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From: awanderin <awanderin@gmail.com>
Newsgroups: comp.sys.apple2.programmer
Subject: Re: packBytes and unpackBytes
Date: Sat, 30 Apr 2016 20:44:02 -0600
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markpmlim@yahoo.com.sg writes:

> Using resources available on the Internet, the equivalent of the Apple
> IIgs misc tool calls packBytes and unpackBytes have been coded in
> Swift.
>
> The Swift playground project demonstrates how to encode and decode
> files using the 2 methods packBytes and unpackBytes. If you wish to
> encode/decode Apple Preferred Format (APF) IIgs graphic files, more
> work needs to be done besides passing the scanlines as parameters to
> the 2 methods.
>
> The project can be downloaded or previewed at
>
> https://github.com/markpmlim/PackBytesAndUnpackBytes
>
> A major part of the credit should go to the authors of packBytes and
> unpackBytes who had made their source code available.
>
> Although Xcode 7.01 was used, it is possible to convert the code to a
> lower version.
>
> Regards.
>
> Mark


Interesting.   FWIW, here's unpackBytes in Python that I wrote several
years back:


def unpackBytes(data):
    """
    data is a bytearray

    PackBytes is a IIgs Toolset method.  It is used to compress
    various formats of Super Hi-res pictures.

     00xxxxxx  - 1 to 64 bytes follow - all different
       bits 0-5 are the count - 1
     01xxxxxx  - 3, 5, 6, or 7 repeats of next byte
       bits 0-5 are 2, 4, 5, or 6.  Add one to get count.
       [I've read some other stuff stating that these can have other
        values, but the Apple documentation does state these limitations.]
     10xxxxxx  - 1 to 64 repeats of next 4 bytes
       bits 0-5 are count - 1.
     11xxxxxx  - 1 to 64 repeats of next byte taken as 4 bytes
       bits 0-5 are count - 1
           In other words, this repeats the byte 4 - 256 times

     See: http://www.fadden.com/techmisc/hdc/lesson02.htm
          http://web.pdx.edu/~heiss/technotes/iigs/tn.iigs.094.html
    """
    out = bytearray()
    L = len(data)
    i = 0

    while i < L:
        b = data[i]
        i += 1

        op = (b & 0xc0) >> 6
        count = (b & 0x3f) + 1

        if op == 0:
            out += data[i:i+count]
            i += count
        elif op == 1:
            #if count not in [3, 5, 6, 7]:
            #    sys.stderr.write('COUNT %d not in [3,5,6,7] '
            #                      'at offset %d (0x%x)\n'
            #                      % (count, i-1, i-1))
            #    # assert count in [3, 5, 6, 7]
            out += data[i:i + 1] * count
            i += 1
        elif op == 2:
            out += data[i:i + 4] * count
            i += 4
        elif op == 3:
            out += data[i:i + 1] * (4 * count)
            i += 1
        else:
            assert False
    return out


--
Jerry    awanderin at gmail dot com