From: Massimiliano Tomassoli <lists@...>
Date: 2012-05-05T23:39:42+09:00
Subject: Little quiz for beginners

The first chapter of the book "The Ruby Programming Language" presents a
class named Sequence. Suppose that a server uses this class in the
following way:

  # from, to, by are provided by the client.
  s = Sequence.new from, to, by

  if s.length < 100
      s.each do
          |x| puts x * x
      end
  else
      puts 'Sequence too long'
  end

Would you be able to mount a denial of service?

Here's the class:

#
# This class represents a sequence of numbers characterized by the three
# parameters from, to, and by. The numbers x in the sequence obey the
# following two constraints:
#
#    from <= x <= to
#    x = from + n*by, where n is an integer
#
class Sequence
  # This is an enumerable class; it defines an each iterator below.
  include Enumerable   # Include the methods of this module in this
class

  # The initialize method is special; it is automatically invoked to
  # initialize newly created instances of the class
  def initialize(from, to, by)
    # Just save our parameters into instance variables for later use
    @from, @to, @by = from, to, by  # Note parallel assignment and @
prefix
  end

  # This is the iterator required by the Enumerable module
  def each
    x = @from       # Start at the starting point
    while x <= @to  # While we haven't reached the end
      yield x       # Pass x to the block associated with the iterator
      x += @by      # Increment x
    end
  end

  # Define the length method (following arrays) to return the number of
  # values in the sequence
  def length
    return 0 if @from > @to       # Note if used as a statement modifier
    Integer((@to-@from)/@by) + 1  # Compute and return length of
sequence
  end

  # Define another name for the same method.
  # It is common for methods to have multiple names in Ruby
  alias size length  # size is now a synonym for length

  # Override the array-access operator to give random access to the
sequence
  def[](index)
    return nil if index < 0 # Return nil for negative indexes
    v = @from + index*@by   # Compute the value
    if v <= @to             # If it is part of the sequence
      v                     # Return it
    else                    # Otherwise...
      nil                   # Return nil
    end
  end

  # Override arithmetic operators to return new Sequence objects
  def *(factor)
    Sequence.new(@from*factor, @to*factor, @by*factor)
  end

  def +(offset)
    Sequence.new(@from+offset, @to+offset, @by)
  end
end

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