unit Unit1;

{$mode objfpc}{$H+}

interface

uses

  Classes, SysUtils, FileUtil, Forms, Controls, Graphics, Dialogs, ExtCtrls;

type

  { TForm1 }

  TForm1 = class(TForm)

    Timer1: TTimer;

    Timer2: TTimer;

    procedure FormCreate(Sender: TObject);

    procedure FormMouseMove(Sender: TObject; Shift: TShiftState; X, Y: Integer);

    procedure FormPaint(Sender: TObject);

    procedure Timer1Timer(Sender: TObject);

    procedure Timer2Timer(Sender: TObject);

  private

    { private declarations }

  public

    { public declarations }

  end;

const

  pointR = 5;

  ballR = 40;

  g = 0.002;

var

  Form1: TForm1;

  maxPoints: integer = 100;

  points: array of TPoint;

  bx, by, vx, vy: single;

implementation

{$R *.lfm}

procedure addPoint(x, y: integer);

var

  i, l: integer;

begin

  if maxPoints <= 0 then exit;

  l := Length(points);

  if l < maxPoints then begin

    SetLength(points, l+1);

    points[l].X := x;

    points[l].Y := y;

  end else begin

    for i:=1 to l-1 do begin

      points[i-1] := points[i];

    end;

    points[l-1].X := x;

    points[l-1].Y := y;

  end;

end;

procedure TForm1.FormCreate(Sender: TObject);

begin

  bx := 200;

  by := 100;

  vx := 0.1;

  vy := 0;

end;

procedure TForm1.FormMouseMove(Sender: TObject; Shift: TShiftState; X,

  Y: Integer);

begin

  if ssLeft in Shift then begin

    addPoint(x, y);

    // todo: addPointChain

  end;

end;

procedure TForm1.FormPaint(Sender: TObject);

var

  i: integer;

begin

  Canvas.Pen.Color := clBlack;

  Canvas.Pen.Width := pointR * 2;

  for i:=0 to length(points)-1 do begin

    Canvas.MoveTo(points[i].x, points[i].y);

    Canvas.LineTo(points[i].x, points[i].y);

  end;

  Canvas.Pen.Color := clBlue;

  Canvas.Pen.Width := ballR * 2;

  Canvas.MoveTo(round(bx), round(by));

  Canvas.LineTo(round(bx), round(by));

end;

procedure TForm1.Timer1Timer(Sender: TObject);

var

  i: integer;

  dx, dy, sqlen, minSqlen, dv: single;

  collision: boolean;

begin

  collision := false;

  bx := bx + vx;

  by := by + vy;

  if bx > ClientWidth  - ballR then begin vx := -abs(vx); collision := true; end;

  if by > ClientHeight - ballR then begin vy := -abs(vy); collision := true; end;

  if bx < ballR                then begin vx :=  abs(vx); collision := true; end;

  if by < ballR                then begin vy :=  abs(vy); collision := true; end;

  minSqlen := sqr(ballR + pointR);

  for i := 0 to length(points)-1 do begin

    dx := points[i].X-bx;

    dy := points[i].Y-by;

    sqlen := sqr(dx) + sqr(dy);

    if (sqlen > 0.1*minSqlen) and (minSqlen > sqlen) then begin

      dv := -2*(dx*vx + dy*vy)/sqlen;

      if dv < 0 then begin

        vx := vx + dv*dx;

        vy := vy + dv*dy;

        collision := true;

      end;

    end;

  end;

  if not collision then vy := vy + g;

  if collision then refresh;

end;

procedure TForm1.Timer2Timer(Sender: TObject);

begin

  Refresh;

end;

end.