{-----------------------------------------------------------------------+
|                                                                       |
|       Screen test                                                     |
|                                                                       |
+-----------------------------------------------------------------------}
gcl0x

{
  This is the original Easter Egg "hidden" in ROM v1.
  So the names are obscured, and originally this file was uncommented.

  The image data is a stream of packed 6-bit pixels from ROM.
  The pixel data is XORed with all-ones (63).
  The value after a raw zero is a repeat count for 1..63 white pixels.
  If the repeat count is 0, we are at the end of the image.
}

$0101 ShiftControl=

[def
  { Select random screen memory position `q': }
  { X ranges from 0..255 and Y from 8..127 }
  [do
    \SYS_Random_34 _sysFn= 34!! q=
    q>, 120- if>=0loop]
  128+ q>.

  q r= 72+ r<.                  {End of line `r', 72 pixels wide image}
  0 i=                          {Index in unpacked byte buffer}
  n=                            {Count, or -1 when reading new count}

  { Loop until end of image stream }
  [do
    { Fill buffer with next group of 4 bytes when needed }
    n [if<=0
      i [if=0                   {Reached end of unpack buffer}
        p 0?? \sysArgs0.        {Packed byte 0}
        p 1?? \sysArgs1.        {Packed byte 1}
        p 2?? \sysArgs2.        {Packed byte 2}
        p 3+ p= 255&            {Advance read pointer}
        249^ [if=0 p 7+ p=]     {Skip trampolines}
        \SYS_Unpack_56 _sysFn=
        56!!                    {Unpack 3 to 4 bytes}
      ]
      n
    ]

    [if<0                       {n == -1}
      { Read repeat count for white pixels }
      \sysArgs0 i+ peek n=
      [if=0 ret]                {End of image}
      i 1+ 3& i=                {Rotate through 4-byte unpack buffer}
    else
      [if>0                     {n > 0}
        { Force white pixel }
        63 q.
        n 1- n=
        q<++                    {Update X, but don't update i}
      else                      {n == 0, normal operation}
        { Consume next pixel from stream }
        \sysArgs0 i+ peek [if<>0
          63^ q.                {New pixel, not white}
          q<++
        else
          -1 n=                 {Next value is a repeat count}
        ]
        i 1+ 3& i=              {Rotate through 4-byte unpack buffer}
      ]
    ]

    { Wrap write pointer }
    q r- [if=0
       0 q.                     {1 pixel border/shadow on the right side}
       q 72- q<. q>++           {Next line}
       q [if<0 $8800 q+ q=]     {Wrap around bottom of screen memory}
       q>, r>.
    ]

    \frameCount, ShiftControl.  {Smooth scrolling}
  loop]
] Transfer=

{--- Run ---}

[do
  \gigatronRaw p=               {Pointer to packed image data}
                                { Contains the packed image data for}
                                { Marcel's and Walter's portraits}
  Transfer!                     {Unpack and draw first image}
  Transfer!                     {Unpack and draw second image}
  loop]                         {Repeat forever}

{-----------------------------------------------------------------------+
|       End                                                             |
+-----------------------------------------------------------------------}