S2: get bounding box of center point (in degrees) and radius

main.go

package main

import (
	"fmt"
	"math"
	"strconv"

	"github.com/golang/geo/s2"
)

const earthRadiusM = 6371000 // per https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html
const hashLength = 8         // < 1km per https://github.com/rh389/dynamodb-geo.js/blob/master/test/integration/hashKeyLength.ts

func main() {
	lowPrefix := uint64(0)
	highPrefix := uint64(0)
	ctrLat := 52.225730 // Cambridge UK
	ctrLng := 0.149593

	boundingSq := squareFromCenterAndRadius(ctrLat, ctrLng, 500)
	fmt.Printf("\nBounding sq %+v\n", boundingSq)

	coveringCells := getCoveringCells(boundingSq)
	fmt.Printf("Covering Cells (%d):\n", len(coveringCells))

	for idx, cell := range coveringCells {
		// cell is the UUID of the center of this cell
		fullHash, hashPrefix := genCellIntPrefix(cell)
		if 0 == idx {
			lowPrefix = hashPrefix
			highPrefix = hashPrefix
		} else if hashPrefix < lowPrefix {
			lowPrefix = hashPrefix
		} else if hashPrefix > highPrefix {
			highPrefix = hashPrefix
		}

		fmt.Printf("\tID:%19v uint64: %-19d prefix: %-10d Range: %-19d - %-19d\n", cell, fullHash, hashPrefix, uint64(cell.RangeMin()), uint64(cell.RangeMax()))
	}

	fmt.Printf("\tPrefix Range from loop: %-10d - %-10d\n", lowPrefix, highPrefix)

	// TODO: Assuming covering cells are sorted.  Correct assumption?
	_, lowPrefix = genCellIntPrefix(coveringCells[0].RangeMin())
	_, highPrefix = genCellIntPrefix(coveringCells[len(coveringCells)-1].RangeMax())

	fmt.Printf("\tPrefix Range direct:    %-10d - %-10d\n", lowPrefix, highPrefix)
}

// Get bounding box square from center point and radius
// Boundnig box is not extremely accurate to the radiusMeters passed in
// @see https://gis.stackexchange.com/questions/80809/calculating-bounding-box-coordinates-based-on-center-and-radius
func squareFromCenterAndRadius(centerLatDegrees float64, centerLngDegrees float64, radiusMeters float32) s2.Rect {
	latLng := s2.LatLngFromDegrees(centerLatDegrees, centerLngDegrees)

	deltaLng := float64(360 * radiusMeters / earthRadiusM) //Search Radius, difference in lat
	deltaLat := deltaLng * math.Cos(latLng.Lng.Radians())  //Search Radius, difference in lng

	lowerLeftLatDeg := centerLatDegrees - deltaLat
	lowerLeftLngDeg := centerLngDegrees - deltaLng
	lowerLeft := s2.LatLngFromDegrees(lowerLeftLatDeg, lowerLeftLngDeg) // AKA s2.Rect.Lo

	upperRightLatDeg := centerLatDegrees + deltaLat
	upperRightLngDeg := centerLngDegrees + deltaLng
	upperRight := s2.LatLngFromDegrees(upperRightLatDeg, upperRightLngDeg) // AKA s2.Rect.Hi

	boundingSquare := s2.RectFromLatLng(lowerLeft).AddPoint(upperRight)

	return boundingSquare
}

func getCoveringCells(boundingRect s2.Rect) s2.CellUnion {
	// defaults per https://github.com/vekexasia/nodes2-ts/blob/1952d8c1f6cb4a862731ace2d5f74d472ec22e55/src/S2RegionCoverer.ts#L101
	rc := &s2.RegionCoverer{
		MinLevel: 12, // 3km^2 per http://s2geometry.io/resources/s2cell_statistics
		MaxLevel: 20, // 46m^2 per http://s2geometry.io/resources/s2cell_statistics
		MaxCells: 8,
		LevelMod: 1,
	}
	return rc.Covering(boundingRect)
}

func genCellIntPrefix(cell s2.CellID) (hash uint64, prefix uint64) {
	hash = uint64(cell)
	geohashString := strconv.FormatUint(hash, 10)
	denominator := math.Pow10(len(geohashString) - hashLength)
	prefix = hash / uint64(denominator)
	return
}