// Copyright © 2016 Abcum Ltd
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package geof

import (
	"math"

	"github.com/abcum/surreal/sql"
)

const earthr float64 = 6371000

const radian float64 = math.Pi / 180

func Haversine(a, b *sql.Point) float64 {
	lata, lnga := a.LA*radian, a.LO*radian
	latb, lngb := b.LA*radian, b.LO*radian
	latc, lngc := lata-latb, lnga-lngb
	dis := math.Pow(math.Sin(latc/2), 2) + math.Cos(lata)*math.Cos(latb)*math.Pow(math.Sin(lngc/2), 2)
	c := 2 * math.Atan2(math.Sqrt(dis), math.Sqrt(1-dis))
	return earthr * c
}