linux-kernel - Re: [RFC] B+Tree library V2

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Message-Id: <1231445935.8398.18.camel@johannes>
Date:	Thu, 08 Jan 2009 21:18:55 +0100
From:	Johannes Berg <johannes@...solutions.net>
To:	Jörn Engel <joern@...fs.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [RFC] B+Tree library V2

On Thu, 2009-01-08 at 21:02 +0100, Jörn Engel wrote:

> > #define btree_for_each_key(head, geo, key, tmp)	\
> > 	for (key = btree_last(head, geo), tmp = btree_get_prev_key(head, geo, key);
> > 	     key; key = tmp, tmp = btree_get_prev_key(head, geo, key))
> 
> [ Changed the function name above.  It really isn't a lookup, it returns
> a key, not a value.  My fault. ]

Right.

> Looks correct otherwise.  Probably needs a comment that without "tmp" we
> would skip a 0 key.  Or am I the only one who wants to simplify the code
> before spotting this little subtlety?

I, uh, I didn't even realise that. I think the code for
btree_last/btree_get_prev_key isn't correct as is since the 0 key is
valid, but you can't tell whether it returned 0 because it didn't find
anything, or because there was no more entry. Or am I missing something?

> > (and possibly some type-checking variants that hardcode the geo)
> > 
> > Does that seem correct? And would it be possible to provide btree_last()
> > that takes an void ** and fills it with the last entry, and the same for
> > lookup_less(), so we can write btree_for_each_entry() too?
> 
> Not sure what you mean.  Something with the same effect as this?
> 
> #define btree_for_each_val(head, geo, key, val)				\
> 	 for (key = btree_last(head, geo),				\
> 	 		val = btree_lookup(head, geo, key);		\
> 	 		val;						\
> 			key = btree_get_prev_key(head, geo, key),	\
> 			val = btree_lookup(head, geo, key))

Well, that does lots of lookups that don't seem necessary, since a
function like btree_last should be able to return the value right away.
Also, if it was

#define btree_for_each_val(head, geo, key, val)
	for (val = btree_last(head, geo, &key);
	     val;
	     val = btree_get_prev(head, geo, &key))

it would be more correct, I think?

johannes

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