Re: [HACKERS] gperf anyone? - Mailing list pgsql-hackers

* Bruce Momjian <pgman@candle.pha.pa.us> [000118 17:14] wrote:
> [Charset ISO-8859-1 unsupported, filtering to ASCII...]
> > A while ago I played around with gperf (GNU perfect hash function
> > generator), abusing the keyword lookup in parser/keyword.c as playground.
> > Now before I delete this I was wondering if this would perhaps be of use
> > to the general public. I don't know how huge the speed advantage of this
> > is, I'm sure the parser/scanner speed is the least of our problems. But I
> > thunk especially ecpg could benefit from this. Btw., gperf is used by GCC,
> > so it's not a toy.
> 
> keywords are a fixed array, with a binary search to find a match.  Could
> gperf be faster?  

yes:

~ % gperf 
/* starting time is 21:10:49 */
postgresql 
really
kicks
butt
/* C code produced by gperf version 2.1 (K&R C version) */
/* Command-line: gperf  */



#define MIN_WORD_LENGTH 4
#define MAX_WORD_LENGTH 10
#define MIN_HASH_VALUE 4
#define MAX_HASH_VALUE 10
/*   4 keywords   7 is the maximum key range
*/

static int
hash (str, len)    register char *str;    register unsigned int  len;
{ static unsigned char hash_table[] =   {    10, 10, 10, 10, 10, 10, 10, 10, 10, 10,    10, 10, 10, 10, 10, 10, 10, 10,
10,10,    10, 10, 10, 10, 10, 10, 10, 10, 10, 10,    10, 10, 10, 10, 10, 10, 10, 10, 10, 10,    10, 10, 10, 10, 10, 10,
10,10, 10, 10,    10, 10, 10, 10, 10, 10, 10, 10, 10, 10,    10, 10, 10, 10, 10, 10, 10, 10, 10, 10,    10, 10, 10, 10,
10,10, 10, 10, 10, 10,    10, 10, 10, 10, 10, 10, 10, 10, 10, 10,    10, 10, 10, 10, 10, 10, 10, 10,  0, 10,    10, 10,
10,10, 10, 10, 10,  0,  0, 10,    10, 10,  0, 10,  0,  0,  0, 10, 10, 10,    10,  0, 10, 10, 10, 10, 10, 10,   };
returnlen + hash_table[str[len - 1]] + hash_table[str[0]];
 
}

char *
in_word_set (str, len)    register char *str;    register unsigned int len;
{
 static char * wordlist[] =   {     "", "", "", "",      "butt",      "kicks",      "really",      "", "", "",
"postgresql",   };
 
 if (len <= MAX_WORD_LENGTH && len >= MIN_WORD_LENGTH)   {     register int key = hash (str, len);
     if (key <= MAX_HASH_VALUE && key >= MIN_HASH_VALUE)       {         register char *s = wordlist[key];
         if (*s == *str && !strcmp (str + 1, s + 1))           return s;       }   } return 0;
}
/* ending time is 21:10:58 */

A perfect hash should be much faster at the trivial expense of some space.

>From the distribution:While teaching a data structures course at University of California,Irvine, I developed a
programcalled GPERF that generates perfect hashfunctions for sets of key words.  A perfect hash function is simply:
   A hash function and a data structure that allows         recognition of a key word in a set of words using
exactly1 probe into the data structure.
 


> We also can not distribute GNU code.

I'm pretty sure that the code the gperf outputs is not covered under the
GPL, just gperf itself.

-- 
-Alfred Perlstein - [bright@wintelcom.net|alfred@freebsd.org]