const EARTH_RADIUS_M = 6_371_000;
const radians = degrees => degrees * Math.PI / 180;

export function distanceMeters(a, b) {
  const dLat = radians(b.lat - a.lat);
  const dLon = radians(b.lon - a.lon);
  const lat1 = radians(a.lat);
  const lat2 = radians(b.lat);
  const h = Math.sin(dLat / 2) ** 2 + Math.cos(lat1) * Math.cos(lat2) * Math.sin(dLon / 2) ** 2;
  return 2 * EARTH_RADIUS_M * Math.asin(Math.sqrt(h));
}

export function routeMetrics(points) {
  if (!points.length) throw new Error('Die GPX-Datei enthält keine Trackpunkte.');
  let distance = 0;
  let gain = 0;
  for (let i = 1; i < points.length; i += 1) {
    distance += distanceMeters(points[i - 1], points[i]);
    const previous = points[i - 1].elevation;
    const current = points[i].elevation;
    if (Number.isFinite(previous) && Number.isFinite(current) && current > previous) gain += current - previous;
  }
  const lats = points.map(point => point.lat);
  const lons = points.map(point => point.lon);
  return {
    startLat: points[0].lat,
    startLon: points[0].lon,
    centerLat: (Math.min(...lats) + Math.max(...lats)) / 2,
    centerLon: (Math.min(...lons) + Math.max(...lons)) / 2,
    minLat: Math.min(...lats), minLon: Math.min(...lons),
    maxLat: Math.max(...lats), maxLon: Math.max(...lons),
    distanceM: distance,
    elevationGainM: gain,
    pointCount: points.length
  };
}