ChallengeWhat is the meaning of this change?
We just fixed a bug in RavenDB with the following commit. What is the purpose of this, and what are we fixing?
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First thing that comes to mind - capacity might be negative, in which case you don't want to multiply it.
Seems like you want to avoid a overflow.
Once capacity is big enough that capacity *= 4 or even capacity *= 2 is so big that it overflows, you get a negative result.
With Math.Max you avoid using the overflowed negative value.
Second thing that comes to mind is that this is integral type overflow protection, assuming unchecked context.
Daniel already provided the answer I wanted to give. We also once got a weird bug when our int16 overflowed into the negative space..
seems like negative on overflow, but i'm not sure
Whatever it is, I'd make it more explicit :)
If this is to guard against overflow then shame on you. Obfuscated code.
Actually, the fact that this code is the subject of a puzzle and has people guessing what it means shows that it should not be in production.
@Oded believe it or not the default is unchecked, even if at certain arithmetics workloads you can squeeze 3% extra performance just setting it explicitely (dont know why though, but I presume the JIT has something to do with it).
@FedericoLois your measurements were wrong. There is no difference at the IL level.
Is it because capacity rolls over to a negative number, therefore the Math.Max() ensures the positive value is used?
+1 Daniel Häfele
As others noted it's for avoiding getting negative values because of overflows. I've found the same bug in Android some time ago:
Daniel is right. But the fix is still not bug-free. You will penalize short-running tasks that overflow, while they could still use the double capacity...
Sergei, Thanks, we'll fix that.
@Sergei, that was good. Code review over blog post :)
Is something like:
capacity = capacity < int.MaxValue / 4 ? capacity * 4 : int.MaxValue; capacity = capacity < int.MaxValue / 2 ? capacity * 2 : int.MaxValue;
realy that more expensive as doing Math.Max?
Onion, That is actually more complex. To read, not to execute.
Seems like you definitely want to do Max(int.MaxValue, capacity*4); i.e. if original capacity is (2/3) of int.MaxValue, you've got a lot of unused room left to increase.
Out of interest, why are you not just shifting bits. You could also test against a high order bit mask to see if you were around to shift off significant bits
+1 for bit shift.