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From r..@rkbloom.net
Subject Re: precalculating hash values for apr_hash_t
Date Sat, 17 Apr 2004 17:15:01 GMT

How are you computing your pre-computed hash values?  Did you copy the hashing
algorithm from APR's find_entry?  

My biggest concern with this function is that the caller's hash function must
agree with APR's hash function or the hash table won't work.  What will happen
if APR changes the hash algorithm in a later release, that would break any
program that uses this set function?  For these reasons, I don't think this
function should be included in APR.

A better approach to this, IMHO, is to create a new apr_hash_make function that
allows a user to pass in a hash function.  This way, your program could
pre-compute and cache the hash values.  When apr_hash_set or apr_hash_get are
called, they will call your hashing function, which could then retrieve the
correct value from the hash.

I would assume that the function signature would be something like:

unsigned int hash_func(const char *key, apr_ssize_t klen);

Ryan

Quoting Ami Ganguli <hse_ami@yahoo.co.uk>:

> Hi folks,
> 
> I'm just starting to use the apr library and my first
> task is to implement a hash based dictionary.  I use
> hash keys to speed up string comparisons, so my keys
> all have pre-computed hash values.
> 
> It probably makes little difference for performance,
> but it doesn't feel right that apr_hash_set keeps
> recomputing my hash values, when I've already done the
> work and remembered the hash.
> 
> If I write my own version of apr_hash_set, perhaps:
> 
> void 
> apr_hash_set_precompute (
>   apr_hash_t *ht, 
>   unsigned int hash,
>   const void *key, 
>   apr_ssize_t klen, 
>   const void *val
>   )
> 
> would there be any interest in including it with the
> library?
> 
> Regards,
> Ami.
> 
> 
> 
> 	
> 	
> 		
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