tile38/vendor/github.com/tidwall/geojson/circle.go

package geojson

import (
	"strconv"

	"github.com/tidwall/geojson/geo"
	"github.com/tidwall/geojson/geometry"
)

// Circle ...
type Circle struct {
	object    Object
	center    geometry.Point
	meters    float64
	haversine float64
	steps     int
	km        bool
	extra     *extra
}

// NewCircle returns an circle object
func NewCircle(center geometry.Point, meters float64, steps int) *Circle {
	if steps < 3 {
		steps = 3
	}
	g := new(Circle)
	g.center = center
	g.meters = meters
	g.steps = steps
	if meters > 0 {
		meters = geo.NormalizeDistance(meters)
		g.haversine = geo.DistanceToHaversine(meters)
	}
	return g
}

// AppendJSON ...
func (g *Circle) AppendJSON(dst []byte) []byte {
	dst = append(dst, `{"type":"Feature","geometry":`...)
	dst = append(dst, `{"type":"Point","coordinates":[`...)
	dst = strconv.AppendFloat(dst, g.center.X, 'f', -1, 64)
	dst = append(dst, ',')
	dst = strconv.AppendFloat(dst, g.center.Y, 'f', -1, 64)
	dst = append(dst, `]},"properties":{"type":"Circle","radius":`...)
	dst = strconv.AppendFloat(dst, g.meters, 'f', -1, 64)
	dst = append(dst, `,"radius_units":"m"}}`...)
	return dst
}

// JSON ...
func (g *Circle) JSON() string {
	return string(g.AppendJSON(nil))
}

// String ...
func (g *Circle) String() string {
	return string(g.AppendJSON(nil))
}

// Meters returns the circle's radius
func (g *Circle) Meters() float64 {
	return g.meters
}

// Center returns the circle's center point
func (g *Circle) Center() geometry.Point {
	return g.center
}

// Haversine returns the haversine corresponding to circle's radius
func (g *Circle) Haversine() float64 {
	return g.haversine
}

// HaversineTo returns the haversine from a given point to circle's center
func (g *Circle) HaversineTo(p geometry.Point) float64 {
	return geo.Haversine(p.Y, p.X, g.center.Y, g.center.X)
}

// Within returns true if circle is contained inside object
func (g *Circle) Within(obj Object) bool {
	return obj.Contains(g)
}

// containsPoint returns true if circle contains a given point
func (g *Circle) containsPoint(p geometry.Point) bool {
	h := geo.Haversine(p.Y, p.X, g.center.Y, g.center.X)
	return h <= g.haversine
}

// Contains returns true if the circle contains other object
func (g *Circle) Contains(obj Object) bool {
	switch other := obj.(type) {
	case *Point:
		return g.containsPoint(other.Center())
	case *Circle:
		return other.Distance(g) < (other.meters + g.meters)
	case *LineString:
		for i := 0; i < other.base.NumPoints(); i++ {
			if !g.containsPoint(other.base.PointAt(i)) {
				return false
			}
		}
		return true
	case Collection:
		for _, p := range other.Children() {
			if !g.Contains(p) {
				return false
			}
		}
		return true
	default:
		// No simple cases, so using polygon approximation.
		return g.getObject().Contains(other)
	}
}

// intersectsSegment returns true if the circle intersects a given segment
func (g *Circle) intersectsSegment(seg geometry.Segment) bool {
	start, end := seg.A, seg.B

	// These are faster checks.
	// If they succeed there's no need do complicate things.
	if g.containsPoint(start) || g.containsPoint(end) {
		return true
	}

	// Distance between start and end
	l := geo.DistanceTo(start.Y, start.X, end.Y, end.X)

	// Unit direction vector
	dx := (end.X - start.X) / l
	dy := (end.Y - start.Y) / l

	// Point of the line closest to the center
	t := dx*(g.center.X-start.X) + dy*(g.center.Y-start.Y)
	px := t*dx + start.X
	py := t*dy + start.Y
	if px < start.X || px > end.X || py < start.Y || py > end.Y {
		// closest point is outside the segment
		return false
	}

	// Distance from the closest point to the center
	return g.containsPoint(geometry.Point{X: px, Y: py})
}

// Intersects returns true the circle intersects other object
func (g *Circle) Intersects(obj Object) bool {
	switch other := obj.(type) {
	case *Point:
		return g.containsPoint(other.Center())
	case *Circle:
		return other.Distance(g) <= (other.meters + g.meters)
	case *LineString:
		for i := 0; i < other.base.NumSegments(); i++ {
			if g.intersectsSegment(other.base.SegmentAt(i)) {
				return true
			}
		}
		return false
	case Collection:
		for _, p := range other.Children() {
			if g.Intersects(p) {
				return true
			}
		}
		return false
	default:
		// No simple cases, so using polygon approximation.
		return g.getObject().Intersects(obj)
	}
}

// Empty ...
func (g *Circle) Empty() bool {
	return false
}

// ForEach ...
func (g *Circle) ForEach(iter func(geom Object) bool) bool {
	return iter(g)
}

// NumPoints ...
func (g *Circle) NumPoints() int {
	// should this be g.steps?
	return 1
}

// Distance ...
func (g *Circle) Distance(other Object) float64 {
	return g.getObject().Distance(other)
}

// Rect ...
func (g *Circle) Rect() geometry.Rect {
	return g.getObject().Rect()
}

// Spatial ...
func (g *Circle) Spatial() Spatial {
	return g.getObject().Spatial()
}

func (g *Circle) getObject() Object {
	if g.object != nil {
		return g.object
	}
	return makeCircleObject(g.center, g.meters, g.steps)
}

func makeCircleObject(center geometry.Point, meters float64, steps int) Object {
	if meters <= 0 {
		return NewPoint(center)
	}
	meters = geo.NormalizeDistance(meters)
	var points []geometry.Point
	step := 360.0 / float64(steps)
	i := 0
	for deg := 360.0; deg > 0; deg -= step {
		lat, lon := geo.DestinationPoint(center.Y, center.X, meters, deg)
		points = append(points, geometry.Point{X: lon, Y: lat})
		i++
	}
	// TODO: account for the pole and antimerdian. In most cases only a
	// polygon is needed, but when the circle bounds passes the 90/180
	// lines, we need to create a multipolygon
	points = append(points, points[0])
	return NewPolygon(
		geometry.NewPoly(points, nil, geometry.DefaultIndexOptions),
	)
}