haversine.ts

type GpsCoordinate = number;
type DistanceMeasure = number;
type DistanceValue = number;
type Degrees = number;
type Radians = number;

interface GpsCoordinates {
  lat1: GpsCoordinate;
  long1: GpsCoordinate;
  lat2: GpsCoordinate;
  long2: GpsCoordinate;
}

function convertKmToMiles(km: DistanceMeasure): DistanceMeasure {
  return km * 0.621371;
}

function distanceInKm(distance: DistanceValue): DistanceValue {
  const EARTH_RADIUS_KM = 6371;
  return distance * EARTH_RADIUS_KM;
}

function degreesToRadians(degrees: Degrees): Radians {
  return degrees * (Math.PI / 180);
}

export default function distanceBetweenPointsInMiles(coordinates: GpsCoordinates): DistanceValue {
  const { lat1, long1, lat2, long2 } = coordinates;
  const lat: Radians = degreesToRadians(lat2 - lat1);
  const long: Radians = degreesToRadians(long2 - long1);

  /**
   * This uses the ‘haversine’ formula to calculate the great-circle distance between two points.
   * Read more: https://www.movable-type.co.uk/scripts/latlong.html
   */
  const a: number =
    Math.sin(lat / 2) * Math.sin(lat / 2) +
    Math.cos(degreesToRadians(lat1)) * Math.cos(degreesToRadians(lat2)) *
    Math.sin(long / 2) * Math.sin(long / 2);

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

  return convertKmToMiles(d);
}