code: /*******************************************************************************
   * Copyright (c) 2015 VoyagerSearch
   * All rights reserved. This program and the accompanying materials
   * are made available under the terms of the Apache License, Version 2.0 which
   * accompanies this distribution and is available at
   *    http://www.apache.org/licenses/LICENSE-2.0.txt
   ******************************************************************************/
  
  package org.locationtech.spatial4j.io;
  
  import org.locationtech.spatial4j.context.SpatialContext;
  import org.locationtech.spatial4j.context.SpatialContextFactory;
  import org.locationtech.spatial4j.distance.DistanceUtils;
  import org.locationtech.spatial4j.shape.*;
  import org.locationtech.spatial4j.shape.impl.BufferedLine;
  import org.locationtech.spatial4j.shape.impl.BufferedLineString;
  import org.locationtech.spatial4j.shape.impl.GeoCircle;
  
  import java.io.IOException;
  import java.io.StringWriter;
  import java.io.Writer;
  import java.text.NumberFormat;
  import java.util.Iterator;
  
  import static org.locationtech.spatial4j.io.GeoJSONReader.BUFFER;
  import static org.locationtech.spatial4j.io.GeoJSONReader.BUFFER_UNITS;
  
  public class GeoJSONWriter implements ShapeWriter {
  
    public GeoJSONWriter(SpatialContext ctx, SpatialContextFactory factory) {
  
    }
  
    @Override
    public String getFormatName() {
      return ShapeIO.GeoJSON;
    }
  
    protected void write(Writer output, NumberFormat nf, double... coords) throws IOException {
      output.write('[');
      for (int i = 0; i < coords.length; i++) {
        if (Double.isNaN(coords[i])) {
          break; // empty point or no more coordinates
        }
        if (i > 0) {
          output.append(',');
        }
        output.append(nf.format(coords[i]));
      }
      output.write(']');
    }
  
    @Override
    public void write(Writer output, Shape shape) throws IOException {
      if (shape == null) {
        throw new NullPointerException("Shape can not be null");
      }
      NumberFormat nf = LegacyShapeWriter.makeNumberFormat(6);
      if (shape instanceof Point) {
        Point v = (Point) shape;
        output.append("{\"type\":\"Point\",\"coordinates\":");
        write(output, nf, v.getX(), v.getY());
        output.append('}');
        return;
      }
      if (shape instanceof Rectangle) {
        Rectangle v = (Rectangle) shape;
        output.append("{\"type\":\"Polygon\",\"coordinates\":[[");
        write(output, nf, v.getMinX(), v.getMinY());
        output.append(',');
        write(output, nf, v.getMinX(), v.getMaxY());
        output.append(',');
        write(output, nf, v.getMaxX(), v.getMaxY());
        output.append(',');
        write(output, nf, v.getMaxX(), v.getMinY());
        output.append(',');
        write(output, nf, v.getMinX(), v.getMinY());
        output.append("]]}");
        return;
      }
      if (shape instanceof BufferedLine) {
        BufferedLine v = (BufferedLine) shape;
        output.append("{\"type\":\"LineString\",\"coordinates\":[");
        write(output, nf, v.getA().getX(), v.getA().getY());
        output.append(',');
        write(output, nf, v.getB().getX(), v.getB().getY());
        output.append(',');
        output.append("]");
        if (v.getBuf() > 0) {
          output.append(',');
          output.append("\"buffer\":");
          output.append(nf.format(v.getBuf()));
        }
        output.append('}');
        return;
      }
      if (shape instanceof BufferedLineString) {
        BufferedLineString v = (BufferedLineString) shape;
        output.append("{\"type\":\"LineString\",\"coordinates\":[");
        BufferedLine last = null;
        Iterator<BufferedLine> iter = v.getSegments().iterator();
        while (iter.hasNext()) {
          BufferedLine seg = iter.next();
          if (last != null) {
            output.append(',');
          }
          write(output, nf, seg.getA().getX(), seg.getA().getY());
          last = seg;
        }
        if (last != null) {
          output.append(',');
          write(output, nf, last.getB().getX(), last.getB().getY());
        }
        output.append("]");
        if (v.getBuf() > 0) {
          writeDistance(output, nf, v.getBuf(), shape.getContext().isGeo(), BUFFER, BUFFER_UNITS);
        }
        output.append('}');
        return;
      }
      if (shape instanceof Circle) {
        // See: https://github.com/geojson/geojson-spec/wiki/Proposal---Circles-and-Ellipses-Geoms
        Circle v = (Circle) shape;
        Point center = v.getCenter();
        output.append("{\"type\":\"Circle\",\"coordinates\":");
        write(output, nf, center.getX(), center.getY());
        writeDistance(output, nf, v.getRadius(), v instanceof GeoCircle, "radius", "radius_units");
        output.append("}");
        return;
      }
      if (shape instanceof ShapeCollection) {
        ShapeCollection v = (ShapeCollection) shape;
        output.append("{\"type\":\"GeometryCollection\",\"geometries\":[");
        for (int i = 0; i < v.size(); i++) {
          if (i > 0) {
            output.append(',');
          }
          write(output, v.get(i));
        }
        output.append("]}");
        return;
      }
      output.append("{\"type\":\"Unknown\",\"wkt\":\"");
      output.append(LegacyShapeWriter.writeShape(shape));
      output.append("\"}");
    }
  
    /**
     * Helper method to encode a distance property (with optional unit). 
     * <p>
     *  The distance unit is only encoded when <tt>isGeo</tt> is true, and it is converted to km. 
     * </p>
     * <p>
     *  The distance unit is encoded within a properties object.
     * </p>
     * @param output The writer.
     * @param nf The number format.
     * @param dist The distance value to encode.
     * @param isGeo The flag determining 
     * @param distProperty The distance property name.
     * @param distUnitsProperty The distance unit property name.
     */
    void writeDistance(Writer output, NumberFormat nf, double dist, boolean isGeo, String distProperty, String distUnitsProperty) 
        throws IOException {
      output.append(",\"").append(distProperty).append("\":");
      if (isGeo) {
        double distKm =
            DistanceUtils.degrees2Dist(dist, DistanceUtils.EARTH_MEAN_RADIUS_KM);
        output.append(nf.format(distKm));
        output.append(",\"properties\":{");
        output.append("\"").append(distUnitsProperty).append("\":\"km\"}");
      } else {
        output.append(nf.format(dist));
      }
    }
  
    @Override
    public String toString(Shape shape) {
      try {
        StringWriter buffer = new StringWriter();
        write(buffer, shape);
        return buffer.toString();
      } catch (IOException ex) {
        throw new RuntimeException(ex);
      }
    }
  }
  