package main

import (
    "fmt"
    "math"
)

func haversine(θ float64) float64 {
    return .5 * (1 - math.Cos(θ))
}

type pos struct {
    φ float64 // latitude, radians
    ψ float64 // longitude, radians
}

func degPos(lat, lon float64) pos {
    return pos{lat * math.Pi / 180, lon * math.Pi / 180}
}

const rEarth = 6372.8 // km

func hsDist(p1, p2 pos) float64 {
    return 2 * rEarth * math.Asin(math.Sqrt(haversine(p2.φ-p1.φ)+
        math.Cos(p1.φ)*math.Cos(p2.φ)*haversine(p2.ψ-p1.ψ)))
}

func main() {
    fmt.Println(hsDist(degPos(36.12, -86.67), degPos(33.94, -118.40)))
}