From: Johan Holmberg <johan556@...>
Date: 2006-10-07T05:14:45+09:00
Subject: Bug: right shift (>>) for Bignum?

Hi!

I think I have stumbled on a bug in the right shift method on Bignums.
An expression like

    (1 - 2**i) >> (i+1)

normally gives a value of -1 (i.e. minus one). But when the shift
amount is near a multiple of 32 (or 64 on 64-bit architectures)
it suddenly gives a value of 0 (i.e. zero) instead.

A unit test demonstrating the problem is given below.
On my IMac running OS-X I get an error when i=31.
I am using Ruby 1.8.5.

--------------------------------
require "test/unit"

class TC_bignum_rshift_bug < Test::Unit::TestCase
  def test_rshift_bug
    (1..100).each do |i|
      assert_equal( -1,
                    (1 - 2**i) >> (i+1),
                    "shift steps: #{i}" )
    end
  end
end
--------------------------------

Regards,
Johan Holmberg