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package geo

import "math"

type coordinate struct {
	lat float64
	lon float64
}

// haversin(θ) function
func hsin(theta float64) float64 {
	return math.Pow(math.Sin(theta/2), 2)
}

func Distance(coordinate1, coordinate2 coordinate) float64 {
	var la1, lo1, la2, lo2, r float64
	la1 = coordinate1.lat * math.Pi / 180
	lo1 = coordinate1.lon * math.Pi / 180
	la2 = coordinate2.lat * math.Pi / 180
	lo2 = coordinate2.lon * math.Pi / 180

	r = 6378100 // Earth radius

	// calculate
	h := hsin(la2-la1) + math.Cos(la1)*math.Cos(la2)*hsin(lo2-lo1)

	return 2 * r * math.Asin(math.Sqrt(h))
}