source: josm/src/org/openstreetmap/josm/actions/AlignInCircleAction.java@ 146

package org.openstreetmap.josm.actions;

import static org.openstreetmap.josm.tools.I18n.tr;

import java.awt.event.ActionEvent;
import java.awt.event.KeyEvent;
import java.util.Collection;
import java.util.LinkedList;

import javax.swing.JOptionPane;

import org.openstreetmap.josm.Main;
import org.openstreetmap.josm.command.Command;
import org.openstreetmap.josm.command.MoveCommand;
import org.openstreetmap.josm.command.SequenceCommand;
import org.openstreetmap.josm.data.coor.EastNorth;
import org.openstreetmap.josm.data.coor.LatLon;
import org.openstreetmap.josm.data.osm.Node;
import org.openstreetmap.josm.data.osm.OsmPrimitive;

/**
 * Aligns all selected nodes within a circle. (Usefull for roundabouts)
 * 
 * @author Matthew Newton
 */
public final class AlignInCircleAction extends JosmAction {

        public AlignInCircleAction() {
                super(tr("Align Nodes in Circle"), "aligncircle", tr("Move the selected nodes into a circle."), KeyEvent.VK_O, KeyEvent.CTRL_MASK | KeyEvent.SHIFT_MASK);
        }

        public void actionPerformed(ActionEvent e) {
                Collection<OsmPrimitive> sel = Main.ds.getSelected();
                Collection<Node> nodes = new LinkedList<Node>();
                for (OsmPrimitive osm : sel)
                        if (osm instanceof Node)
                                nodes.add((Node)osm);
                if (nodes.size() < 4) {
                        JOptionPane.showMessageDialog(Main.parent, tr("Please select at least four nodes."));
                        return;
                }

                // Get average position of all nodes
                Node avn = new Node(new LatLon(0,0));
                for (Node n : nodes) {
                        avn.eastNorth = new EastNorth(avn.eastNorth.east()+n.eastNorth.east(), avn.eastNorth.north()+n.eastNorth.north());
                        avn.coor = Main.proj.eastNorth2latlon(avn.eastNorth);
                }
                avn.eastNorth = new EastNorth(avn.eastNorth.east()/nodes.size(), avn.eastNorth.north()/nodes.size());
                avn.coor = Main.proj.eastNorth2latlon(avn.eastNorth);
                // Node "avn" now is central to all selected nodes.

                // Now calculate the average distance to each node from the
                // centre.
                double avdist = 0;
                for (Node n : nodes)
                        avdist += Math.sqrt(avn.eastNorth.distance(n.eastNorth));
                avdist = avdist / nodes.size();

                Collection<Command> cmds = new LinkedList<Command>();
                // Move each node to that distance from the centre.
                for (Node n : nodes) {
                        double dx = n.eastNorth.east() - avn.eastNorth.east();
                        double dy = n.eastNorth.north() - avn.eastNorth.north();
                        double dist = Math.sqrt(avn.eastNorth.distance(n.eastNorth));
                        cmds.add(new MoveCommand(n, (dx * (avdist / dist)) - dx, (dy * (avdist / dist)) - dy));
                }

                Main.main.editLayer().add(new SequenceCommand(tr("Align Nodes in Circle"), cmds));
                Main.ds.setSelected(avn);
                Main.map.repaint();
        }
}