Bug #2121

Updated by marcandre (Marc-Andre Lafortune) about 6 years ago


I've known this for a while, but only now realized this is actually a terrible bug.

The mathn library replaces Fixnum#/ and Bignum#/ causing them to return a different value. When the result of a division is not an integral value, the default versions will return 0. I can think of many algorithms that would use this expectation, and most other languages will not upconvert integral numeric types to floating-point or fractional types without explicit consent by the programmer.

When requiring 'mathn', Fixnum#/ and Bignum#/ are replaced with versions that return a fractional value ('quo') causing a core math operator to return not just a different type, but *a different value*.

No core library should be allowed to modify the return value of core numeric operators, or else those operators are worthless; you can't rely on them to return a specific value *ever* since someone else could require 'mathn' or 'rational'.

Note also that 'rational' destroys Fixnum#quo and Bignum#quo. This is also a bug that should be fixed, though it is less dangerous because they're not commonly-used operators.

The following code should not fail; Fixnum#/ should never return a value of a different magnitude based on which libraries are loaded:

require 'test/unit'

class TestFixnumMath < Test::Unit::TestCase
# 0 to ensure it runs first, for illustration purposes
def test_0_without_mathn
assert_equal 0, 1/3

def test_with_mathn
require 'mathn'
assert_equal 0, 1/3

But it does fail:

~/projects/jruby ➔ ruby test_fixnum_math.rb
Loaded suite test_fixnum_math
Finished in 0.016003 seconds.

1) Failure:
test_with_mathn(TestFixnumMath) [test_fixnum_math.rb:11]:
<0> expected but was

2 tests, 2 assertions, 1 failures, 0 errors