| Andrew Cooke | Contents | Latest | RSS | Twitter | Previous | Next

C[omp]ute

Welcome to my blog, which was once a mailing list of the same name and is still generated by mail. Please reply via the "comment" links.

Always interested in offers/projects/new ideas. Eclectic experience in fields like: numerical computing; Python web; Java enterprise; functional languages; GPGPU; SQL databases; etc. Based in Santiago, Chile; telecommute worldwide. CV; email.

Personal Projects

Lepl parser for Python.

Colorless Green.

Photography around Santiago.

SVG experiment.

Professional Portfolio

Calibration of seismometers.

Data access via web services.

Cache rewrite.

Extending OpenSSH.

C-ORM: docs, API.

Last 100 entries

Python libraries error on OpenSuse; Deserving Trump; And Smugness; McCloskey Economics Trilogy; cmocka - Mocks for C; Concept Creep (Americans); Futhark - OpenCL Language; Moved / Gone; Fan and USB issues; Burgers in Santiago; The Origin of Icosahedral Symmetry in Viruses; autoenum on PyPI; Jars Explains; Tomato Chutney v3; REST; US Elections and Gender: 24 Point Swing; PPPoE on OpenSuse Leap 42.1; SuperMicro X10SDV-TLN4F/F with Opensuse Leap 42.1; Big Data AI Could Be Very Bad Indeed....; Cornering; Postcapitalism (Paul Mason); Black Science Fiction; Git is not a CDN; Mining of Massive Data Sets; Rachel Kaadzi Ghansah; How great republics meet their end; Raspberry, Strawberry and Banana Jam; Interesting Dead Areas of Math; Later Taste; For Sale; Death By Bean; It's Good!; Tomato Chutney v2; Time ATAC MX 2 Pedals - First Impressions; Online Chilean Crafts; Intellectual Variety; Taste + Texture; Time Invariance and Gauge Symmetry; Jodorowsky; Tomato Chutney; Analysis of Support for Trump; Indian SF; TP-Link TL-WR841N DNS TCP Bug; TP-Link TL-WR841N as Wireless Bridge; Sending Email On Time; Maybe run a command; Sterile Neutrinos; Strawberry and Banana Jam; The Best Of All Possible Worlds; Kenzaburo Oe: The Changeling; Peach Jam; Taste Test; Strawberry and Raspberry Jam; flac to mp3 on OpenSuse 42.1; Also, Sebald; Kenzaburo Oe Interview; Otake (Kitani Minoru) move Black 121; Is free speech in British universities under threat?; I am actually good at computers; Was This Mansplaining?; WebFaction / LetsEncrypt / General Disappointment; Sensible Philosophy of Science; George Ellis; Misplaced Intuition and Online Communities; More Reading About Japan; Visibilty / Public Comments / Domestic Violence; Ferias de Santiago; More (Clearly Deliberate); Deleted Obit Post; And then a 50 yo male posts this...; We Have Both Kinds Of Contributors; Free Springer Books; Books on Religion; Books on Linguistics; Palestinan Electronica; Books In Anthropology; Taylor Expansions of Spacetime; Info on Juniper; Efficient Stream Processing; The Moral Character of Crypto; Hearing Aid Info; Small Success With Go!; Re: Quick message - This link is broken; Adding Reverb To The Echo Chamber; Sox Audio Tools; Would This Have Been OK?; Honesty only important economically before institutions develop; Stegangraphy via PS4; OpenCL Mess; More Book Recommendations; Good Explanation of Difference Between Majority + Minority; Musical Chairs - Who's The Privileged White Guy; I can see straight men watching this conversation and laffing; Meta Thread Defending POC Causes POC To Close Account; Indigenous People Of Chile; Curry Recipe; Interesting Link On Marginality; A Nuclear Launch Ordered, 1962; More Book Recs (Better Person); It's Nuanced, And I Tried, So Back Off; Marx

© 2006-2015 Andrew Cooke (site) / post authors (content).

Burrows-Wheeler Transform

From: "andrew cooke" <andrew@...>

Date: Fri, 17 Feb 2006 09:26:10 -0300 (CLST)

This is very neat.  A reversible transform that makes data trivial to
compress.

The Transform:

Take the text, of length n, and generate n different texts from the
rotations available.  Note that the last character of each line is the
prefix to the first.  Then sort and note the index of the "correct"
(unrotated) line.

The transform is the index and the final characters of the sorted list.

The Inverse:

The transform characters have all the charcters in the text.  So the
leading character column can be constructed by sorting.  This gives the
first and last character of each rotated string.  Which is a sequence of
(character, next character pairs).  Which can be chained together to give
the original text (except for an arbitrary rotation, which is corrected by
the index).

The compression:

The first characters of each line are sorted.  The transform characters
are their prefixes.  So the transform will be strongly ordered if
character pairs are strongly correlated.


Wikipedia explanation -
http://en.wikipedia.org/wiki/Burrows-Wheeler_transform

Article - http://dogma.net/markn/articles/bwt/bwt.htm

Paper - http://citeseer.ist.psu.edu/76182.html

(This is the bzip/bzip2 compression method; note that it requires the
whole input - or at least a large block size - and that decompression is
faster than compression).

From http://pmd.sourceforge.net/cpd.html
From someone on pragprog (sorry, lost reference)

Andrew

Second Order Correlations?

From: "andrew cooke" <andrew@...>

Date: Fri, 17 Feb 2006 13:20:36 -0300 (CLST)

Seems to me that the sort on the input is arbitrary.  In the different
descriptions I've read it's lexical.  But why?  It seems to me that you
could fine-tune the algorithm by using a different ordering - one, for
example, that keeps letters that occur together as close as possible.

This would increase compression because it reduces "churn" as the lead
character changes (and, to a lesser extent, for subsequent characters).

Since the sorting is necessary for the inverse there's an additional
problem - the sorting order has to be one of:
- universal
- encoded in the file (extra length)
- inferred from the transform itself

The third option is attractive because the ordering seems to reflect
similar information to the (character, prefix) pair information in the
transform.  But off the top of my head I don't see how it helps.

On the other hand, how much would this help?  As a first approximation
perhaps it is only significant when the first character changes.  That
occurs, proportionally, less and less often in large texts (since the
alphabet is fixed).

Still, it's tempting to play around...

Comment on this post