[Original notes written ~1997, copied from handwritten A4]

Generalised data transformation
===============================
SWI DataConvert_TransformToName
=> R0 = transform number
<= R1-> transform name
   R2-> transform group name

Decodes a transform number to a name.


SWI DataConvert_TransformFromName
=> R0 = transform group base, or -1 if not known
   R1-> transform name, or 0 to get group
   R2-> group name, or ignored if R0 is not -1
<= R0 = transform (group) number

Decodes transform names to numbers.


SWI DataConvert_Transform
=> R0 = transform number
   R1 = flags:
          b0  set for initial call
          b1  set if no more data is pending
   R2-> workspace, or 0 to return values
   R3-> input data block
   R4 = input data length
   R5-> output data block
   R6 = output data length
<= R0 = reason code:
          0  complete
          1  need more input data
          2  need more output buffer
          3  transformation failed (inconsistent input)
          4  transformation failed (transform is impossible)
   R1 preserved
   R2 = workspace required, if R0 was 0
        preserved otherwise
   R3-> next byte that will be read if R2 is not 0
        preserved otherwise
   R4 = minimum input buffer size if R2 is 0
        input length remaining otherwise
   R5-> next byte to write to, if R2 is 0
        output length remaining otherwise

This will apply a transform to a block of data,
allowing for streaming operations. Minimum input length will
be -1 if the buffer must contain all the data due to encoding
limitations.


SWI DataConvert_TransformOption
=> R0 = transform number
   R1-> workspace, or 0 if a global data item
   R2 = option in form &RWGGTTOO
          RW = read if 0, write if 1
          GG = group to access:
                  0 data for this transform
                  1 global data
                  2 transform data
          TT = type of data:
                  0 number
                  1 string
                  2 data block
          OO = option (transform specific)
   R3 = data
   R4 = data length (if applicable) or -1 to read length required.
<= R3 = data
   R4 = data length (if applicable), or -1 for 'any'

This call would be used to set up various parameters for the
transform once it has been initialised. Transform data options:
  0  string  descriptive name
  1  number  version * 100
  2  string  author name
more will probably be added.

Any non-existant, or inconsistent, options will return an error.


Transform group bases
---------------------
  &00000  Encoding - this is 're-writing data in a form that is
          capable of being transferred across other mediums
          safely'.
  &10000  Encryption - this is 'making the data secure such that
          only the intended recipients cna obtain the original'.
  &20000  Compression - this is 'making the data take less space
          than it did'.
  &30000  Hashing - this is 'reducing the data to some keyed
          value'.
[Note: Corruption is not allocated a group!]


Suggested encoding numbers
--------------------------
  &00001  UUcode (24bit->32bit encoding system using 6bit characters)
  &00002  Base64 (as UUcode but safer across Internet gateways)


Suggested encryption numbers
----------------------------
  &10001  Rot13 (shift alphabet 13 characters)
  &10002  DES ("standard" encryption)
  &10003  PGP (publicly available public key encryption)
  
[Note: Encryption methods are covered by certain export laws and
       should be treated with care]


Suggested compression numbers
-----------------------------
  &20001  RLE (simple run length encoding)
  &20002  Compress (aka Squash)
  &20003  Deflate (aka zlib)
  &21001  GSM ("standard" audio compression at 8kHz)

It is suggested that compression be subdivided into:
  &20xxx  Application
  &21xxx  Sound
  &22xxx  Image
  &23xxx  Video