From: Martin Hansen <mail@...>
Date: 2011-02-23T19:12:25+09:00
Subject: Monkey patching class String to add bitwise operators

Hello all,


Unfortunately Ruby don't have bitwise operators for Class String like
Perl do. I am hoping very much that such operators will be implemented
in Ruby one day at a low level so they will fast! But until such a day,
I will roll my own crude operators by monkey patching Class String.
However, I am experiencing a peculiar error from my Unit Tests:



Loaded suite ./test_bits
Started
....F.
Finished in 0.000661 seconds.

  1) Failure:
test_Bits_XOR_with_equal_length_returns_correctly(TestBits)
[./test_bits.rb:27]:
<"00110001"> expected but was
<"\x00\x00\x01\x01\x00\x00\x00\x01">.

6 tests, 9 assertions, 1 failures, 0 errors, 0 skips

Test run options: --seed 36958




Code:



# Monkey patching Class String to add bitwise operators.
# Behaviour matching Perl's:
# http://perldoc.perl.org/perlop.html#Bitwise-String-Operators
class String
  # Method that performs bitwise AND operation where bits
  # are copied if they exists in BOTH operands. If the operand
  # sizes are different, the & operator methods acts as though
  # the longer operand were truncated to the length of the shorter.
  def &(str)
    new = ""

    (0 ... [self.length, str.length].min).each do |i|
      new << (self[i].ord & str[i].ord)
    end

    new
  end

  # Method that performs bitwise OR operation where bits
  # are copied if they exists in EITHER operands. If the operand
  # sizes differ, the shorter operand is extended with the terminal
  # part of the longer operand.
  def |(str)
    new = ""

    min = [self.length, str.length].min

    (0 ... min).each do |i|
      new << (self[i].ord | str[i].ord)
    end

    if self.length > str.length
      new << self[min ... self.length]
    elsif self.length < str.length
      new << str[min ... str.length]
    end

    new
  end

  # Method that performs bitwise XOR operation where bits
  # are copied if they exists in ONE BUT NOT BOTH operands.
  def ^(str)
    new = ""

    (0 ... [self.length, str.length].min).each do |i|
      new << (self[i].ord ^ str[i].ord)
    end

    new
  end
end


Tests:


#!/usr/bin/env ruby

require 'bits'
require 'test/unit'
require 'pp'

class TestBits < Test::Unit::TestCase
  def test_Bits_AND_with_equal_length_returns_correctly
    assert_equal("00001100", "00111100" & "00001101")
  end

  def test_Bits_AND_with_unequal_length_returns_correctly
    assert_equal("JAPH\n", "japh\nJunk" & '_____')
    assert_equal("JAPH\n", '_____' & "japh\nJunk")
  end

  def test_Bits_OR_with_equal_length_returns_correctly
    assert_equal("00111101", "00111100" | "00001101")
  end

  def test_Bits_OR_with_unequal_length_returns_correctly
    assert_equal("japh\n", "JA" | "  ph\n")
    assert_equal("japh\n", "  ph\n" | "JA")
  end

  def test_Bits_XOR_with_equal_length_returns_correctly
    assert_equal("00110001", "00111100" ^ "00001101")
  end

  def test_Bits_XOR_with_unequal_length_returns_correctly
    assert_equal("JAPH", "j p \n" ^ " a h")
    assert_equal("JAPH", " a h" ^ "j p \n")
  end
end


So, what is this error I see?



Cheers,


Martin

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