[go] Having issues getting the bounding CellIDs from a loop / polygon

[Chance Dinkins]

Hey folks,

First of all, thank you for the library! I appreciate all of the hard work you have put into it. 

Second, I'm having trouble getting the bounding CellIDs. I am looking to obtain the largest possible cell IDs possible, hopefully of various sizes, to completely fill the given polygons. I assume that  CellUnionBound() would do the trick but I'm getting the same output for all polygons. I'm not sure where I'm going wrong or if my assumption on which func to use is accurate. Thank you for taking the time to read this! 

The output I'm getting is: 

1152921504606846976, 3458764513820540928, 5764607523034234880, 8070450532247928832, 10376293541461622784, 12682136550675316736

gist link: https://gist.github.com/chanced/f3a93ad383eddb5ce7c40379bb205cb9

package main

import (

"encoding/json"

"fmt"

"os"

"github.com/golang/geo/s2"

"github.com/twpayne/go-geom"

"github.com/twpayne/go-geom/encoding/geojson"

)

type Location struct {

Geometry json.RawMessage `json:"geometry"`

}

func main() {

f, err := os.Open("./geo.json")

if err != nil {

panic(err)

}

s, _ := f.Stat()

b := make([]byte, s.Size())

f.Read(b)

loc := &Location{}

var g geom.T

err = json.Unmarshal(b, &loc)

if err != nil {

panic(err)

}

err = geojson.Unmarshal(loc.Geometry, &g)

if err != nil {

panic(err)

}

mp := g.(*geom.MultiPolygon)

c := mp.Coords()

for _, polygon := range c {

var xv float64

var xi = -1

points := []s2.Point{}

for _, l := range polygon {

for i := 0; i < len(l)-1; i++ {

p := l[i]

x := p.X()

latlng := s2.LatLngFromDegrees(p.Y(), x)

point := s2.PointFromLatLng(latlng)

points = append(points, point)

if xi == -1 || x > xv {

xv = x

xi = i

}

}

}

if xi == -1 {

continue

}

points = append(points[xi:], points[:xi]...)

loop := s2.LoopFromPoints(points)

printUnionBound(loop.CellUnionBound())

polygon := s2.PolygonFromLoops([]*s2.Loop{loop})

printUnionBound(polygon.CellUnionBound())

}

}

func printUnionBound(cellIDs []s2.CellID) {

l := len(cellIDs)

for i := 0; i < l; i++ {

cellID := cellIDs[i]

if i == l-1 {

fmt.Printf("%d\n", cellID)

} else {

fmt.Printf("%d, ", cellID)

}

}

}