From: Ruby Quiz <james@...>
Date: 2007-08-31T22:19:54+09:00
Subject: [QUIZ] Twisting a Rope (#137)

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by John Miller

This week's task is to implement the Rope data structure as a Ruby class.  This
topic comes out of the ICFP programming competition
(http://www.icfpcontest.com/) which had competitors manipulating a 7.5 million
character string this year.

	What is a Rope:

You may not realize it, but for many changes the content to a String, Ruby
creates a new copy of the original with the modifications applied.  For small
strings that are created once and read often this is actually a very efficient
way to do thing, but what happens when the string starts to get long, and is
undergoing a lot of changes?  First, the program will spend more and more of its
processing cycles just copying bits around.  Second, the garbage collector will
be called more and more often to pick up the little stringy scraps you've left
all over the memory.

Ropes (the name is a pun for a heavy duty string) are tree structures where a
node represents the concatenation of its left branch with its right, and leaves
are flat strings. (This is a little sloppy. A rope may contain shared subtrees,
and is thus really a directed acyclic graph, where the out-edges of each vertex
are ordered. We will continue to be sloppy.)  E.g. To prepend Text A to Text B,
one creates a Node (call it N1) with A as its left branch and B as its right.  
To further append Text C create a new Node N2 with its left branch pointing to 
N1 and its right to C.  Easy, right?  To find out more see Boehm, Atkinson and 
Plass "Ropes: an Alternative to Strings" at:

	http://rubyurl.com/2FRbO

The task comes in three parts, each increasing in difficulty:

Part one:

Create a Rope class that can do the following:

	a. 'append' or 'prepend' a String or another Rope 
	   (alias the << operator to the append function)     
	b. Return the fully concatenated text with 'to_s' 
	c. define 'slice' to call to_s.slice  
	d. provide a 'length' method

Part two:

Add the following:

	a. Add the ability to 'index' a single character given a 0-based offset
	   from the beginning of the string.
	b. Add the ability to 'shift' a single character from the front of a Rope.
	   (Remove and return the character) 
	c. Add your own 'slice' method that returns a String. Implement as many of 
	   the String method's forms as possible.  To run the example code this
	   function will only need to understand the slice(offset,length) form.
	   Major Bonus for Regex and Substring forms. 
	d. "Balance" the tree with a 'normalize' method. 
	   (see Boehm, Atkinson and Plass 1319 Rebalancing)

Part three: (bonus)

Add the following:

	a. Change the 'slice' method to return a Rope. Ideally this method should
	   do as little string copying as possible. (Doing this will well
	   dramatically increase the speed of the example code)
	b. Allow 'shift' to optionally accept an integer number of characters to
	   remove and return.
	c. modify the '<<' operator so that can efficiently append a few
	   characters at a time. (see Boehm, Atkinson and Plass 1318 para. 4)
	d. *Major Bonus* Add the ability to append and prepend IO classes in a
	   lazy fashion. (see Boehm, Atkinson and Plass 1318 para. 2)

The following code may help you explore how efficient your code is and show
where Ropes are useful.  `ruby -r /path/to/your/rope/class this_script.rb Rope`
will run the test with your code.  Run the script without arguments to see how
well String does.  Also play around with the SIZE and CHUNKS constants to get a
feel for how they affect performance.

	require 'benchmark'
	
	#This code make a String/Rope of  CHUNCKS chunks of text
	#each chunck is SIZE bytes long.  Each chunck starts with 
	#an 8 byte number.  Initially the chuncks are shuffled the
	#qsort method sorts them into ascending order.
	#
	#pass the name of the class to use as a parameter
	#ruby -r rope.rb this_file Rope
	
	puts 'preparing data...'
	TextClass = Object.const_get(ARGV.shift || :String)
	
	def qsort(text)
	  return TextClass.new if text.length == 0
	  pivot = text.slice(0,8).to_s.to_i
	  less = TextClass.new
	  more = TextClass.new
	  offset = 8+SIZE
	  while (offset < text.length)
	    i = text.slice(offset,8).to_s.to_i
	    (i < pivot ? less : more) << text.slice(offset,8+SIZE)
	    offset = offset + 8+SIZE
	  end
	  print "*"
	  return qsort(less) << text.slice(0,8+SIZE) << qsort(more)
	end
	
	SIZE  = 512 * 1024
	CHUNCKS = 128
	CHARS = %w[R O P E]
	data = TextClass.new
	bulk_string = 
	  TextClass.new(Array.new(SIZE) { CHARS[rand(4)] }.join)
	puts 'Building Text...'
	build = Benchmark.measure do
	  (0..CHUNCKS).sort_by { rand }.each do |n| 
	    data<< sprintf("%08i",n) << bulk_string
	  end 
	  data.normalize  if data.respond_to? :normalize
	end
	GC.start
	sort = Benchmark.measure do
	  puts "Sorting Text..."
	  qsort(data)
	  puts"\nEND"
	end
	
	puts "Build: #{build}Sort: #{sort}"