feat: add route selection functionality and improve station handling

- Added `vitest` for testing and created initial tests for route utilities.
- Implemented route selection logic in the App component, allowing users to select start and end stations.
- Updated the NetworkMap component to reflect focused and selected stations, including visual indicators for start and end stations.
- Enhanced the route panel UI to display selected route information and estimated lengths.
- Introduced utility functions for building track adjacency and computing routes based on selected stations.
- Improved styling for route selection and station list items to enhance user experience.

TODO.md

@@ -16,12 +16,12 @@
 - [x] Define geographic bounding boxes and filtering rules for importing real-world stations from OpenStreetMap.
 - [x] Implement an import script/CLI that pulls OSM station data and normalizes it to the PostGIS schema.
 - [x] Expose backend CRUD endpoints for stations (create, update, archive) with validation and geometry handling.
-- [ ] Build React map tooling for selecting a station.
+- [x] Build React map tooling for selecting a station.
-- [ ] Build tools for station editing, including form validation.
+- [ ] Enhance map UI to support selecting two stations and previewing the rail corridor between them.
 - [ ] Define track selection criteria and tagging rules for harvesting OSM rail segments within target regions.
 - [ ] Extend the importer to load track geometries and associate them with existing stations.
 - [ ] Implement backend track-management APIs with length/speed validation and topology checks.
-- [ ] Create a frontend track-drawing workflow (polyline editor, snapping to stations, undo/redo).
+- [ ] Implement track path mapping along existing OSM rail segments between chosen stations.
 - [ ] Design train connection manager requirements (link trains to operating tracks, manage consist data).
 - [ ] Implement backend services and APIs to attach trains to routes and update assignments.
 - [ ] Add UI flows for managing train connections, including visual feedback on the map.

frontend/package-lock.json

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-        "vite": "^5.4.0"
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+        "vitest": "^1.6.0"
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@@ -1722,6 +1846,19 @@
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@@ -1942,6 +2079,16 @@
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@@ -2069,6 +2216,16 @@
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@@ -2150,6 +2307,25 @@
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+        },
+        "@vitest/ui": {
+          "optional": true
+        },
+        "happy-dom": {
+          "optional": true
+        },
+        "jsdom": {
+          "optional": true
+        }
+      }
+    },
     "node_modules/which": {
       "version": "2.0.2",
       "resolved": "https://registry.npmjs.org/which/-/which-2.0.2.tgz",
@@ -5884,6 +6576,23 @@
         "url": "https://github.com/sponsors/ljharb"
       }
     },
+    "node_modules/why-is-node-running": {
+      "version": "2.3.0",
+      "resolved": "https://registry.npmjs.org/why-is-node-running/-/why-is-node-running-2.3.0.tgz",
+      "integrity": "sha512-hUrmaWBdVDcxvYqnyh09zunKzROWjbZTiNy8dBEjkS7ehEDQibXJ7XvlmtbwuTclUiIyN+CyXQD4Vmko8fNm8w==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "siginfo": "^2.0.0",
+        "stackback": "0.0.2"
+      },
+      "bin": {
+        "why-is-node-running": "cli.js"
+      },
+      "engines": {
+        "node": ">=8"
+      }
+    },
     "node_modules/word-wrap": {
       "version": "1.2.5",
       "resolved": "https://registry.npmjs.org/word-wrap/-/word-wrap-1.2.5.tgz",

frontend/package.json

@@ -9,6 +9,7 @@
     "preview": "vite preview",
     "lint": "eslint \"src/**/*.{ts,tsx}\"",
     "format": "prettier --write \"src/**/*.{ts,tsx,css}\"",
+    "test": "vitest run",
     "test:e2e": "playwright test"
   },
   "dependencies": {
@@ -34,6 +35,7 @@
     "eslint-plugin-react-hooks": "^4.6.2",
     "prettier": "^3.3.3",
     "typescript": "^5.5.3",
-    "vite": "^5.4.0"
+    "vite": "^5.4.0",
+    "vitest": "^1.6.0"
   }
 }

frontend/src/App.tsx

@@ -1,38 +1,114 @@
 import './styles/global.css';
 
+import type { LatLngExpression } from 'leaflet';
 import { useEffect, useMemo, useState } from 'react';
 
 import { LoginForm } from './components/auth/LoginForm';
 import { NetworkMap } from './components/map/NetworkMap';
 import { useNetworkSnapshot } from './hooks/useNetworkSnapshot';
 import { useAuth } from './state/AuthContext';
+import type { Station } from './types/domain';
+import { buildTrackAdjacency, computeRoute } from './utils/route';
 
 function App(): JSX.Element {
   const { token, user, status: authStatus, logout } = useAuth();
   const isAuthenticated = authStatus === 'authenticated' && token !== null;
   const { data, status, error } = useNetworkSnapshot(isAuthenticated ? token : null);
-  const [selectedStationId, setSelectedStationId] = useState<string | null>(null);
+  const [focusedStationId, setFocusedStationId] = useState<string | null>(null);
+  const [routeSelection, setRouteSelection] = useState<{
+    startId: string | null;
+    endId: string | null;
+  }>({ startId: null, endId: null });
 
   useEffect(() => {
     if (status !== 'success' || !data?.stations.length) {
-      setSelectedStationId(null);
+      setFocusedStationId(null);
+      setRouteSelection({ startId: null, endId: null });
       return;
     }
 
-    if (
+    if (!focusedStationId || !hasStation(data.stations, focusedStationId)) {
-      !selectedStationId ||
+      setFocusedStationId(data.stations[0].id);
-      !data.stations.some((station) => station.id === selectedStationId)
-    ) {
-      setSelectedStationId(data.stations[0].id);
     }
-  }, [status, data, selectedStationId]);
+  }, [status, data, focusedStationId]);
 
-  const selectedStation = useMemo(() => {
+  useEffect(() => {
-    if (!data || !selectedStationId) {
+    if (status !== 'success' || !data) {
+      return;
+    }
+
+    setRouteSelection((current) => {
+      const startExists = current.startId
+        ? hasStation(data.stations, current.startId)
+        : false;
+      const endExists = current.endId
+        ? hasStation(data.stations, current.endId)
+        : false;
+
+      return {
+        startId: startExists ? current.startId : null,
+        endId: endExists ? current.endId : null,
+      };
+    });
+  }, [status, data]);
+
+  const stationById = useMemo(() => {
+    if (!data) {
+      return new Map<string, Station>();
+    }
+    const lookup = new Map<string, Station>();
+    for (const station of data.stations) {
+      lookup.set(station.id, station);
+    }
+    return lookup;
+  }, [data]);
+
+  const trackAdjacency = useMemo(
+    () => buildTrackAdjacency(data ? data.tracks : []),
+    [data]
+  );
+
+  const routeComputation = useMemo(() => {
+    const core = computeRoute({
+      startId: routeSelection.startId,
+      endId: routeSelection.endId,
+      stationById,
+      adjacency: trackAdjacency,
+    });
+
+    const segments = core.stations ? buildSegmentsFromStations(core.stations) : [];
+
+    return {
+      ...core,
+      segments,
+    };
+  }, [routeSelection, stationById, trackAdjacency]);
+
+  const focusedStation = useMemo(() => {
+    if (!data || !focusedStationId) {
       return null;
     }
-    return data.stations.find((station) => station.id === selectedStationId) ?? null;
+    return stationById.get(focusedStationId) ?? null;
-  }, [data, selectedStationId]);
+  }, [data, focusedStationId, stationById]);
+
+  const handleStationSelection = (stationId: string) => {
+    setFocusedStationId(stationId);
+    setRouteSelection((current) => {
+      if (!current.startId || (current.startId && current.endId)) {
+        return { startId: stationId, endId: null };
+      }
+
+      if (current.startId === stationId) {
+        return { startId: stationId, endId: null };
+      }
+
+      return { startId: current.startId, endId: stationId };
+    });
+  };
+
+  const clearRouteSelection = () => {
+    setRouteSelection({ startId: null, endId: null });
+  };
 
   return (
     <div className="app-shell">
@@ -65,10 +141,71 @@ function App(): JSX.Element {
                 <div className="map-wrapper">
                   <NetworkMap
                     snapshot={data}
-                    selectedStationId={selectedStationId}
+                    focusedStationId={focusedStationId}
-                    onSelectStation={(id) => setSelectedStationId(id)}
+                    startStationId={routeSelection.startId}
+                    endStationId={routeSelection.endId}
+                    routeSegments={routeComputation.segments}
+                    onStationClick={handleStationSelection}
                   />
                 </div>
+                <div className="route-panel">
+                  <div className="route-panel__header">
+                    <h3>Route Selection</h3>
+                    <button
+                      type="button"
+                      className="ghost-button"
+                      onClick={clearRouteSelection}
+                      disabled={!routeSelection.startId && !routeSelection.endId}
+                    >
+                      Clear
+                    </button>
+                  </div>
+                  <p className="route-panel__hint">
+                    Click a station to set the origin, then click another station to
+                    preview the rail corridor between them.
+                  </p>
+                  <dl className="route-panel__meta">
+                    <div>
+                      <dt>Origin</dt>
+                      <dd>
+                        {routeSelection.startId
+                          ? (stationById.get(routeSelection.startId)?.name ??
+                            'Unknown station')
+                          : 'Choose a station'}
+                      </dd>
+                    </div>
+                    <div>
+                      <dt>Destination</dt>
+                      <dd>
+                        {routeSelection.endId
+                          ? (stationById.get(routeSelection.endId)?.name ??
+                            'Unknown station')
+                          : 'Choose a station'}
+                      </dd>
+                    </div>
+                    <div>
+                      <dt>Estimated Length</dt>
+                      <dd>
+                        {routeComputation.totalLength !== null
+                          ? `${(routeComputation.totalLength / 1000).toFixed(2)} km`
+                          : 'N/A'}
+                      </dd>
+                    </div>
+                  </dl>
+                  {routeComputation.error && (
+                    <p className="route-panel__error">{routeComputation.error}</p>
+                  )}
+                  {!routeComputation.error && routeComputation.stations && (
+                    <div className="route-panel__path">
+                      <span>Path:</span>
+                      <ol>
+                        {routeComputation.stations.map((station) => (
+                          <li key={`route-station-${station.id}`}>{station.name}</li>
+                        ))}
+                      </ol>
+                    </div>
+                  )}
+                </div>
                 <div className="grid">
                   <div>
                     <h3>Stations</h3>
@@ -78,12 +215,20 @@ function App(): JSX.Element {
                           <button
                             type="button"
                             className={`station-list-item${
-                              station.id === selectedStationId
+                              station.id === focusedStationId
                                 ? ' station-list-item--selected'
                                 : ''
+                            }${
+                              station.id === routeSelection.startId
+                                ? ' station-list-item--start'
+                                : ''
+                            }${
+                              station.id === routeSelection.endId
+                                ? ' station-list-item--end'
+                                : ''
                             }`}
-                            aria-pressed={station.id === selectedStationId}
+                            aria-pressed={station.id === focusedStationId}
-                            onClick={() => setSelectedStationId(station.id)}
+                            onClick={() => handleStationSelection(station.id)}
                           >
                             <span className="station-list-item__name">
                               {station.name}
@@ -92,6 +237,14 @@ function App(): JSX.Element {
                               {station.latitude.toFixed(3)},{' '}
                               {station.longitude.toFixed(3)}
                             </span>
+                            {station.id === routeSelection.startId && (
+                              <span className="station-list-item__badge">Origin</span>
+                            )}
+                            {station.id === routeSelection.endId && (
+                              <span className="station-list-item__badge">
+                                Destination
+                              </span>
+                            )}
                           </button>
                         </li>
                       ))}
@@ -120,43 +273,43 @@ function App(): JSX.Element {
                     </ul>
                   </div>
                 </div>
-                {selectedStation && (
+                {focusedStation && (
                   <div className="selected-station">
-                    <h3>Selected Station</h3>
+                    <h3>Focused Station</h3>
                     <dl>
                       <div>
                         <dt>Name</dt>
-                        <dd>{selectedStation.name}</dd>
+                        <dd>{focusedStation.name}</dd>
                       </div>
                       <div>
                         <dt>Coordinates</dt>
                         <dd>
-                          {selectedStation.latitude.toFixed(5)},{' '}
+                          {focusedStation.latitude.toFixed(5)},{' '}
-                          {selectedStation.longitude.toFixed(5)}
+                          {focusedStation.longitude.toFixed(5)}
                         </dd>
                       </div>
-                      {selectedStation.code && (
+                      {focusedStation.code && (
                         <div>
                           <dt>Code</dt>
-                          <dd>{selectedStation.code}</dd>
+                          <dd>{focusedStation.code}</dd>
                         </div>
                       )}
-                      {typeof selectedStation.elevationM === 'number' && (
+                      {typeof focusedStation.elevationM === 'number' && (
                         <div>
                           <dt>Elevation</dt>
-                          <dd>{selectedStation.elevationM.toFixed(1)} m</dd>
+                          <dd>{focusedStation.elevationM.toFixed(1)} m</dd>
                         </div>
                       )}
-                      {selectedStation.osmId && (
+                      {focusedStation.osmId && (
                         <div>
                           <dt>OSM ID</dt>
-                          <dd>{selectedStation.osmId}</dd>
+                          <dd>{focusedStation.osmId}</dd>
                         </div>
                       )}
                       <div>
                         <dt>Status</dt>
                         <dd>
-                          {(selectedStation.isActive ?? true) ? 'Active' : 'Inactive'}
+                          {(focusedStation.isActive ?? true) ? 'Active' : 'Inactive'}
                         </dd>
                       </div>
                     </dl>
@@ -172,3 +325,20 @@ function App(): JSX.Element {
 }
 
 export default App;
+
+function hasStation(stations: Station[], id: string): boolean {
+  return stations.some((station) => station.id === id);
+}
+
+function buildSegmentsFromStations(stations: Station[]): LatLngExpression[][] {
+  const segments: LatLngExpression[][] = [];
+  for (let index = 0; index < stations.length - 1; index += 1) {
+    const current = stations[index];
+    const next = stations[index + 1];
+    segments.push([
+      [current.latitude, current.longitude],
+      [next.latitude, next.longitude],
+    ]);
+  }
+  return segments;
+}

frontend/src/components/map/NetworkMap.tsx

@@ -15,8 +15,11 @@ import 'leaflet/dist/leaflet.css';
 
 interface NetworkMapProps {
   readonly snapshot: NetworkSnapshot;
-  readonly selectedStationId?: string | null;
+  readonly focusedStationId?: string | null;
-  readonly onSelectStation?: (stationId: string) => void;
+  readonly startStationId?: string | null;
+  readonly endStationId?: string | null;
+  readonly routeSegments?: LatLngExpression[][];
+  readonly onStationClick?: (stationId: string) => void;
 }
 
 interface StationPosition {
@@ -29,8 +32,11 @@ const DEFAULT_CENTER: LatLngExpression = [51.505, -0.09];
 
 export function NetworkMap({
   snapshot,
-  selectedStationId,
+  focusedStationId,
-  onSelectStation,
+  startStationId,
+  endStationId,
+  routeSegments = [],
+  onStationClick,
 }: NetworkMapProps): JSX.Element {
   const stationPositions = useMemo<StationPosition[]>(() => {
     return snapshot.stations.map((station) => ({
@@ -86,12 +92,12 @@ export function NetworkMap({
     ] as LatLngBoundsExpression;
   }, [stationPositions]);
 
-  const selectedPosition = useMemo(() => {
+  const focusedPosition = useMemo(() => {
-    if (!selectedStationId) {
+    if (!focusedStationId) {
       return null;
     }
-    return stationLookup.get(selectedStationId) ?? null;
+    return stationLookup.get(focusedStationId) ?? null;
-  }, [selectedStationId, stationLookup]);
+  }, [focusedStationId, stationLookup]);
 
   return (
     <MapContainer
@@ -104,35 +110,52 @@ export function NetworkMap({
         attribution='&copy; <a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a> contributors'
         url="https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png"
       />
-      {selectedPosition ? <StationFocus position={selectedPosition} /> : null}
+      {focusedPosition ? <StationFocus position={focusedPosition} /> : null}
       {trackSegments.map((segment, index) => (
         <Polyline
           key={`track-${index}`}
           positions={segment}
-          pathOptions={{ color: '#38bdf8', weight: 4 }}
+          pathOptions={{ color: '#334155', weight: 3, opacity: 0.8 }}
+        />
+      ))}
+      {routeSegments.map((segment, index) => (
+        <Polyline
+          key={`route-${index}`}
+          positions={segment}
+          pathOptions={{ color: '#facc15', weight: 6, opacity: 0.9 }}
         />
       ))}
       {stationPositions.map((station) => (
         <CircleMarker
           key={station.id}
           center={station.position}
-          radius={station.id === selectedStationId ? 9 : 6}
+          radius={station.id === focusedStationId ? 9 : 6}
           pathOptions={{
-            color: station.id === selectedStationId ? '#34d399' : '#f97316',
+            color: resolveMarkerStroke(
-            fillColor: station.id === selectedStationId ? '#6ee7b7' : '#fed7aa',
+              station.id,
-            fillOpacity: 0.95,
+              startStationId,
-            weight: station.id === selectedStationId ? 3 : 1,
+              endStationId,
+              focusedStationId
+            ),
+            fillColor: resolveMarkerFill(
+              station.id,
+              startStationId,
+              endStationId,
+              focusedStationId
+            ),
+            fillOpacity: 0.96,
+            weight: station.id === focusedStationId ? 3 : 1,
           }}
           eventHandlers={{
             click: () => {
-              onSelectStation?.(station.id);
+              onStationClick?.(station.id);
             },
           }}
         >
           <Tooltip
             direction="top"
             offset={[0, -8]}
-            permanent={station.id === selectedStationId}
+            permanent={station.id === focusedStationId}
             sticky
           >
             {station.name}
@@ -152,3 +175,39 @@ function StationFocus({ position }: { position: LatLngExpression }): null {
 
   return null;
 }
+
+function resolveMarkerStroke(
+  stationId: string,
+  startStationId?: string | null,
+  endStationId?: string | null,
+  focusedStationId?: string | null
+): string {
+  if (stationId === startStationId) {
+    return '#38bdf8';
+  }
+  if (stationId === endStationId) {
+    return '#fb923c';
+  }
+  if (stationId === focusedStationId) {
+    return '#22c55e';
+  }
+  return '#f97316';
+}
+
+function resolveMarkerFill(
+  stationId: string,
+  startStationId?: string | null,
+  endStationId?: string | null,
+  focusedStationId?: string | null
+): string {
+  if (stationId === startStationId) {
+    return '#bae6fd';
+  }
+  if (stationId === endStationId) {
+    return '#fed7aa';
+  }
+  if (stationId === focusedStationId) {
+    return '#bbf7d0';
+  }
+  return '#ffe4c7';
+}

frontend/src/styles/global.css

@@ -102,6 +102,7 @@ body {
     background-color 0.18s ease,
     border-color 0.18s ease,
     transform 0.18s ease;
+  flex-wrap: wrap;
 }
 
 .station-list-item:hover,
@@ -118,6 +119,16 @@ body {
   box-shadow: 0 8px 18px -10px rgba(45, 212, 191, 0.65);
 }
 
+.station-list-item--start {
+  border-color: rgba(56, 189, 248, 0.8);
+  background: rgba(14, 165, 233, 0.2);
+}
+
+.station-list-item--end {
+  border-color: rgba(249, 115, 22, 0.8);
+  background: rgba(234, 88, 12, 0.18);
+}
+
 .station-list-item__name {
   font-weight: 600;
 }
@@ -128,6 +139,27 @@ body {
   font-family: 'Fira Code', 'Source Code Pro', monospace;
 }
 
+.station-list-item__badge {
+  font-size: 0.75rem;
+  font-weight: 600;
+  text-transform: uppercase;
+  letter-spacing: 0.05em;
+  padding: 0.1rem 0.45rem;
+  border-radius: 999px;
+  background: rgba(148, 163, 184, 0.18);
+  color: rgba(226, 232, 240, 0.85);
+}
+
+.station-list-item--start .station-list-item__badge {
+  background: rgba(56, 189, 248, 0.35);
+  color: #0ea5e9;
+}
+
+.station-list-item--end .station-list-item__badge {
+  background: rgba(249, 115, 22, 0.35);
+  color: #f97316;
+}
+
 .grid h3 {
   margin-bottom: 0.5rem;
   font-size: 1.1rem;
@@ -151,6 +183,89 @@ body {
   width: 100%;
 }
 
+.route-panel {
+  display: grid;
+  gap: 0.85rem;
+  padding: 1.1rem 1.35rem;
+  border-radius: 12px;
+  border: 1px solid rgba(250, 204, 21, 0.3);
+  background: rgba(161, 98, 7, 0.16);
+}
+
+.route-panel__header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 1rem;
+}
+
+.route-panel__hint {
+  font-size: 0.9rem;
+  color: rgba(226, 232, 240, 0.78);
+}
+
+.route-panel__meta {
+  display: grid;
+  gap: 0.45rem;
+}
+
+.route-panel__meta > div {
+  display: flex;
+  justify-content: space-between;
+  align-items: baseline;
+  gap: 0.75rem;
+}
+
+.route-panel__meta dt {
+  font-size: 0.8rem;
+  text-transform: uppercase;
+  letter-spacing: 0.06em;
+  color: rgba(226, 232, 240, 0.65);
+}
+
+.route-panel__meta dd {
+  font-size: 0.95rem;
+  color: rgba(226, 232, 240, 0.92);
+}
+
+.route-panel__error {
+  color: #f87171;
+  font-weight: 600;
+}
+
+.route-panel__path {
+  display: flex;
+  gap: 0.6rem;
+  align-items: baseline;
+}
+
+.route-panel__path span {
+  font-size: 0.85rem;
+  color: rgba(226, 232, 240, 0.7);
+  text-transform: uppercase;
+  letter-spacing: 0.06em;
+}
+
+.route-panel__path ol {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 0.4rem;
+  list-style: none;
+  padding: 0;
+  margin: 0;
+}
+
+.route-panel__path li::after {
+  content: '→';
+  margin-left: 0.35rem;
+  color: rgba(250, 204, 21, 0.75);
+}
+
+.route-panel__path li:last-child::after {
+  content: '';
+  margin: 0;
+}
+
 .selected-station {
   margin-top: 1rem;
   padding: 1rem 1.25rem;

frontend/src/utils/route.test.ts

@@ -0,0 +1,141 @@
+import { describe, expect, it } from 'vitest';
+
+import { buildTrackAdjacency, computeRoute } from './route';
+import type { Station, Track } from '../types/domain';
+
+const baseTimestamps = {
+  createdAt: '2024-01-01T00:00:00Z',
+  updatedAt: '2024-01-01T00:00:00Z',
+};
+
+describe('route utilities', () => {
+  it('finds a multi-hop path across connected tracks', () => {
+    const stations: Station[] = [
+      {
+        id: 'station-a',
+        name: 'Alpha',
+        latitude: 51.5,
+        longitude: -0.1,
+        ...baseTimestamps,
+      },
+      {
+        id: 'station-b',
+        name: 'Bravo',
+        latitude: 51.52,
+        longitude: -0.11,
+        ...baseTimestamps,
+      },
+      {
+        id: 'station-c',
+        name: 'Charlie',
+        latitude: 51.54,
+        longitude: -0.12,
+        ...baseTimestamps,
+      },
+      {
+        id: 'station-d',
+        name: 'Delta',
+        latitude: 51.55,
+        longitude: -0.15,
+        ...baseTimestamps,
+      },
+    ];
+
+    const tracks: Track[] = [
+      {
+        id: 'track-ab',
+        startStationId: 'station-a',
+        endStationId: 'station-b',
+        lengthMeters: 1200,
+        maxSpeedKph: 120,
+        ...baseTimestamps,
+      },
+      {
+        id: 'track-bc',
+        startStationId: 'station-b',
+        endStationId: 'station-c',
+        lengthMeters: 1500,
+        maxSpeedKph: 110,
+        ...baseTimestamps,
+      },
+      {
+        id: 'track-cd',
+        startStationId: 'station-c',
+        endStationId: 'station-d',
+        lengthMeters: 900,
+        maxSpeedKph: 115,
+        ...baseTimestamps,
+      },
+    ];
+
+    const stationById = new Map(stations.map((station) => [station.id, station]));
+    const adjacency = buildTrackAdjacency(tracks);
+
+    const result = computeRoute({
+      startId: 'station-a',
+      endId: 'station-d',
+      stationById,
+      adjacency,
+    });
+
+    expect(result.error).toBeNull();
+    expect(result.stations?.map((station) => station.id)).toEqual([
+      'station-a',
+      'station-b',
+      'station-c',
+      'station-d',
+    ]);
+    expect(result.tracks.map((track) => track.id)).toEqual([
+      'track-ab',
+      'track-bc',
+      'track-cd',
+    ]);
+    expect(result.totalLength).toBe(1200 + 1500 + 900);
+  });
+
+  it('returns an error when no path exists', () => {
+    const stations: Station[] = [
+      {
+        id: 'station-a',
+        name: 'Alpha',
+        latitude: 51.5,
+        longitude: -0.1,
+        ...baseTimestamps,
+      },
+      {
+        id: 'station-b',
+        name: 'Bravo',
+        latitude: 51.6,
+        longitude: -0.2,
+        ...baseTimestamps,
+      },
+    ];
+
+    const tracks: Track[] = [
+      {
+        id: 'track-self',
+        startStationId: 'station-a',
+        endStationId: 'station-a',
+        lengthMeters: 0,
+        maxSpeedKph: 80,
+        ...baseTimestamps,
+      },
+    ];
+
+    const stationById = new Map(stations.map((station) => [station.id, station]));
+    const adjacency = buildTrackAdjacency(tracks);
+
+    const result = computeRoute({
+      startId: 'station-a',
+      endId: 'station-b',
+      stationById,
+      adjacency,
+    });
+
+    expect(result.stations).toBeNull();
+    expect(result.tracks).toHaveLength(0);
+    expect(result.error).toBe(
+      'No rail connection found between the selected stations.'
+    );
+  });
+});

frontend/src/utils/route.ts

@@ -0,0 +1,185 @@
+import type { Station, Track } from '../types/domain';
+
+export interface NeighborEdge {
+  readonly neighborId: string;
+  readonly track: Track;
+}
+
+export type TrackAdjacency = Map<string, NeighborEdge[]>;
+
+export interface ComputeRouteParams {
+  readonly startId?: string | null;
+  readonly endId?: string | null;
+  readonly stationById: Map<string, Station>;
+  readonly adjacency: TrackAdjacency;
+}
+
+export interface RouteComputation {
+  readonly stations: Station[] | null;
+  readonly tracks: Track[];
+  readonly totalLength: number | null;
+  readonly error: string | null;
+}
+
+export function buildTrackAdjacency(tracks: readonly Track[]): TrackAdjacency {
+  const adjacency: TrackAdjacency = new Map();
+
+  const register = (fromId: string, toId: string, track: Track) => {
+    if (!adjacency.has(fromId)) {
+      adjacency.set(fromId, []);
+    }
+    adjacency.get(fromId)!.push({ neighborId: toId, track });
+  };
+
+  for (const track of tracks) {
+    register(track.startStationId, track.endStationId, track);
+    register(track.endStationId, track.startStationId, track);
+  }
+
+  return adjacency;
+}
+
+export function computeRoute({
+  startId,
+  endId,
+  stationById,
+  adjacency,
+}: ComputeRouteParams): RouteComputation {
+  if (!startId || !endId) {
+    return emptyResult();
+  }
+
+  if (!stationById.has(startId) || !stationById.has(endId)) {
+    return {
+      stations: null,
+      tracks: [],
+      totalLength: null,
+      error: 'Selected stations are no longer available.',
+    };
+  }
+
+  if (startId === endId) {
+    const station = stationById.get(startId);
+    return {
+      stations: station ? [station] : null,
+      tracks: [],
+      totalLength: 0,
+      error: null,
+    };
+  }
+
+  const visited = new Set<string>();
+  const queue: string[] = [];
+  const parent = new Map<string, { prev: string | null; via: Track | null }>();
+
+  queue.push(startId);
+  visited.add(startId);
+  parent.set(startId, { prev: null, via: null });
+
+  while (queue.length > 0) {
+    const current = queue.shift()!;
+    if (current === endId) {
+      break;
+    }
+
+    const neighbors = adjacency.get(current) ?? [];
+    for (const { neighborId, track } of neighbors) {
+      if (visited.has(neighborId)) {
+        continue;
+      }
+      visited.add(neighborId);
+      parent.set(neighborId, { prev: current, via: track });
+      queue.push(neighborId);
+    }
+  }
+
+  if (!parent.has(endId)) {
+    return {
+      stations: null,
+      tracks: [],
+      totalLength: null,
+      error: 'No rail connection found between the selected stations.',
+    };
+  }
+
+  const stationPath: string[] = [];
+  const trackSequence: Track[] = [];
+  let cursor: string | null = endId;
+
+  while (cursor) {
+    const details = parent.get(cursor);
+    if (!details) {
+      break;
+    }
+    stationPath.push(cursor);
+    if (details.via) {
+      trackSequence.push(details.via);
+    }
+    cursor = details.prev;
+  }
+
+  stationPath.reverse();
+  trackSequence.reverse();
+
+  const stations = stationPath
+    .map((id) => stationById.get(id))
+    .filter((station): station is Station => Boolean(station));
+
+  const totalLength = computeTotalLength(trackSequence, stations);
+
+  return {
+    stations,
+    tracks: trackSequence,
+    totalLength,
+    error: null,
+  };
+}
+
+function computeTotalLength(tracks: Track[], stations: Station[]): number | null {
+  if (tracks.length === 0 && stations.length <= 1) {
+    return 0;
+  }
+
+  const hasTrackLengths = tracks.every(
+    (track) =>
+      typeof track.lengthMeters === 'number' && Number.isFinite(track.lengthMeters)
+  );
+
+  if (hasTrackLengths) {
+    return tracks.reduce((total, track) => total + (track.lengthMeters ?? 0), 0);
+  }
+
+  if (stations.length < 2) {
+    return null;
+  }
+
+  let total = 0;
+  for (let index = 0; index < stations.length - 1; index += 1) {
+    total += haversineDistance(stations[index], stations[index + 1]);
+  }
+  return total;
+}
+
+function haversineDistance(a: Station, b: Station): number {
+  const R = 6371_000;
+  const toRad = (value: number) => (value * Math.PI) / 180;
+  const dLat = toRad(b.latitude - a.latitude);
+  const dLon = toRad(b.longitude - a.longitude);
+  const lat1 = toRad(a.latitude);
+  const lat2 = toRad(b.latitude);
+
+  const sinDLat = Math.sin(dLat / 2);
+  const sinDLon = Math.sin(dLon / 2);
+  const root = sinDLat * sinDLat + Math.cos(lat1) * Math.cos(lat2) * sinDLon * sinDLon;
+  const c = 2 * Math.atan2(Math.sqrt(root), Math.sqrt(1 - root));
+  return R * c;
+}
+
+function emptyResult(): RouteComputation {
+  return {
+    stations: null,
+    tracks: [],
+    totalLength: null,
+    error: null,
+  };
+}

frontend/tsconfig.json

@@ -15,7 +15,7 @@
     "isolatedModules": true,
     "noEmit": true,
     "jsx": "react-jsx",
-    "types": ["vite/client"]
+    "types": ["vite/client", "vitest"]
   },
   "include": ["src"]
 }

frontend/vitest.config.ts

@@ -0,0 +1,8 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    include: ['src/**/*.test.ts'],
+    environment: 'node',
+  },
+});