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From: David LaRue <huey.dll@tampabay.rr.com>
Newsgroups: comp.lang.c
Subject: Re: simple compression mathod to code itself?
Date: Sat, 6 Apr 2024 15:39:11 -0000 (UTC)
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fir <fir@grunge.pl> wrote in news:uurd34$8ga0$1@i2pn2.org:

> David LaRue wrote:
>> fir <fir@grunge.pl> wrote in news:uupflr$5t1n$1@i2pn2.org:
>>
>>> i not code at all in recent years
>>> (recently i coded half of my compuler with a plan to write second half
>>> but its to ambitious to lazy coding mood i got recently)
>>> but recently i started this slamm coding modd and found
>>> ite pleasurable
>>>
>>> searching for lazy side things i could code in such mood
>>> i thought maybe i wopuld like to have compression routine
>>> but i would like to write it myself sortla like i use quicksort
>>> or so but i also want to write it myself
>>>
>>> so is there maybe some method i could use i mean some simple
>>> routine that compresses and uncompresses an array of bytes but
>>> maybe something a bit more complex than RLE (rune length encoding)
>>> - only thing i know from this domain
>>>
>>
>> Hello fir,
>>
>> A method a bit more complex that you might try builds a table of all
>> bytes as you scan them from the input.  The compressed output is a
>> reference to the table you just built (wnhem repeated byte strings are
>> found, otherwise feed the input to the compressed image so that the
>> expansion method can build the sme dynamic table the encoder built. 
>> The table generally has a limit on the number of entries (usually a
>> good size) and allows the table of bytes to dynamically change as new
>> patterns are read from the input. 
>>
>> This is a well known and documented compression/expansion algorithm. 
>> PKZIP and other engines use this as one of their compression methodz. 
>> Look for the description of that if you need more details to figure out
>> what you need to write.
>>
>> Expansion is the reverse.  Read the source (now the compressed image)
>> and build the compression table from the bytes.  As encoded references
>> to the compression table are read from the compressed image output the
>> source byte sequences.  The output should be the same as what your
>> encoder originally read.
>>
>> A good check on the final code is to compare the original input with
>> the eventual output and make sure they agree exactly.
>>
>> Have fun,
>>
>> David
>>
> this could be good but i dont quite understood that .. but eventually 
> could be good...
> 
> i thinged something abut that if rle search for repetitions of 1
> byte then maybe after that search for repetitions of 2 bytes, then 3 
> bytes, 4 bytes and so on.. then do some "report" how many found and then
> find a way to encode that
> 
>   need to think a bit becouse if rle only stores repetitions that are 
> one after another then this method should store  repetitions that have
> various distances among them
> 
> 
> i also do nto want to spend a much time on this 1-2 days eventually
> 

Hello fir,

The method above can do the same thing for each repetition of a single 
byte.  Alone it has the advantage that compression and decompression are 
single pass operations.  It is similar to RLE but allows for more 
variations to be used along the way.  The encoder also needs some table 
searching to determine best matches, new byte sequences, and improved 
length sequences.  An ordered table of byte strings is typically used.  
I've also seen more advanced search methods that use branch tables to 
represent the encoded sequences with the final entry being the value to put 
in the compressed output.  Very easy to search and gives some improvements 
to the basic search design to make performance better.  This can be done in 
a few days or less, depending on how much time you spend on deciding the 
table size and so on.  I've also seen the search part as a coding 
requirement for a one hour test to see how the coder breaks the idea down.

I like all the ideas you have.

David