# BigInteger.bitLength() can return negative result

Dmitry Nadezhin Dmitry.Nadezhin at Sun.COM
Tue Oct 20 04:18:10 UTC 2009

```I'm preparing a fix which restricts the range of BigInteger so that
bitLength() always
return correct result.

I can't choose between two variants of range of valid BigInteger.

A) Two's complement range
[-2^Integer.MAX_VALUE, 2^Integer.MAX_VALUE-1]
This is exactly those numbers with bitLength() <= Integer.MAX_VALUE.

B) Symmetrical range
[-2^Integer.MAX_VALUE+1, 2^Integer.MAX_VALUE-1]

In variant A)
- x.neg() and x.abs() will throw ArithmeticException on minimum
BigInteger value, because the exact result is out of range;
- bit operations or(), and(), xor(), andNot() will always return valid
result.

In variant B)
- x.neg() and x.abs() will never throw ArithmeticException;
- and(), andNot(), xor() may throw ArithmeticException for certain
arguments.

Which variant do you prefer ?

-Dima

/
/>>>>/ d) All BigInteger constructors ensure that the bit length is no
/>>>>/ larger than 2^31 - 1;
/>>>/
/>>>/ d) is arguably the most correct approach to address the problem.
/>>>/ However, I think the practical consequences of this flaw are low.
/>>/ Nevertheless, may I use this unimportant flaw as a lesson of Java bug
/>>/ fixing ?
/>>/
/>
>No fix for the bug has yet been offered.
>
>You're independently rediscovered a bug that has been in the platform
>for about 12 years.
>
>-Joe

```

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