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Date: Sun, 3 Nov 2013 19:40:36 -0800
Subject: Re: Algorithm that makes maximum compression of completly diffused data.
From: Mark Janssen <dreamingforward@gmail.com>
To: Joshua Landau <joshua@landau.ws>
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Xref: csiph.com comp.lang.python:58426

> Note that I *can* make a "compression" algorithm that takes any
> length-n sequence and compresses all but one sequence by at least one
> bit, and does not ever expand the data.
>
> "00" -> ""
> "01" -> "0"
> "10" -> "1"
> "11" -> "00"
>
> This, obviously, is just 'cause the length is an extra piece of data,
> but sometimes you have to store that anyway ;).

And how many bits will you use to store the length?

> So if I have a list of
> N length-Y lists containing only 1s or 0s, I can genuinely compress
> the whole structure by N log2 Y items.

But you cheated by using a piece of information from "outside the
system": length.  A generic compression algorithm doesn't have this
information beforehand.
-- 
MarkJ
Tacoma, Washington