Re: Little tutorial about streams

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Tue, 01 Mar 2011 13:56:40 +0000, Simon Wright wrote:

> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
> 
>> You can also mention chain code as a technique to implement
>> S'Read/Write in a platform independent way.
>
> ?
> 
> Wikipedia says "A chain code is a lossless compression algorithm for
> monochrome images."

I guess this is because chain codes were initially deployed for encoding
pixels and directions (left, top-left etc) in image processing. But chain
code is useful for any encoding of variable-length data without an explicit
length count. E.g. UTF-8 is a chain code. 

Naturals of any length can be portably encoded using chain code like this:

0 = 16#00#
1 = 16#01#
127= 16#7F#
128= 16#80# 16#01# 
129= 16#81# 16#01#
... 

(assuming that octets are atomic). The high order bit when 0 indicates end
of the chain. The lower order bits encode the value.

Signed integer is encoded as above multiplied by 2 using the lowest bit for
the sign.

Floating point numbers are encoded as integer fraction (normalized,
multiplied by 2**p) followed by the integer normalized exponent - p. p is
the required precision.

Enumerations are encoded as E'Pos.

Strings are as the length and the body. (Nobody uses wide-strings of course
(:-)).

That basically is.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de