autocomplete_distance.go

package main

import (
	"errors"
	"math"
	"strconv"
	"strings"
)

type Position struct {
	Latitude  float64 `json:"stop_lat"`
	Longitude float64 `json:"stop_lon"`
}

func PositionFromString(data string) (Position, error) {
	var err error
	var position Position

	split := strings.Split(data, ",")
	if len(split) != 2 {
		return position, errors.New("incorrect format")
	}

	if position.Latitude, err = strconv.ParseFloat(split[0], 64); err != nil {
		return position, err
	}
	if position.Longitude, err = strconv.ParseFloat(split[1], 64); err != nil {
		return position, err
	}

	return position, nil
}

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

// Distance function returns the distance (in meters) between two points of
//     a given longitude and latitude relatively accurately (using a spherical
//     approximation of the Earth) through the Haversin Distance Formula for
//     great arc distance on a sphere with accuracy for small distances
//
// point coordinates are supplied in degrees and converted into rad. in the func
//
// distance returned is METERS!!!!!!
// http://en.wikipedia.org/wiki/Haversine_formula
func Distance(a Position, b Position) float64 {
	// convert to radians
	// must cast radius as float to multiply later
	var la1, lo1, la2, lo2, r float64
	la1 = a.Latitude * math.Pi / 180
	lo1 = a.Longitude * math.Pi / 180
	la2 = b.Latitude * math.Pi / 180
	lo2 = b.Longitude * math.Pi / 180

	r = 6378100 // Earth radius in METERS

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

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