src/utils/geoUtils.ts

/**
 * Geographic utility functions.
 *
 * All calculations use the Haversine formula and operate on
 * WGS-84 (standard GPS) coordinates.
 */

const EARTH_RADIUS_KM = 6371;

/**
 * Calculate the straight-line distance between two points using the
 * Haversine formula.
 *
 * @returns Distance in kilometers
 */
export function calculateDistance(
  lat1: number,
  lng1: number,
  lat2: number,
  lng2: number,
): number {
  const toRad = (deg: number) => (deg * Math.PI) / 180;

  const dLat = toRad(lat2 - lat1);
  const dLng = toRad(lng2 - lng1);

  const a =
    Math.sin(dLat / 2) * Math.sin(dLat / 2) +
    Math.cos(toRad(lat1)) *
      Math.cos(toRad(lat2)) *
      Math.sin(dLng / 2) *
      Math.sin(dLng / 2);

  const c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));

  return EARTH_RADIUS_KM * c;
}

/**
 * Calculate the initial bearing (forward azimuth) from point 1 to point 2.
 *
 * @returns Bearing in degrees (0–360), where 0/360 = North, 90 = East, etc.
 */
export function calculateBearing(
  lat1: number,
  lng1: number,
  lat2: number,
  lng2: number,
): number {
  const toRad = (deg: number) => (deg * Math.PI) / 180;
  const toDeg = (rad: number) => (rad * 180) / Math.PI;

  const dLng = toRad(lng2 - lng1);
  const y = Math.sin(dLng) * Math.cos(toRad(lat2));
  const x =
    Math.cos(toRad(lat1)) * Math.sin(toRad(lat2)) -
    Math.sin(toRad(lat1)) * Math.cos(toRad(lat2)) * Math.cos(dLng);

  const bearing = toDeg(Math.atan2(y, x));
  return (bearing + 360) % 360;
}

/**
 * Get a simple compass direction label from a bearing in degrees.
 */
export function bearingToCompass(bearing: number): string {
  const dirs = ['N', 'NE', 'E', 'SE', 'S', 'SW', 'W', 'NW'];
  const index = Math.round(bearing / 45) % 8;
  return dirs[index];
}

/**
 * Calculate the geographic midpoint between two coordinates.
 */
export function midpoint(
  lat1: number,
  lng1: number,
  lat2: number,
  lng2: number,
): { lat: number; lng: number } {
  const toRad = (deg: number) => (deg * Math.PI) / 180;
  const toDeg = (rad: number) => (rad * 180) / Math.PI;

  const dLng = toRad(lng2 - lng1);

  const lat1Rad = toRad(lat1);
  const lat2Rad = toRad(lat2);
  const lng1Rad = toRad(lng1);

  const bx = Math.cos(lat2Rad) * Math.cos(dLng);
  const by = Math.cos(lat2Rad) * Math.sin(dLng);

  const lat =
    toDeg(
      Math.atan2(
        Math.sin(lat1Rad) + Math.sin(lat2Rad),
        Math.sqrt(
          (Math.cos(lat1Rad) + bx) * (Math.cos(lat1Rad) + bx) + by * by,
        ),
      ),
    );
  const lng = toDeg(lng1Rad + Math.atan2(by, Math.cos(lat1Rad) + bx));

  return { lat, lng };
}

/**
 * Check if a point is within a given radius of another point.
 */
export function isWithinRadius(
  lat1: number,
  lng1: number,
  lat2: number,
  lng2: number,
  radiusKm: number,
): boolean {
  return calculateDistance(lat1, lng1, lat2, lng2) <= radiusKm;
}

/**
 * Calculate approximate bounding box around a center point.
 *
 * @param lat - Center latitude
 * @param lng - Center longitude
 * @param radiusKm - Radius in kilometers
 * @returns Bounding box { north, south, east, west }
 */
export function boundingBox(
  lat: number,
  lng: number,
  radiusKm: number,
): { north: number; south: number; east: number; west: number } {
  const toRad = (deg: number) => (deg * Math.PI) / 180;
  const toDeg = (rad: number) => (rad * 180) / Math.PI;

  const latDelta = toDeg(radiusKm / EARTH_RADIUS_KM);
  const lngDelta = toDeg(
    radiusKm / (EARTH_RADIUS_KM * Math.cos(toRad(lat))),
  );

  return {
    north: lat + latDelta,
    south: lat - latDelta,
    east: lng + lngDelta,
    west: lng - lngDelta,
  };
}