function VectorLength(x, y: Extended): Extended;
begin
  VectorLength := sqrt(x*x + y*y);
end;

function AdjustBounds(x: Extended): Extended;
begin
  if x < 0 then
    AdjustBounds := 0.0
  else if x > 1 then
    AdjustBounds := 1.0
  else
    AdjustBounds := x;
end;

procedure TForm1.AdvanceForces(Sender: TObject);
function Signum(x: Extended): Extended;
begin
  if x > 0 then Signum := 1
  else if x = 0 then Signum := 0
  else Signum := -1;
end;
procedure Repulsion(v1, v2: TVertex; var fx, fy, k: Extended);
var dx, dy, d, f: Extended;
begin
  dx := v2.PosX - v1.PosX;
  dy := v2.PosY - v1.PosY;
  d := VectorLength(dx, dy);
  f := -k*k/d;
  fx := fx + f * dx/d;
  fy := fy + f * dy/d;
end;
procedure Attraction(v1, v2: TVertex; var fx, fy, k: Extended);
var dx, dy, d, f: Extended;
begin
  dx := v2.PosX - v1.PosX;
  dy := v2.PosY - v1.PosY;
  d := VectorLength(dx, dy);
  f := d*d/k;
  fx := fx + f * dx/d;
  fy := fy + f * dy/d;
end;
var x, y, vx, vy, fx, fy, k: Extended;
  v1, v2: TVertex;
  n, i, j: Integer;
begin
  if Graph <> nil then
  begin
    n := Graph.Vertices.Count;
    k := 0.5*sqrt(1/n);
    for i := 0 to n-1 do
    begin
      fx := 0;
      fy := 0;
      v1 := Graph.Vertices[i];

      for j := 0 to n-1 do
      begin
        v2 := Graph.Vertices[j];
        if i <> j then
          Repulsion(v1, v2, fx, fy, k);
      end;

      for j := 0 to n-1 do
      begin
        v2 := Graph.Vertices[j];
        if ((i <> j) and (Graph.Incidency[i, j])) then
          Attraction(v1, v2, fx, fy, k);
      end;

      v1.VX := (v1.VX + 0.5 * fx) * DampingC;
      v1.VY := (v1.VY + 0.5 * fy) * DampingC;

      v1.PosX := AdjustBounds(v1.PosX + 0.5 * v1.VX);
      v1.PosY := AdjustBounds(v1.PosY + 0.5 * v1.VY);
    end;
  end;
end;