RFR 8057793 BigDecimal is no longer effectively immutable

[Joe Darcy]

Hi Brian,

Other than removing the

     // bug 8057793

comment on the new test method, this looks good to be pushed.

Thanks,

-Joe

On 9/13/2014 7:56 AM, Brian Burkhalter wrote:
> I forgot to add setScaleDoesNotMutateTest() to main() in ZeroScalingTests. D’oh! Here’s the corrected webrev:
>
> http://cr.openjdk.java.net/~bpb/8057793/webrev.01/
>
> Thanks,
>
> Brian
>
> On Sep 12, 2014, at 4:54 PM, Brian Burkhalter <brian.burkhalter at oracle.com> wrote:
>
>> Hello,
>>
>> I created a formal webrev:
>>
>> Issue:	https://bugs.openjdk.java.net/browse/JDK-8057793
>> Webrev:	http://cr.openjdk.java.net/~bpb/8057793/webrev.00/
>>
>> Based on manual inspection of the revised code the patch looks good to me. The test submitted with the issue now succeeds as do all regression tests in jdk/test/java/math to which I also added the code from the test case in the issue report.
>>
>> Note that this webrev is with respect to JDK 9.
>>
>> Thanks,
>>
>> Brian
>>
>> On Sep 11, 2014, at 6:35 PM, Joe Darcy <joe.darcy at oracle.com> wrote:
>>
>>> Hello,
>>>
>>> Hmmm. I haven't dived into the details of the code, but setScale calls out to divide functionality so it is plausible a bug in divide could cause a problem in setScale.
>>>
>>> Thanks for the bug report,
>>>
>>> -Joe
>>>
>>> On 9/9/2014 1:30 AM, Robert Gibson wrote:
>>>>
>>>> Hi there,
>>>>
>>>> I came across a case in BigDecimal division where the dividend ends up getting mutated, which is rather strange for a seemingly immutable class! (It's a subset of the cases where the Burnikel-Ziegler algorithm is used, I haven't done the analysis to find out under which exact conditions it's triggered.)
>>>>
>>>> The attached patch - against the JDK8 version - should fix the problem, at the cost of an extra array copy.  Could somebody review and/or comment please?
>>>>
>>>> Thanks,
>>>> Robert
>>>>
>>>> --- MutableBigInteger.java      2014-09-04 09:42:23.426815000 +0200
>>>> +++ MutableBigInteger.java.patched      2014-09-04 09:46:21.344132000 +0200
>>>> @@ -1261,19 +1261,20 @@
>>>>              int sigma = (int) Math.max(0, n32 - b.bitLength());   // step 3: sigma = max{T | (2^T)*B < beta^n}
>>>>              MutableBigInteger bShifted = new MutableBigInteger(b);
>>>>              bShifted.safeLeftShift(sigma);   // step 4a: shift b so its length is a multiple of n
>>>> -            safeLeftShift(sigma);     // step 4b: shift this by the same amount
>>>> +            MutableBigInteger aShifted = new MutableBigInteger (this);
>>>> +            aShifted.safeLeftShift(sigma);     // step 4b: shift a by the same amount
>>>> -            // step 5: t is the number of blocks needed to accommodate this plus one additional bit
>>>> -            int t = (int) ((bitLength()+n32) / n32);
>>>> +            // step 5: t is the number of blocks needed to accommodate a plus one additional bit
>>>> +            int t = (int) ((aShifted.bitLength()+n32) / n32);
>>>>              if (t < 2) {
>>>>                  t = 2;
>>>>              }
>>>> -            // step 6: conceptually split this into blocks a[t-1], ..., a[0]
>>>> -            MutableBigInteger a1 = getBlock(t-1, t, n);   // the most significant block of this
>>>> +            // step 6: conceptually split a into blocks a[t-1], ..., a[0]
>>>> +            MutableBigInteger a1 = aShifted.getBlock(t-1, t, n);   // the most significant block of a
>>>>              // step 7: z[t-2] = [a[t-1], a[t-2]]
>>>> -            MutableBigInteger z = getBlock(t-2, t, n);    // the second to most significant block
>>>> +            MutableBigInteger z = aShifted.getBlock(t-2, t, n);    // the second to most significant block
>>>>              z.addDisjoint(a1, n);   // z[t-2]
>>>>              // do schoolbook division on blocks, dividing 2-block numbers by 1-block numbers
>>>> @@ -1284,7 +1285,7 @@
>>>>                  ri = z.divide2n1n(bShifted, qi);
>>>>                  // step 8b: z = [ri, a[i-1]]
>>>> -                z = getBlock(i-1, t, n);   // a[i-1]
>>>> +                z = aShifted.getBlock(i-1, t, n);   // a[i-1]
>>>>                  z.addDisjoint(ri, n);
>>>>                  quotient.addShifted(qi, i*n);   // update q (part of step 9)
>>>>              }
>>>> @@ -1292,7 +1293,7 @@
>>>>              ri = z.divide2n1n(bShifted, qi);
>>>>              quotient.add(qi);
>>>> -           ri.rightShift(sigma);   // step 9: this and b were shifted, so shift back
>>>> +           ri.rightShift(sigma);   // step 9: a and b were shifted, so shift back
>>>>              return ri;
>>>>          }
>>>>      }