package main

import (
	"fmt"
	"math"
)

//:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
//:::                                                                         :::
//:::  This routine calculates the distance between two points (given the     :::
//:::  latitude/longitude of those points). It is being used to calculate     :::
//:::  the distance between two locations using GeoDataSource (TM) products  :::
//:::                                                                         :::
//:::  Definitions:                                                           :::
//:::    South latitudes are negative, east longitudes are positive           :::
//:::                                                                         :::
//:::  Passed to function:                                                    :::
//:::    lat1, lon1 = Latitude and Longitude of point 1 (in decimal degrees)  :::
//:::    lat2, lon2 = Latitude and Longitude of point 2 (in decimal degrees)  :::
//:::    unit = the unit you desire for results                               :::
//:::           where: 'M' is statute miles (default)                         :::
//:::                  'K' is kilometers                                      :::
//:::                  'N' is nautical miles                                  :::
//:::                                                                         :::
//:::  Worldwide cities and other features databases with latitude longitude  :::
//:::  are available at https://www.geodatasource.com                         :::
//:::                                                                         :::
//:::  For enquiries, please contact sales@geodatasource.com                  :::
//:::                                                                         :::
//:::  Official Web site: https://www.geodatasource.com                       :::
//:::                                                                         :::
//:::               GeoDataSource.com (C) All Rights Reserved 2022            :::
//:::                                                                         :::
//:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

func distance(lat1 float64, lng1 float64, lat2 float64, lng2 float64, unit ...string) float64 {
	const PI float64 = 3.141592653589793
	
	radlat1 := float64(PI * lat1 / 180)
	radlat2 := float64(PI * lat2 / 180)
	
	theta := float64(lng1 - lng2)
	radtheta := float64(PI * theta / 180)
	
	dist := math.Sin(radlat1) * math.Sin(radlat2) + math.Cos(radlat1) * math.Cos(radlat2) * math.Cos(radtheta)
	
	if dist > 1 {
		dist = 1
	}
	
	dist = math.Acos(dist)
	dist = dist * 180 / PI
	dist = dist * 60 * 1.1515
	
	if len(unit) > 0 {
		if unit[0] == "K" {
			dist = dist * 1.609344
		} else if unit[0] == "N" {
			dist = dist * 0.8684
		}
	}
	
	return dist
}

func main() {
	fmt.Printf("%f Miles\n", distance(32.9697, -96.80322, 29.46786, -98.53506, "M"))
	fmt.Printf("%f Kilometers\n", distance(32.9697, -96.80322, 29.46786, -98.53506, "K"))
	fmt.Printf("%f Nautical Miles\n", distance(32.9697, -96.80322, 29.46786, -98.53506, "N"))
}