// Package geo turns a timezone or an IP into a point on the globe and picks
// the nearest of a set of AWS regions (nearest-region spec §6). It holds
// points only for the length of one call: nothing here stores, logs or
// returns a caller's coordinates.
package geo

import "math"

// Point is a latitude and longitude in degrees.
type Point struct{ Lat, Lon float64 }

// earthRadiusKm is the IUGG mean Earth radius.
const earthRadiusKm = 6371.0088

// DistanceKm is the great-circle (haversine) distance between a and b.
func DistanceKm(a, b Point) float64 {
	rad := func(d float64) float64 { return d * math.Pi / 180 }
	dLat := rad(b.Lat - a.Lat)
	dLon := rad(b.Lon - a.Lon)
	h := math.Sin(dLat/2)*math.Sin(dLat/2) +
		math.Cos(rad(a.Lat))*math.Cos(rad(b.Lat))*math.Sin(dLon/2)*math.Sin(dLon/2)
	return 2 * earthRadiusKm * math.Asin(math.Sqrt(h))
}