Index: trunk/src/org/openstreetmap/josm/actions/CreateCircleAction.java
===================================================================
--- trunk/src/org/openstreetmap/josm/actions/CreateCircleAction.java	(revision 996)
+++ trunk/src/org/openstreetmap/josm/actions/CreateCircleAction.java	(revision 996)
@@ -0,0 +1,183 @@
+// License: GPL. Copyright 2007 by Immanuel Scholz and others
+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.AddCommand;
+import org.openstreetmap.josm.command.ChangeCommand;
+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;
+import org.openstreetmap.josm.data.osm.Way;
+
+/**
+ * Create a new circle from three selected nodes--or a way with 3 nodes. (Useful for roundabouts)
+ * Note: If a way is selected, it is changed. If nodes are selected a new way is created.
+ *       So if you've got a way with 3 nodes it makes a difference between running this on the way or the nodes!
+ * BTW: Someone might want to implement projection corrections for this...
+ *
+ * @author Henry Loenwind, based on much copy&Paste from other Actions.
+ */
+public final class CreateCircleAction extends JosmAction {
+
+	public CreateCircleAction() {
+		super(tr("Create Circle"), "createcircle", tr("Create a circle from three selected nodes."), KeyEvent.VK_O, KeyEvent.CTRL_MASK, true);
+	}
+
+	private double calcang(double xc, double yc, double x, double y) {
+		// calculate the angle from xc|yc to x|y
+		if (xc == x && yc == y) {
+			return 0; // actually invalid, but we won't have this case in this context
+		}
+		double yd = Math.abs(y - yc);
+		if (yd == 0 && xc < x) {
+			return 0;
+		}
+		if (yd == 0 && xc > x) {
+			return Math.PI;
+		}
+		double xd = Math.abs(x - xc);
+		double a = Math.atan2(xd, yd);
+		if (y > yc) {
+			a = Math.PI - a;
+		}
+		if (x < xc) {
+			a = -a;
+		}
+		a = 1.5*Math.PI + a;
+		if (a < 0) {
+			a += 2*Math.PI;
+		}
+		if (a >= 2*Math.PI) {
+			a -= 2*Math.PI;
+		}
+		return a;
+	}
+
+	public void actionPerformed(ActionEvent e) {
+		int numberOfNodesInCircle = Integer.parseInt(Main.pref.get("createcircle.nodecount", "8"));
+		if (numberOfNodesInCircle < 1) {
+			numberOfNodesInCircle = 1;
+		} else if (numberOfNodesInCircle > 100) {
+			numberOfNodesInCircle = 100;
+		}
+
+		Collection<OsmPrimitive> sel = Main.ds.getSelected();
+		Collection<Node> nodes = new LinkedList<Node>();
+		Way existingWay = null;
+
+		for (OsmPrimitive osm : sel)
+			if (osm instanceof Node)
+				nodes.add((Node)osm);
+
+		// special case if no single nodes are selected and exactly one way is:
+		// then use the way's nodes
+		if ((nodes.size() == 0) && (sel.size() == 1))
+			for (OsmPrimitive osm : sel)
+				if (osm instanceof Way) {
+					existingWay = ((Way)osm);
+					for (Node n : ((Way)osm).nodes)
+					{
+						if(!nodes.contains(n))
+							nodes.add(n);
+					}
+				}
+
+		if (nodes.size() != 3) {
+			JOptionPane.showMessageDialog(Main.parent, tr("Please select exactly three nodes or one way with exactly three nodes."));
+			return;
+		}
+
+		// let's get some shorter names
+		Node   n1 = ((Node)nodes.toArray()[0]);
+		double x1 = n1.eastNorth.east();
+		double y1 = n1.eastNorth.north();
+		Node   n2 = ((Node)nodes.toArray()[1]);
+		double x2 = n2.eastNorth.east();
+		double y2 = n2.eastNorth.north();
+		Node   n3 = ((Node)nodes.toArray()[2]);
+		double x3 = n3.eastNorth.east();
+		double y3 = n3.eastNorth.north();
+
+		// calculate the center (xc/yc)
+		double s = 0.5*((x2 - x3)*(x1 - x3) - (y2 - y3)*(y3 - y1));
+		double sUnder = (x1 - x2)*(y3 - y1) - (y2 - y1)*(x1 - x3);
+
+		if (sUnder == 0) {
+			JOptionPane.showMessageDialog(Main.parent, tr("Those nodes are not in a circle."));
+			return;
+		}
+
+		s /= sUnder;
+
+		double xc = 0.5*(x1 + x2) + s*(y2 - y1);
+		double yc = 0.5*(y1 + y2) + s*(x1 - x2);
+
+		// calculate the radius (r)
+		double r = Math.sqrt(Math.pow(xc-x1,2) + Math.pow(yc-y1,2));
+
+		// find where to put the existing nodes
+		double a1 = calcang(xc, yc, x1, y1);
+		double a2 = calcang(xc, yc, x2, y2);
+		double a3 = calcang(xc, yc, x3, y3);
+		if (a1 < a2) { double at = a1; Node nt = n1; a1 = a2; n1 = n2; a2 = at; n2 = nt; }
+		if (a2 < a3) { double at = a2; Node nt = n2; a2 = a3; n2 = n3; a3 = at; n3 = nt; }
+		if (a1 < a2) { double at = a1; Node nt = n1; a1 = a2; n1 = n2; a2 = at; n2 = nt; }
+
+		// now we can start doing thigs to OSM data
+		Collection<Command> cmds = new LinkedList<Command>();
+
+		// build a way for the circle
+		Way wayToAdd;
+		if (existingWay == null) {
+			wayToAdd = new Way();
+		} else {
+			// re-use existing way if it was selected
+			wayToAdd = new Way(existingWay);
+			wayToAdd.nodes.clear();
+		}
+		for (int i = 1; i <= numberOfNodesInCircle; i++) {
+			double a = 2*Math.PI*(1.0 - i/(double)numberOfNodesInCircle); // "1-" to get it clock-wise
+			// insert existing nodes if they fit before this new node (999 means "already added this node")
+			if (a1 < 999 && a1 > a) {
+				wayToAdd.nodes.add(n1);
+				a1 = 999;
+			}
+			if (a2 < 999 && a2 > a) {
+				wayToAdd.nodes.add(n2);
+				a2 = 999;
+			}
+			if (a3 < 999 && a3 > a) {
+				wayToAdd.nodes.add(n3);
+				a3 = 999;
+			}
+			// get the position of the new node and insert it
+			double x = xc + r*Math.cos(a);
+			double y = yc + r*Math.sin(a);
+			Node n = new Node(Main.proj.eastNorth2latlon(new EastNorth(x,y)));
+			wayToAdd.nodes.add(n);
+			cmds.add(new AddCommand(n));
+		}
+		wayToAdd.nodes.add(wayToAdd.nodes.get(0)); // close the circle
+		if (existingWay == null) {
+			cmds.add(new AddCommand(wayToAdd));
+		} else {
+			cmds.add(new ChangeCommand(existingWay, wayToAdd));
+		}
+
+		Main.main.undoRedo.add(new SequenceCommand(tr("Create Circle"), cmds));
+		Main.map.repaint();
+	}
+}
Index: trunk/src/org/openstreetmap/josm/gui/MainMenu.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/MainMenu.java	(revision 995)
+++ trunk/src/org/openstreetmap/josm/gui/MainMenu.java	(revision 996)
@@ -22,4 +22,5 @@
 import org.openstreetmap.josm.actions.CombineWayAction;
 import org.openstreetmap.josm.actions.CopyAction;
+import org.openstreetmap.josm.actions.CreateCircleAction;
 import org.openstreetmap.josm.actions.DeleteAction;
 import org.openstreetmap.josm.actions.DownloadAction;
@@ -102,4 +103,5 @@
 	public final JosmAction alignInLine = new AlignInLineAction();
 	public final JosmAction alignInRect = new AlignInRectangleAction();
+	public final JosmAction createCircle = new CreateCircleAction();
 	public final JosmAction mergeNodes = new MergeNodesAction();
 	public final JosmAction joinNodeWay = new JoinNodeWayAction();
@@ -229,4 +231,7 @@
 		current.setAccelerator(alignInRect.shortCut);
 		toolsMenu.addSeparator();
+		current = toolsMenu.add(createCircle);
+		current.setAccelerator(createCircle.shortCut);
+		toolsMenu.addSeparator();
 		current = toolsMenu.add(mergeNodes);
 		current.setAccelerator(mergeNodes.shortCut);