Re: [PATCH] Add hash_contains function

[Colin Watson]

On Sun, Feb 14, 2010 at 03:02:14PM +0100, Jim Meyering wrote:
> Colin Watson wrote:
> > I looked at converting man-db to use Gnulib's hash implementation rather
> > than its own.  One obstacle seems to be that there is one place where
> > man-db cares about the difference between a key not being in the hash at
> > all, and a key being in the hash but with a NULL value (in Perl syntax,
> > the difference between 'not exists $hash{$key}' and 'not defined
> > $hash{$key}').  I haven't spent much time looking at the relevant code
> > and so it may well be that it can be rewritten to avoid relying on this
> > distinction, but in general it does seem like a useful distinction to
> > draw; most high-level languages' built-in hash implementations allow you
> > to ask whether a key exists without testing the truth of its value.
> 
> Hi Colin,
> 
> It's good to hear that man-db might use gnulib's hash function.
> 
> If the hash module were really unable to distinguish those cases, we
> would have found it too limiting (and fixed it) long ago ;-)

I confess I did not check how new the module was; written in 1992, I can
see your point. ;-)

> The API for hash_insert requires that you pass it a non-NULL-pointer
> (usually to a key/value pair).

Oops.  My bad for only code-inspecting the lookup functions ... the
hash_insert documentation is of course perfectly clear.

> Note that if you want to use the hash table as a simple set, there is
> no need to have a separate key,value struct, since the value would be
> irrelevant.  In that case, you can save storage and indirection costs
> by passing the key string directly to hash_insert (always assuming the
> hash_compare and hash_hash functions are aware).

Indeed, in the case at hand it looks as though I just need a set, so
you're right, setting key == value will be quite adequate.

> Perhaps this can be attributed to the lack of documentation and/or
> examples?  For reference, there are many uses in gnulib itself
> and in the tar and coreutils packages.

I think the biggest thing I found missing in this regard was an overview
of the generic data structure modules provided by Gnulib.  For instance,
I wasted some time fiddling around with oset before realising that I
didn't really need an *ordered* set in most cases and that a hash would
do just fine.  Some kind of index of this category of modules would be
wonderful, for orientation purposes.

Thanks for the cluebat,

-- 
Colin Watson                                       address@hidden