a interesting Parallel Programing Problem: asciify-string

Xah Lee xahlee at gmail.com
Tue Mar 6 14:11:40 CET 2012


here's a interesting problem that we are discussing at comp.lang.lisp.

〈Parallel Programing Problem: asciify-string〉
http://xahlee.org/comp/parallel_programing_exercise_asciify-string.html

here's the plain text. Code example is emacs lisp, but the problem is
general.

for a bit python relevancy… is there any python compiler that's
parallel-algorithm aware?

-----------------------------------------------
Parallel Programing Problem: asciify-string

Here's a interesting parallel programing problem.

Problem Description

The task is to change this function so it's parallelable. (code
example in emacs lisp)

(defun asciify-string (inputStr)
  "Make Unicode string into equivalent ASCII ones."
  (setq inputStr (replace-regexp-in-string "á\\|à\\|â\\|ä" "a"
inputStr))
  (setq inputStr (replace-regexp-in-string "é\\|è\\|ê\\|ë" "e"
inputStr))
  (setq inputStr (replace-regexp-in-string "í\\|ì\\|î\\|ï" "i"
inputStr))
  (setq inputStr (replace-regexp-in-string "ó\\|ò\\|ô\\|ö" "o"
inputStr))
  (setq inputStr (replace-regexp-in-string "ú\\|ù\\|û\\|ü" "u"
inputStr))
  inputStr
  )

Here's a more general description of the problem.

You are given a Unicode text file that's a few peta bytes. For certain
characters in the file, they need to be changed to different char.
(For example of practical application, see: IDN homograph attack ◇
Duplicate characters in Unicode.)

One easy solution is to simply use regex, as the above sample code, to
search thru the file sequentially, and perform the transfrom of a
particular set of chars, then repeat for each char chat needs to be
changed.

But your task is to use a algorithm parallelizable. That is, in a
parallel-algorithm aware language (e.g. Fortress), the compiler will
automatically span the computation to multiple processors.

Refer to Guy Steele's video talk if you haven't seen already. See: Guy
Steele on Parallel Programing.
Solution Suggestions

A better way to write it for parallel programing, is to map a char-
transform function to each char in the string. Here's a pseudo-code in
lisp by Helmut Eller:

(defun asciify-char (c)
  (case c
    ((? ? ? ?) ?a)
    ((? ? ? ?) ?e)
    ((? ? ? ?) ?i)
    ((? ? ? ?) ?o)
    ((? ? ? ?) ?u)
    (t c)))

(defun asciify-string (string) (map 'string #'asciify-string string))

One problem with this is that the function “asciify-char” itself is
sequential, and not 100% parallelizable. (we might assume here that
there are billions of chars in Unicode that needs to be transformed)

It would be a interesting small project, if someone actually use a
parallel-algorithm-aware language to work on this problem, and report
on the break-point of file-size of parallel-algorithm vs sequential-
algorithm.

Anyone would try it? Perhaps in Fortress, Erlang, Ease, Alice, X10, or
other? Is the Clojure parallel aware?

 Xah



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