/**
 * Converts numeric degrees to radians
 * @param val
 * @returns {number}
 */
function toRad (val: number): number {
  return val * Math.PI / 180
}

/**
 * This function takes in latitude and longitude of two location and returns the distance between them as the crow flies (in km)
 * @param lat1
 * @param lon1
 * @param lat2
 * @param lon2
 * @returns {number}
 */
function sphericDistance (lat1: number, lon1: number, lat2: number, lon2: number): number {
  const R = 6371 // km
  const dLat = toRad(lat2 - lat1)
  const dLon = toRad(lon2 - lon1)
  lat1 = toRad(lat1)
  lat2 = toRad(lat2)

  const a = Math.sin(dLat / 2) * Math.sin(dLat / 2) +
    Math.sin(dLon / 2) * Math.sin(dLon / 2) * Math.cos(lat1) * Math.cos(lat2)
  const c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a))
  return R * c
}

export default sphericDistance