public transport route."""
+    if tags.get("type") == "route_master":
+        return {
+            "error": "is_route_master",
+            "message": (
+                f"Relation {relation_id} is a route_master relation. "
+                "Select one of its individual routes."
+            ),
+        }, 422
+    if tags.get("type") != "route":
+        kind = tags.get("type", "unknown")
+        return {
+            "error": "not_public_transport",
+            "message": (
+                f"Relation {relation_id} is a {kind!r} relation, not a route relation. "
+                "This tool only supports public transport route relations."
+            ),
+        }, 422
+    route = tags.get("route")
+    if not route:
+        return {
+            "error": "not_public_transport",
+            "message": (
+                f"Relation {relation_id} is a route relation but has no 'route' tag. "
+                "Cannot determine the route type."
+            ),
+        }, 422
+    if route not in PUBLIC_TRANSPORT_ROUTE_TYPES:
+        supported = ", ".join(sorted(PUBLIC_TRANSPORT_ROUTE_TYPES))
+        return {
+            "error": "not_public_transport",
+            "route_type": route,
+            "message": (
+                f"Relation {relation_id} is a {route!r} route, not a public transport route. "
+                f"This tool supports: {supported}."
+            ),
+        }, 422
+    return None
+
+
+@app.route("/")
+def index() -> ResponseReturnValue:
+    """Render the landing page with no relation loaded."""
+    error = request.args.get("error")
+    return render_template("index.html", relation_id=None, error=error)
+
+
+@app.route("/<int:relation_id>")
+def route_page(relation_id: int) -> ResponseReturnValue:
+    """Render the page with a relation pre-loaded."""
+    return render_template("index.html", relation_id=relation_id, error=None)
+
+
+@app.route("/load", methods=["POST"])
+def load() -> ResponseReturnValue:
+    """Parse user input and redirect to the relation page."""
+    raw = request.form.get("relation", "").strip()
+    relation_id = parse_relation_id(raw)
+    if relation_id is None:
+        return redirect(url_for("index", error="Could not find a relation ID in that input."))
+    return redirect(url_for("route_page", relation_id=relation_id))
+
+
+@app.route("/api/route/<int:relation_id>")
+def api_route(relation_id: int) -> ResponseReturnValue:
+    """Return the full route as GeoJSON plus a list of stops."""
+    try:
+        data = fetch_relation_full(relation_id)
+        nodes, ways, stop_ids, way_ids, tags = parse_elements(data, relation_id)
+    except OsmError as e:
+        return jsonify({"error": "osm_error", "message": str(e)}), e.status_code
+
+    err = check_route_tags(tags, relation_id)
+    if err:
+        return jsonify(err[0]), err[1]
+
+    route_coords = build_route_coords(way_ids, ways, nodes)
+    geojson = make_geojson(route_coords, stop_ids, nodes, tags)
+
+    stops = []
+    for sid in stop_ids:
+        if sid in nodes:
+            n = nodes[sid]
+            stops.append({"name": node_name(n), "lat": n["lat"], "lon": n["lon"]})
+
+    other_directions = fetch_sibling_routes(relation_id)
+
+    return jsonify({
+        "name": tags.get("name", str(relation_id)),
+        "ref": tags.get("ref"),
+        "stops": stops,
+        "geojson": geojson,
+        "other_directions": other_directions,
+    })
+
+
+@app.route("/api/segment/<int:relation_id>")
+def api_segment(relation_id: int) -> ResponseReturnValue:
+    """Return GeoJSON for the segment between two named stops."""
+    from_name = request.args.get("from", "").strip()
+    to_name = request.args.get("to", "").strip()
+    include_stops = request.args.get("stops", "1") != "0"
+
+    if not from_name or not to_name:
+        return jsonify({
+            "error": "missing_params",
+            "message": "Both 'from' and 'to' parameters are required.",
+        }), 400
+
+    try:
+        data = fetch_relation_full(relation_id)
+        nodes, ways, stop_ids, way_ids, tags = parse_elements(data, relation_id)
+    except OsmError as e:
+        return jsonify({"error": "osm_error", "message": str(e)}), e.status_code
+
+    err = check_route_tags(tags, relation_id)
+    if err:
+        return jsonify(err[0]), err[1]
+
+    route_coords = build_route_coords(way_ids, ways, nodes)
+
+    def find_stop(name: str) -> int | None:
+        """Return the node ID of the stop matching name (case-insensitive), or None."""
+        for sid in stop_ids:
+            if sid in nodes and node_name(nodes[sid]).lower() == name.lower():
+                return sid
+        return None
+
+    sid_from = find_stop(from_name)
+    sid_to = find_stop(to_name)
+
+    missing = [n for n, s in ((from_name, sid_from), (to_name, sid_to)) if s is None]
+    if missing:
+        available = [node_name(nodes[s]) for s in stop_ids if s in nodes]
+        return jsonify({
+            "error": "station_not_found",
+            "message": f"Station(s) not found: {', '.join(repr(m) for m in missing)}",
+            "available": available,
+        }), 404
+
+    assert sid_from is not None and sid_to is not None
+    idx_from = nearest_coord_index(nodes[sid_from]["lon"], nodes[sid_from]["lat"], route_coords)
+    idx_to = nearest_coord_index(nodes[sid_to]["lon"], nodes[sid_to]["lat"], route_coords)
+
+    geojson = make_geojson(
+        route_coords, stop_ids, nodes, tags,
+        idx_from=idx_from, idx_to=idx_to,
+        no_stops=not include_stops,
+    )
+    return jsonify(geojson)
+
+
+@app.route("/api/route_master/<int:relation_id>")
+def api_route_master(relation_id: int) -> ResponseReturnValue:
+    """Return all member routes of a route_master with their GeoJSON geometries."""
+    try:
+        rm_tags, routes = fetch_route_master_routes(relation_id)
+    except OsmError as e:
+        return jsonify({"error": "osm_error", "message": str(e)}), e.status_code
+    return jsonify({
+        "name": rm_tags.get("name", str(relation_id)),
+        "ref": rm_tags.get("ref"),
+        "routes": routes,
+    })
+
+
+if __name__ == "__main__":
+    app.run(debug=True)

web/static/app.js

@@ -0,0 +1,467 @@
+/**
+ * osm-pt-geojson web frontend
+ * Handles map rendering, stop selection, segment preview and GeoJSON download.
+ */
+
+'use strict';
+
+// ── Map setup ──────────────────────────────────────────────────────────────
+
+const map = L.map('map').setView([20, 0], 2);
+
+L.tileLayer('https://tile.openstreetmap.org/{z}/{x}/{y}.png', {
+  attribution: '© <a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a> contributors',
+  maxZoom: 19,
+}).addTo(map);
+
+// Leaflet layers
+let fullRouteLayer = null;   // thin grey polyline for the full route
+let segmentLayer = null;     // bold coloured polyline for the selected segment
+let markerLayer = null;      // all stop markers
+let routeMasterLayers = [];  // coloured polylines when viewing a route_master
+
+// ── State ─────────────────────────────────────────────────────────────────
+
+let routeData = null;        // response from /api/route/
+let segmentGeoJson = null;   // last response from /api/segment/ (always includes stops)
+let activeSlot = 'from';     // 'from' | 'to' | null
+let selectedFrom = null;     // stop name string or null
+let selectedTo = null;       // stop name string or null
+
+// ── Helpers ────────────────────────────────────────────────────────────────
+
+/**
+ * Show an error alert in the sidebar.
+ * @param {string} msg
+ */
+function showError(msg) {
+  const el = document.getElementById('js-alert');
+  document.getElementById('js-alert-msg').textContent = msg;
+  el.classList.remove('d-none');
+}
+
+/**
+ * Trigger a file download of text content.
+ * @param {string} content
+ * @param {string} filename
+ */
+function downloadText(content, filename) {
+  const blob = new Blob([content], { type: 'application/geo+json' });
+  const url = URL.createObjectURL(blob);
+  const a = document.createElement('a');
+  a.href = url;
+  a.download = filename;
+  a.click();
+  URL.revokeObjectURL(url);
+}
+
+/**
+ * Return a safe filename fragment from a stop name.
+ * @param {string} name
+ * @returns {string}
+ */
+function safeName(name) {
+  return name.replace(/[^a-zA-Z0-9_\-]/g, '_').replace(/_+/g, '_');
+}
+
+// ── Slot UI ────────────────────────────────────────────────────────────────
+
+/**
+ * Update the From/To slot boxes to reflect current state.
+ */
+function renderSlots() {
+  const slots = { from: selectedFrom, to: selectedTo };
+  for (const [id, value] of Object.entries(slots)) {
+    const box = document.getElementById(`slot-${id}`);
+    const text = document.getElementById(`slot-${id}-text`);
+    const pencil = document.getElementById(`slot-${id}-pencil`);
+    const isActive = activeSlot === id;
+
+    box.classList.toggle('active', isActive);
+    pencil.classList.toggle('d-none', !isActive);
+
+    if (value) {
+      text.textContent = value;
+      text.classList.remove('placeholder');
+    } else {
+      text.textContent = 'Click a stop…';
+      text.classList.add('placeholder');
+    }
+  }
+}
+
+/**
+ * Set which slot is active. Pass null to deactivate both.
+ * @param {'from'|'to'|null} slot
+ */
+function setActiveSlot(slot) {
+  activeSlot = slot;
+  renderSlots();
+}
+
+// ── Stop selection ─────────────────────────────────────────────────────────
+
+/**
+ * Handle a stop being selected (from list click or map marker click).
+ * @param {string} name
+ */
+function selectStop(name) {
+  if (activeSlot === 'from') {
+    selectedFrom = name;
+    setActiveSlot('to');
+  } else if (activeSlot === 'to') {
+    selectedTo = name;
+    setActiveSlot(null);
+  } else {
+    // Neither slot active — do nothing, user must click a slot first.
+    return;
+  }
+  renderStopList();
+  updateMarkers();
+  if (selectedFrom && selectedTo) {
+    loadSegment();
+  }
+}
+
+// ── Stop list ──────────────────────────────────────────────────────────────
+
+/**
+ * Render the sidebar stop list from routeData.
+ */
+function renderStopList() {
+  if (!routeData) return;
+  const list = document.getElementById('stop-list');
+  document.getElementById('stop-count').textContent = `(${routeData.stops.length})`;
+  list.innerHTML = '';
+  for (const stop of routeData.stops) {
+    const div = document.createElement('div');
+    div.className = 'stop-item';
+    div.textContent = stop.name;
+    if (stop.name === selectedFrom) div.classList.add('is-from');
+    if (stop.name === selectedTo) div.classList.add('is-to');
+    div.addEventListener('click', () => selectStop(stop.name));
+    list.appendChild(div);
+  }
+}
+
+// ── Map rendering ──────────────────────────────────────────────────────────
+
+/** Marker colours for stop states. */
+const MARKER_COLOURS = {
+  from:    '#198754',  // green
+  to:      '#dc3545',  // red
+  segment: '#0d6efd',  // blue
+  default: '#6c757d',  // grey
+};
+
+/**
+ * Create a small circular Leaflet marker.
+ * @param {number} lat
+ * @param {number} lon
+ * @param {string} colour  hex colour
+ * @param {string} title
+ * @returns {L.CircleMarker}
+ */
+function makeMarker(lat, lon, colour, title) {
+  return L.circleMarker([lat, lon], {
+    radius: 6,
+    color: colour,
+    fillColor: colour,
+    fillOpacity: 0.9,
+    weight: 2,
+  }).bindTooltip(title);
+}
+
+/**
+ * Determine which stops fall within the current segment (by index).
+ * Returns a Set of stop names.
+ * @returns {Set<string>}
+ */
+function segmentStopNames() {
+  if (!segmentGeoJson) return new Set();
+  return new Set(
+    segmentGeoJson.features
+      .filter(f => f.geometry.type === 'Point')
+      .map(f => f.properties.name)
+  );
+}
+
+/**
+ * Redraw all stop markers to reflect current selection state.
+ */
+function updateMarkers() {
+  if (!routeData) return;
+  if (markerLayer) markerLayer.remove();
+  markerLayer = L.layerGroup();
+  const inSegment = segmentStopNames();
+
+  for (const stop of routeData.stops) {
+    let colour;
+    if (stop.name === selectedFrom)       colour = MARKER_COLOURS.from;
+    else if (stop.name === selectedTo)    colour = MARKER_COLOURS.to;
+    else if (inSegment.has(stop.name))    colour = MARKER_COLOURS.segment;
+    else                                  colour = MARKER_COLOURS.default;
+
+    const marker = makeMarker(stop.lat, stop.lon, colour, stop.name);
+    marker.on('click', () => selectStop(stop.name));
+    markerLayer.addLayer(marker);
+  }
+  markerLayer.addTo(map);
+}
+
+/**
+ * Draw or redraw the segment layer from segmentGeoJson, respecting the stops toggle.
+ */
+function renderSegment() {
+  if (segmentLayer) segmentLayer.remove();
+  if (!segmentGeoJson) return;
+
+  const includeStops = document.getElementById('include-stops').checked;
+
+  // Build a filtered GeoJSON: always keep LineString, conditionally keep Points.
+  const filtered = {
+    type: 'FeatureCollection',
+    features: segmentGeoJson.features.filter(f =>
+      f.geometry.type === 'LineString' || (f.geometry.type === 'Point' && includeStops)
+    ),
+  };
+
+  segmentLayer = L.geoJSON(filtered, {
+    style: { color: '#0d6efd', weight: 5, opacity: 0.85 },
+    pointToLayer: (feature, latlng) =>
+      makeMarker(latlng.lat, latlng.lng, MARKER_COLOURS.segment, feature.properties.name),
+  }).addTo(map);
+
+  updateMarkers();
+  updateDownloadButtons();
+}
+
+// ── Download buttons ───────────────────────────────────────────────────────
+
+/**
+ * Enable/disable the segment download button and wire up both download buttons.
+ */
+function updateDownloadButtons() {
+  const btn = document.getElementById('btn-download-segment');
+  if (segmentGeoJson && selectedFrom && selectedTo) {
+    btn.classList.remove('disabled');
+  } else {
+    btn.classList.add('disabled');
+  }
+}
+
+document.getElementById('btn-download-segment').addEventListener('click', () => {
+  if (!segmentGeoJson || !selectedFrom || !selectedTo) return;
+  const includeStops = document.getElementById('include-stops').checked;
+  const geojson = {
+    type: 'FeatureCollection',
+    features: segmentGeoJson.features.filter(f =>
+      f.geometry.type === 'LineString' || (f.geometry.type === 'Point' && includeStops)
+    ),
+  };
+  const ref = routeData.ref || 'route';
+  const filename = `${safeName(ref)}-${safeName(selectedFrom)}-${safeName(selectedTo)}.geojson`;
+  downloadText(JSON.stringify(geojson, null, 2), filename);
+});
+
+document.getElementById('btn-download-full').addEventListener('click', () => {
+  if (!routeData) return;
+  const includeStops = document.getElementById('include-stops').checked;
+  const geojson = {
+    type: 'FeatureCollection',
+    features: routeData.geojson.features.filter(f =>
+      f.geometry.type === 'LineString' || (f.geometry.type === 'Point' && includeStops)
+    ),
+  };
+  const ref = routeData.ref || 'route';
+  downloadText(JSON.stringify(geojson, null, 2), `${safeName(ref)}-full.geojson`);
+});
+
+// ── Clear button ───────────────────────────────────────────────────────────
+
+document.getElementById('btn-clear').addEventListener('click', () => {
+  selectedFrom = null;
+  selectedTo = null;
+  segmentGeoJson = null;
+  if (segmentLayer) { segmentLayer.remove(); segmentLayer = null; }
+  setActiveSlot('from');
+  renderStopList();
+  updateMarkers();
+  updateDownloadButtons();
+});
+
+// ── Stops toggle ───────────────────────────────────────────────────────────
+
+document.getElementById('include-stops').addEventListener('change', () => {
+  renderSegment();   // re-filters existing data, no network request
+});
+
+// ── Slot click handlers ────────────────────────────────────────────────────
+
+document.getElementById('slot-from').addEventListener('click', () => setActiveSlot('from'));
+document.getElementById('slot-to').addEventListener('click', () => setActiveSlot('to'));
+
+// ── API calls ──────────────────────────────────────────────────────────────
+
+/**
+ * Fetch the full route and render it on the map.
+ * @param {number} relationId
+ */
+async function loadRoute(relationId) {
+  try {
+    const resp = await fetch(`/api/route/${relationId}`);
+    const data = await resp.json();
+    if (!resp.ok) {
+      if (data.error === 'is_route_master') {
+        await loadRouteMaster(relationId);
+        return;
+      }
+      showError(data.message || `Error loading relation ${relationId}.`);
+      return;
+    }
+    routeData = data;
+    // Clean up any previous route_master view
+    for (const l of routeMasterLayers) l.remove();
+    routeMasterLayers = [];
+    document.getElementById('route-master-panel').classList.add('d-none');
+    selectedFrom = null;
+    selectedTo = null;
+    segmentGeoJson = null;
+    setActiveSlot('from');
+
+    // Show route panel
+    document.getElementById('route-panel').classList.remove('d-none');
+    document.getElementById('route-name').textContent = data.name;
+
+    // Draw full route in grey
+    if (fullRouteLayer) fullRouteLayer.remove();
+    fullRouteLayer = L.geoJSON(data.geojson, {
+      style: { color: '#adb5bd', weight: 3, opacity: 0.7 },
+      pointToLayer: () => null,   // don't render stop points here
+      filter: f => f.geometry.type === 'LineString',
+    }).addTo(map);
+
+    map.fitBounds(fullRouteLayer.getBounds(), { padding: [20, 20] });
+
+    renderStopList();
+    updateMarkers();
+    updateDownloadButtons();
+
+    // Other directions (sibling routes from the same route_master)
+    const dirPanel = document.getElementById('other-directions-panel');
+    const dirList = document.getElementById('other-directions-list');
+    dirList.innerHTML = '';
+    if (data.other_directions && data.other_directions.length > 0) {
+      dirPanel.classList.remove('d-none');
+      for (const dir of data.other_directions) {
+        const a = document.createElement('a');
+        a.href = `/${dir.id}`;
+        a.className = 'stop-item d-block text-decoration-none';
+        a.textContent = dir.name;
+        dirList.appendChild(a);
+      }
+    } else {
+      dirPanel.classList.add('d-none');
+    }
+  } catch (e) {
+    showError('Network error loading route.');
+  }
+}
+
+/**
+ * Fetch the segment GeoJSON and render it.
+ */
+async function loadSegment() {
+  if (!selectedFrom || !selectedTo || !routeData) return;
+  const rid = RELATION_ID;
+  const params = new URLSearchParams({ from: selectedFrom, to: selectedTo, stops: '1' });
+  try {
+    const resp = await fetch(`/api/segment/${rid}?${params}`);
+    const data = await resp.json();
+    if (!resp.ok) {
+      showError(data.message || 'Error loading segment.');
+      return;
+    }
+    segmentGeoJson = data;
+    renderSegment();
+  } catch (e) {
+    showError('Network error loading segment.');
+  }
+}
+
+/**
+ * Fetch and display all member routes of a route_master relation.
+ * @param {number} relationId
+ */
+async function loadRouteMaster(relationId) {
+  try {
+    const resp = await fetch(`/api/route_master/${relationId}`);
+    const data = await resp.json();
+    if (!resp.ok) {
+      showError(data.message || `Error loading route master ${relationId}.`);
+      return;
+    }
+
+    // Clear individual route state
+    routeData = null;
+    selectedFrom = null;
+    selectedTo = null;
+    segmentGeoJson = null;
+    if (fullRouteLayer) { fullRouteLayer.remove(); fullRouteLayer = null; }
+    if (segmentLayer) { segmentLayer.remove(); segmentLayer = null; }
+    if (markerLayer) { markerLayer.remove(); markerLayer = null; }
+    for (const l of routeMasterLayers) l.remove();
+    routeMasterLayers = [];
+
+    // Hide individual route panel, show route_master panel
+    document.getElementById('route-panel').classList.add('d-none');
+    document.getElementById('route-master-panel').classList.remove('d-none');
+    document.getElementById('route-master-name').textContent = data.name;
+
+    const colours = ['#0d6efd', '#dc3545', '#198754', '#fd7e14', '#6f42c1'];
+    const list = document.getElementById('route-master-list');
+    list.innerHTML = '';
+
+    data.routes.forEach((route, i) => {
+      const colour = colours[i % colours.length];
+
+      if (route.geojson) {
+        const layer = L.geoJSON(route.geojson, {
+          style: { color: colour, weight: 4, opacity: 0.85 },
+        }).addTo(map);
+        routeMasterLayers.push(layer);
+      }
+
+      const div = document.createElement('div');
+      div.className = 'stop-item d-flex align-items-center gap-2';
+
+      const dot = document.createElement('span');
+      dot.style.cssText =
+        `display:inline-block;width:10px;height:10px;border-radius:50%;` +
+        `background:${colour};flex-shrink:0`;
+
+      const a = document.createElement('a');
+      a.href = `/${route.id}`;
+      a.className = 'text-decoration-none text-reset flex-grow-1';
+      a.textContent = route.name;
+
+      div.appendChild(dot);
+      div.appendChild(a);
+      list.appendChild(div);
+    });
+
+    if (routeMasterLayers.length > 0) {
+      let bounds = routeMasterLayers[0].getBounds();
+      for (const l of routeMasterLayers.slice(1)) bounds = bounds.extend(l.getBounds());
+      map.fitBounds(bounds, { padding: [20, 20] });
+    }
+  } catch (e) {
+    showError('Network error loading route master.');
+  }
+}
+
+// ── Init ───────────────────────────────────────────────────────────────────
+
+if (RELATION_ID) {
+  loadRoute(RELATION_ID);
+}

web/static/favicon.svg

@@ -0,0 +1,6 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32">
+  <rect width="32" height="32" rx="6" fill="#212529"/>
+  <line x1="7" y1="23" x2="25" y2="9" stroke="#adb5bd" stroke-width="2.5" stroke-linecap="round"/>
+  <circle cx="7" cy="23" r="4" fill="#198754"/>
+  <circle cx="25" cy="9" r="4" fill="#dc3545"/>
+</svg>

web/static/style.css

@@ -0,0 +1,68 @@
+html, body {
+  height: 100%;
+  overflow: hidden;
+}
+
+#main-row {
+  height: calc(100vh - 56px); /* subtract navbar height */
+}
+
+#sidebar {
+  height: 100%;
+  overflow-y: auto;
+  padding: 1rem;
+}
+
+#map {
+  height: 100%;
+}
+
+.slot-box {
+  border: 2px solid #dee2e6;
+  border-radius: 6px;
+  padding: 0.4rem 0.6rem;
+  cursor: pointer;
+  min-height: 2.2rem;
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  transition: border-color 0.15s;
+}
+
+.slot-box.active {
+  border-color: #0d6efd;
+  background: #f0f5ff;
+}
+
+.slot-box .placeholder {
+  color: #adb5bd;
+  font-style: italic;
+}
+
+.slot-box .pencil {
+  color: #0d6efd;
+  font-size: 0.8rem;
+}
+
+#stop-list .stop-item {
+  cursor: pointer;
+  padding: 0.25rem 0.5rem;
+  border-radius: 4px;
+  font-size: 0.9rem;
+}
+
+#stop-list .stop-item:hover {
+  background: #e9ecef;
+}
+
+#stop-list .stop-item.is-from {
+  background: #d1e7dd;
+}
+
+#stop-list .stop-item.is-to {
+  background: #f8d7da;
+}
+
+.leaflet-container {
+  font-family: inherit;
+}

web/templates/index.html

@@ -0,0 +1,126 @@
+<!doctype html>
+<html lang="en">
+<head>
+  <meta charset="utf-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1">
+  <title>OSM Public Transport → GeoJSON</title>
+  <link rel="icon" type="image/svg+xml" href="{{ url_for('static', filename='favicon.svg') }}">
+  <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap@5.3.3/dist/css/bootstrap.min.css">
+  <link rel="stylesheet" href="https://unpkg.com/leaflet@1.9.4/dist/leaflet.css">
+  <link rel="stylesheet" href="{{ url_for('static', filename='style.css') }}">
+</head>
+<body>
+
+<nav class="navbar navbar-dark bg-dark px-3" style="height:56px">
+  <a class="navbar-brand" href="/">OSM Public Transport → GeoJSON</a>
+</nav>
+
+<div class="container-fluid h-100 p-0">
+  <div class="row g-0 h-100" id="main-row">
+
+    <!-- Sidebar -->
+    <div class="col-3 border-end" id="sidebar">
+
+      <!-- Load form -->
+      <form method="post" action="/load" class="mb-3">
+        <label class="form-label fw-semibold small">Relation ID or OSM URL</label>
+        <div class="input-group input-group-sm">
+          <input type="text" name="relation" class="form-control"
+                 placeholder="e.g. 15083963"
+                 value="{{ relation_id or '' }}">
+          <button class="btn btn-primary" type="submit">Load</button>
+        </div>
+      </form>
+
+      <!-- Error alert -->
+      {% if error %}
+      <div class="alert alert-danger alert-dismissible py-2 small" role="alert">
+        {{ error }}
+        <button type="button" class="btn-close btn-sm" data-bs-dismiss="alert"></button>
+      </div>
+      {% endif %}
+      <div id="js-alert" class="alert alert-danger alert-dismissible py-2 small d-none" role="alert">
+        <span id="js-alert-msg"></span>
+        <button type="button" class="btn-close btn-sm" data-bs-dismiss="alert"></button>
+      </div>
+
+      <!-- Route master panel (shown when a route_master relation is loaded) -->
+      <div id="route-master-panel" class="d-none">
+        <div class="mb-3">
+          <div class="fw-semibold" id="route-master-name"></div>
+        </div>
+        <div class="fw-semibold small mb-1">Routes</div>
+        <div id="route-master-list"></div>
+      </div>
+
+      <!-- Route info (hidden until loaded) -->
+      <div id="route-panel" class="d-none">
+        <div class="mb-3">
+          <div class="fw-semibold" id="route-name"></div>
+        </div>
+
+        <!-- From / To slots -->
+        <div class="mb-1">
+          <label class="form-label fw-semibold small mb-1">From</label>
+          <div class="slot-box active" id="slot-from" title="Click to make active, then click a stop">
+            <span class="placeholder" id="slot-from-text">Click a stop…</span>
+            <span class="pencil" id="slot-from-pencil">✎</span>
+          </div>
+        </div>
+        <div class="mb-3">
+          <label class="form-label fw-semibold small mb-1">To</label>
+          <div class="slot-box" id="slot-to" title="Click to make active, then click a stop">
+            <span class="placeholder" id="slot-to-text">Click a stop…</span>
+            <span class="pencil d-none" id="slot-to-pencil">✎</span>
+          </div>
+        </div>
+
+        <!-- Clear button -->
+        <button class="btn btn-outline-secondary btn-sm w-100 mb-3" id="btn-clear" type="button">
+          ✕ Clear selection
+        </button>
+
+        <!-- Include stops toggle -->
+        <div class="form-check mb-3">
+          <input class="form-check-input" type="checkbox" id="include-stops" checked>
+          <label class="form-check-label small" for="include-stops">Include stops in GeoJSON</label>
+        </div>
+
+        <!-- Download buttons -->
+        <div class="d-grid gap-2 mb-3">
+          <button class="btn btn-outline-secondary btn-sm disabled" id="btn-download-segment">
+            ↓ Download segment
+          </button>
+          <button class="btn btn-outline-secondary btn-sm" id="btn-download-full">
+            ↓ Download full route
+          </button>
+        </div>
+
+        <!-- Other directions -->
+        <div id="other-directions-panel" class="d-none mb-3">
+          <div class="fw-semibold small mb-1">Other directions</div>
+          <div id="other-directions-list"></div>
+        </div>
+
+        <!-- Stop list -->
+        <div class="fw-semibold small mb-1">Stops <span id="stop-count" class="text-muted"></span></div>
+        <div id="stop-list"></div>
+      </div>
+
+    </div><!-- /sidebar -->
+
+    <!-- Map -->
+    <div class="col-9" id="map"></div>
+
+  </div>
+</div>
+
+<script src="https://cdn.jsdelivr.net/npm/bootstrap@5.3.3/dist/js/bootstrap.bundle.min.js"></script>
+<script src="https://unpkg.com/leaflet@1.9.4/dist/leaflet.js"></script>
+<script>
+  // Relation ID injected by Flask; null if none.
+  const RELATION_ID = {{ relation_id | tojson }};
+</script>
+<script src="{{ url_for('static', filename='app.js') }}"></script>
+</body>
+</html>