| 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.

Last 100 entries

SQL Performance Explained; The Little Manual of API Design; Multiple Word Sizes; CRC - Next Steps; FizzBuzz; Update on CRCs; Decent Links / Discussion Community; Automated Reasoning About LLVM Optimizations and Undefined Behavior; A Painless Guide To CRC Error Detection Algorithms; Tests in Julia; Dave Eggers: what's so funny about peace, love and Starship?; Cello - High Level C Programming; autoreconf needs tar; Will Self Goes To Heathrow; Top 5 BioInformatics Papers; Vasovagal Response; Good Food in Vina; Chilean Drug Criminals Use Subsitution Cipher; Adrenaline; Stiglitz on the Impact of Technology; Why Not; How I Am 5; Lenovo X240 OpenSuse 13.1; NSA and GCHQ - Psychological Trolls; Finite Fields in Julia (Defining Your Own Number Type); Julian Assange; Starting Qemu on OpenSuse; Noisy GAs/TMs; Venezuela; Reinstalling GRUB with EFI; Instructions For Disabling KDE Indexing; Evolving Speakers; Changing Salt Size in Simple Crypt 3.0.0; Logarithmic Map (Moved); More Info; Words Found in Voynich Manuscript; An Inventory Of 3D Space-Filling Curves; Foxes Using Magnetic Fields To Hunt; 5 Rounds RC5 No Rotation; JP Morgan and Madoff; Ori - Secure, Distributed File System; Physical Unclonable Functions (PUFs); Prejudice on Reddit; Recursion OK; Optimizing Julia Code; Cash Handouts in Brazil; Couple Nice Music Videos; It Also Works!; Adaptive Plaintext; It Works!; RC5 Without Rotation (2); 8 Years...; Attack Against Encrypted Linux Disks; Pushing Back On NSA At IETF; Summary of Experimental Ethics; Very Good Talk On Security, Snowden; Locusts are Grasshoppers!; Vagrant (OpenSuse and IDEs); Interesting Take On Mandela's Context; Haskell Cabal O(n^2) / O(n) Fix; How I Am 4; Chilean Charity Supporting Women; Doing SSH right; Festival of Urban Intervention; Neat Idea - Wormholes Provide Entanglement; And a Link....; Simple Encryption for Python 2.7; OpenSuse 13.1 Is Better!; Little Gain...; More Details on Technofull Damage; Palmrest Cracked Too....; Tecnofull (Lenovo Support) Is Fucking Useless; The Neuroscientist Who Discovered He Was a Psychopath; Interpolating Polynomials; Bottlehead Crack as Pre-amp; Ooops K702!; Bottlehead Crack, AKG K701; Breaking RC5 Without Rotation; Great post thank you; Big Balls of Mud; Phabricator - Tools for working together; Amazing Julia RC5 Code Parameterized By Word Size; Chi-Square Can Be Two-Sided; Why Do Brits Accept Surveillance?; Statistics Done Wrong; Mesas Trape from Bravo; European Report on Crypto Primitives and Protocols; Interesting Omissions; Oryx And Crake (Margaret Atwood); Music and Theory; My Arduino Programs; Elliptic Curve Crypto; Re: Licensing Interpreted Code; Licensing Interpreted Code; ASUS 1015E-DS03 OpenSuse 12.3 SSD; translating lettuce feature files into stub steps files; Re: translating lettuce feature files into stub steps files; A Tale of Two Psychiatrists; The Real Reason the Poor Go Without Bank Accounts; The Rational Choices of Crack Addicts; Forgot grouped

© 2006-2013 Andrew Cooke (site) / post authors (content).

Tying the knot in Clojure

From: andrew cooke <andrew@...>

Date: Wed, 18 Jul 2012 23:41:56 -0400

(defn stream [seed]
  (defn helper
    ([slow] (helper (first slow) seed (rest slow)))
    ([c fast slow]
      (let [fast (seq fast)]
        (if fast
          (lazy-seq (cons (str c (first fast)) (helper c (rest fast) slow)))
          (helper slow)))))
  (declare delayed)
  (let [slow (cons "" (lazy-seq delayed))]
    (def delayed (helper slow))
    delayed))

(take 5 (stream ["a" "b" "c"]))
("a" "b" "c" "aa" "ab" "ac" "ba" "bb" "bc" "ca" "cb" "cc" "aaa" 
"aab" "aac" "abb" "abc" "aca" "acb" "acc" "baa" "bab" "bac" "bba")

From http://stackoverflow.com/a/11553140/181772

The idea is pretty simple: I take the output from the sequence and feed it
back on itself. For each value in the output (which is also the input), I
generate a new output by appending each of the letters in the seed
sequence. Since this is circular it just keeps on going.

The process of feeding the output into the input is called "tying the knot" in
a fairly famous paper for Haskell. But it's harder to do in Clojure because
it's an eager language (and even lazy sequences aren't "lazy enough") - the
only solution I could find was that mess with def (I suspect someone might do
better with delay and force, but I had no luck).

https://github.com/andrewcooke/clojure-lab/blob/master/src/org/acooke/lab/perms.clj

Andrew

Improved Code

From: andrew cooke <andrew@...>

Date: Thu, 19 Jul 2012 15:13:14 -0400

(defn stream [seed]
      (defn helper [slow]
        (concat (map #(str (first slow) %) seed) (lazy-seq (helper (rest
      slow)))))
      (declare delayed)
      (let [slow (cons "" (lazy-seq delayed))]
        (def delayed (helper slow))
        delayed))
    
    (take 25 (stream ["a" "b" "c"]))
    ("a" "b" "c" "aa" "ab" "ac" "ba" "bb" "bc" "ca" "cb" "cc" "aaa" "aab"
    "aac" "aba" "abb" "abc" "aca" "acb" "acc" "baa" "bab" "bac" "bba")

Also: http://stackoverflow.com/questions/11567625/tying-the-knot-in-clojure

Andrew

PS And I had a typo in teh earlier code - should have been "take 25".

iterate

From: Javier Neira Sanchez <atreyu.bbb@...>

Date: Thu, 19 Jul 2012 23:11:05 +0200

I think tying the knot is about corecursion and the more elegant to
make it in clojure is with iterate

(def abc (map str (map char (range 65 91))))
    (defn cols [seed]
      (let [next #(for [x %] (for [y seed] (str x y)))]
        (->> (iterate #(apply concat (next %)) seed)
             (mapcat identity))))

    (time (first (drop 475254 (cols abc))))
    "Elapsed time: 356.879148 msecs"
    "AAAAA"

    (doc iterate)
    -------------------------
    clojure.core/iterate
    ([f x])
      Returns a lazy sequence of x, (f x), (f (f x)) etc. f must be
free of side-effects

Comment on this post