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polygon.go
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polygon.go
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package circletopolygon
import (
"fmt"
geojson "github.com/paulmach/go.geojson"
)
// ErrPolygonNotClosed is returned when the first and last points don't match:
var ErrPolygonNotClosed = fmt.Errorf("Polygons must start and finish on the same point")
// Polygon is a shape made up of points:
type Polygon []*Point
// GeoJSON renders a polygon as a GeoJSON geometry:
func (p Polygon) GeoJSON() ([]byte, error) {
// Validate first:
if err := p.Validate(); err != nil {
return nil, err
}
// Build up a slice of points for the GeoJSON library:
geometryPoints := [][]float64{}
for _, point := range p {
geometryPoints = append(geometryPoints, []float64{
float64(point.Longitude),
float64(point.Latitude),
})
}
// Use them to make a GeoJSON polygon:
geoJSONPolygon := geojson.NewPolygonGeometry([][][]float64{
geometryPoints,
})
return geoJSONPolygon.MarshalJSON()
}
// Validate that our Polygon makes sense:
func (p Polygon) Validate() error {
// Get the points:
points := p
// Make sure the first point is the same as the last (that we have a "closed" geometry):
if points[0].Latitude != points[len(points)-1].Latitude || points[0].Longitude != points[len(points)-1].Longitude {
return ErrPolygonNotClosed
}
// Validate all of the points:
for _, point := range points {
if err := point.Validate(); err != nil {
return err
}
}
return nil
}