class Concurrent::AtomicMarkableReference

def compare_and_set(expected_val, new_val, expected_mark, new_mark)
  # Memoize a valid reference to the current AtomicReference for
  # later comparison.
  current             = reference
  curr_val, curr_mark = current
  # Ensure that that the expected marks match.
  return false unless expected_mark == curr_mark
  if expected_val.is_a? Numeric
    # If the object is a numeric, we need to ensure we are comparing
    # the numerical values
    return false unless expected_val == curr_val
  else
    # Otherwise, we need to ensure we are comparing the object identity.
    # Theoretically, this could be incorrect if a user monkey-patched
    # `Object#equal?`, but they should know that they are playing with
    # fire at that point.
    return false unless expected_val.equal? curr_val
  end
  prospect = immutable_array(new_val, new_mark)
  compare_and_set_reference current, prospect
end