diff --git a/halfedge.pde b/halfedge.pde
new file mode 100644
index 0000000..0aa484f
--- /dev/null
+++ b/halfedge.pde
@@ -0,0 +1,275 @@
+class HalfEdge {
+  HalfEdge prev;
+  HalfEdge next;
+  HalfEdge twin;
+  Vertex v;
+  
+  HalfEdge(Vertex vv) {
+    v =vv;
+  }
+  
+  void connectTo(HalfEdge h) {
+    next = h;
+    h.prev = this;
+  }
+  
+  // Disconnect both a halfedge and its twin.
+  void detach() {
+    if (v.isLeaf()) { 
+      v.h = null;
+    } 
+    else {
+      prev.connectTo(twin.next);
+      v.h = twin.next;
+    }
+    if (twin.v.isLeaf()) { 
+      twin.v.h = null; 
+    }
+    else {
+      twin.prev.connectTo(next);
+      twin.v.h = next;
+    }
+  }
+  
+  void draw() {
+    strokeWeight(2);
+    stroke(100,100,200);
+    Vertex u = twin.v;
+    float x1 = v.x;
+    float x2 = u.x;
+    float y1 = v.y;
+    float y2 = u.y;
+    float dx = x2 - x1;
+    float dy = y2 - y1;
+    float d = sqrt(dx * dx + dy * dy);
+    float offset = 4;
+    float t = 0.75;
+    line(x1 + offset*(dx+dy)/d, y1 + offset*(dy-dx)/d, x1 + offset*dy/d + t*dx, y1 + t*dy - offset*dx/d);
+  }  
+}
+
+class Vertex {
+  float x,y;
+  HalfEdge h;
+  
+  Vertex(float xx, float yy) {
+    x = xx;
+    y = yy;
+    h = null;
+  }
+  
+  void draw(){
+    fill(0);
+    noStroke();
+    ellipse(x,y,2,2);
+  }
+
+  HalfEdge handle(Vertex u) {
+    if (isIsolated() || isLeaf()) { return h; }
+    HalfEdge h1 = h, h2 = h.prev.twin;
+    while (!ordered(h1.twin.v,u,h2.twin.v)) {
+      h1 = h2;
+      h2 = h1.prev.twin;      
+    }
+    return h1;
+  }
+  
+  boolean isIsolated() {
+    return (h == null);
+  }
+  
+  boolean isLeaf() {
+    return (!isIsolated()) && (h.twin == h.prev);
+  }
+  
+  boolean ccw(Vertex a, Vertex b) {
+    return ((a.y-y) * (b.x-x) - (a.x-x) * (b.y-y) >= 0);
+  }
+  
+  boolean ordered(Vertex a, Vertex b, Vertex c) {
+    boolean I   = ccw(a,b);
+    boolean II  = ccw(b,c);
+    boolean III = ccw(c,a);
+    return ((I && (II || III)) || (II && III)); // at least two must be true
+  } 
+  
+  void reconnect() {
+    ArrayList<Vertex> nbrs = new ArrayList<Vertex>();
+    while(!isIsolated()) {
+      nbrs.add(h.twin.v);
+      h.detach();
+    }
+    for (Vertex n : nbrs) { attach(this, n); }
+  }
+}
+
+class Edge {
+  Vertex s,t;
+  
+  Edge(Vertex ss, Vertex tt) {
+    s = ss;
+    t = tt;
+  }
+  
+  void draw() {
+    strokeWeight(2);
+    stroke(256,150,150);
+    line(s.x, s.y, t.x, t.y);
+  }
+  
+}
+
+ArrayList<Vertex> vertex = new ArrayList<Vertex>();  
+ArrayList<Edge> edge = new ArrayList<Edge>();
+boolean draggingVertex = false;
+boolean edgeStarted = false;
+HalfEdge activeHalfEdge;
+Vertex activeVertex;
+float xOffset;
+float yOffset;
+
+void setup() {
+  size(800, 800);
+  smooth();
+  ellipseMode(RADIUS);
+  
+  Vertex u = newVertex(100, 100);
+  Vertex v = newVertex(200,300);
+  newEdge(u,v);
+  activeHalfEdge = u.h;
+}
+
+void draw() {
+  background(255);
+  for(Edge e :edge) {
+    e.draw();
+  }
+  for(Vertex v : vertex) {
+    v.draw();
+  }
+  
+  activeHalfEdge.draw();
+  
+  Vertex v = whichVertex(mouseX, mouseY);
+  if(v != null) {
+    strokeWeight(2);
+    noFill();
+    stroke(150,150,256);
+    ellipse(v.x,v.y,5,5);
+//    Vertex u = activeHalfEdge.v;
+//    HalfEdge h = v.handle(u);
+//    if (h != null) { h.draw(); }
+//    h = u.handle(v);
+//    if (h != null) { h.draw(); }
+  } 
+  if(edgeStarted) {
+    strokeWeight(2);
+    noFill();
+    stroke(150,150,256);
+    line(activeVertex.x, activeVertex.y, mouseX, mouseY);
+  }
+}
+
+Vertex newVertex(float x, float y) {
+  Vertex v = new Vertex(x,y);
+  vertex.add(v);
+  return v;
+}
+
+Edge newEdge(Vertex s, Vertex t) {
+  // todo: check if edge already exists.
+  Edge e = new Edge(s, t);
+  edge.add(e);
+  attach(s,t);
+  return e;
+}
+
+void attach(Vertex s, Vertex t) {
+  HalfEdge h1 = new HalfEdge(s);
+  HalfEdge h2 = new HalfEdge(t);
+  h1.twin = h2;
+  h2.twin = h1;
+  if (s.h == null) {
+    h2.connectTo(h1);
+    s.h = h1;
+  }
+  if (t.h == null) {
+    h1.connectTo(h2);
+    t.h = h2;    
+  }
+  HalfEdge sh = s.handle(t);
+  HalfEdge th = t.handle(s);
+  sh.prev.connectTo(h1);
+  th.prev.connectTo(h2);
+  h2.connectTo(sh);
+  h1.connectTo(th);  
+}
+
+Vertex whichVertex(float x, float y) {
+  if(vertex.isEmpty()) { return null; }
+  Vertex nearest = null;
+  float d_min = 50;
+  float d; 
+  for(Vertex v: vertex) {
+    d = squaredDistance(v.x, v.y, x,y);
+    if(d < d_min) {
+      d_min = d;
+      nearest = v;
+    } 
+  }
+  return nearest;
+}
+
+void keyPressed() {
+  if(key == 'p') { activeHalfEdge = activeHalfEdge.prev; } 
+  if(key == 'n') { activeHalfEdge = activeHalfEdge.next; } 
+  if(key == 't') { activeHalfEdge = activeHalfEdge.twin; } 
+}
+
+void mousePressed() {
+  Vertex v = whichVertex(mouseX,mouseY);
+  if(v == null) { 
+    v = newVertex(mouseX, mouseY);
+    if (edgeStarted) {
+      newEdge(activeVertex, v);
+      edgeStarted = false;
+    }
+  }  
+  else {
+    if (edgeStarted) {
+      newEdge(activeVertex, v);
+      edgeStarted = false;
+    }
+    else {
+      edgeStarted = true;
+      draggingVertex = true;
+      activeVertex = v;
+      xOffset = v.x - mouseX;
+      yOffset = v.y - mouseY;
+    }
+  } 
+}
+
+void mouseDragged () {
+  edgeStarted = false;
+  if(draggingVertex) {
+    activeVertex.x = mouseX + xOffset;
+    activeVertex.y = mouseY + yOffset; 
+    activeVertex.reconnect(); 
+
+  }
+}
+
+void mouseReleased() {
+  if(draggingVertex) { 
+    draggingVertex = false;
+  }
+}
+
+float squaredDistance(float x1, float y1, float x2, float y2) {
+  return squaredNorm(x1-x2, y1-y2);
+}
+
+float squaredNorm(float x, float y) {
+  return x*x + y*y;
+}