gcl0x

[def
  { X     dX    Y   dY  }
  ##$0800 ##0   ##0 ##-$80
  ##$1000 ##0   ##0 ##-$100
  ##$1800 ##0   ##0 ##-$180
  ##$2000 ##0   ##0 ##-$200
  ##$2800 ##0   ##0 ##-$280
  ##$3000 ##0   ##0 ##-$300
  ##$3800 ##0   ##0 ##-$380
] Points=

[def
  ##$4450 ##$4450 ##$4450
  ##$4450 ##$4450 ##$4450
  ##$4450
] Current=

{ Clear screen }
$800 [do
 p= $2020 p:
 p 2+ if>0loop]

[do
 { Draw points }
 Points p=
 Current c=
 0 d=
 14 [do i=
   c; q= d q.                   {Plot last position in color}
   p 1+ peek $50+ <q.
   p 5+ peek $44+ >q.           {Let Y point down for now}
   q c:                         {New position}
   63 q.                        {White}
   d 23+ d=                     {New color}
   c 2+ c= p 8+ p=              {Next point}
   i 2- if>0loop]

 { Update coordinates }
 Points p=
 14 [do i=
                                {p[0] += p[1]}
   p       deek u=
   p 2+ q= deek v=
   u+ p:
                                {p[1] -= p[0]/256}
   p 1+ peek 128^ 128- u=       {Sign extend}
   v u- q:

   p 4+ p=                      {Next point}
   i 1- if>0loop]
 loop]