Re: [PATCH] rbtree: undo augmented damage -v2
From: Venkatesh Pallipadi
Date: Wed Jun 02 2010 - 21:16:12 EST
On Wed, Jun 2, 2010 at 2:11 PM, Suresh Siddha <suresh.b.siddha@xxxxxxxxx> wrote:
> On Tue, 2010-06-01 at 11:42 -0700, Peter Zijlstra wrote:
>> On Tue, 2010-06-01 at 11:09 -0700, Venkatesh Pallipadi wrote:
>> > On Tue, Jun 1, 2010 at 10:42 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>> > > On Tue, 2010-06-01 at 10:34 -0700, Venkatesh Pallipadi wrote:
>> > >>
>> > >> Yes. This will cover all the cases on insert. But on erase, there is
>> > >> still a case where a rotate of sibling node is done during the
>> > >> re-coloration process. There we have a child change on sibling's
>> > >> child. I am not able to think of any easy way to handle that case.
>> > >
>> > > Let me go draw some figures with pen and paper to match up the erase
>> > > path with the rb_augment_erase_begin() code, because I can't quite spot
>> > > the case we're missing.
>> > >
>> > > If you have it handy, ascii art might help..
>> >
>> > It is this case
>> >
>> > P
>> > / \
>> > N S
>> > / \
>> > SL SR
>> >
>> > changing to
>> >
>> > P
>> > / \
>> > N SL
>> > \
>> > S
>> > \
>> > SR
>>
>> Right, but see: http://en.wikipedia.org/wiki/Red-black_tree
>> That is delete_case5, however then we fall into delete_case6 and perform
>> a left rotation.
>>
>> So suppose we start with the tree:
>>
>> P P P SL
>> / \ / \ / \ / \
>> D S --> N S --> N SL --> P S
>> \ / \ / \ \ / \
>> N SL SR SL* SR S* N SR
>> \
>> SR
>>
>> and then remove D, delete case 5 and finally delete case 6, * marks red.
>>
>> rb_augment_erase_begin(D) will return N, and then rb_augment_path(N)
>> will re-augment: N, P, SL and S.
>
>
> P SL
> / \ / \
> N S ---> N S
> / / \ / \
> C SL SR C SR
>
> If P needs to be removed, we need to re-augment S also in this case,
> right? It looks like we are not handling this case.
>
rb_augment_erase_begin() should take care of that. In this case, it
will return S as the deepest node and we start the walk-back-to-root
from there.
Thanks,
Venki
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