From
hv.h:
/* hash a key */
/* FYI: This is the "One-at-a-Time" algorithm by Bob Jenkins
* from requirements by Colin Plumb.
* (http://burtleburtle.net/bob/hash/doobs.html) */
/* The use of a temporary pointer and the casting games
* is needed to serve the dual purposes of
* (a) the hashed data being interpreted as "unsigned char" (new since
+ 5.8,
* a "char" can be either signed or unsigned, depending on the com
+piler)
* (b) catering for old code that uses a "char"
*
* The "hash seed" feature was added in Perl 5.8.1 to perturb the resu
+lts
* to avoid "algorithmic complexity attacks".
*
* If USE_HASH_SEED is defined, hash randomisation is done by default
* If USE_HASH_SEED_EXPLICIT is defined, hash randomisation is done
* only if the environment variable PERL_HASH_SEED is set.
* For maximal control, one can define PERL_HASH_SEED.
* (see also perl.c:perl_parse()).
*/
#ifndef PERL_HASH_SEED
# if defined(USE_HASH_SEED) || defined(USE_HASH_SEED_EXPLICIT)
# define PERL_HASH_SEED PL_hash_seed
# else
# define PERL_HASH_SEED 0
# endif
#endif
#define PERL_HASH(hash,str,len) \
STMT_START { \
register const char * const s_PeRlHaSh_tmp = str; \
register const unsigned char *s_PeRlHaSh = (const unsigned char *)
+s_PeRlHaSh_tmp; \
register I32 i_PeRlHaSh = len; \
register U32 hash_PeRlHaSh = PERL_HASH_SEED; \
while (i_PeRlHaSh--) { \
hash_PeRlHaSh += *s_PeRlHaSh++; \
hash_PeRlHaSh += (hash_PeRlHaSh << 10); \
hash_PeRlHaSh ^= (hash_PeRlHaSh >> 6); \
} \
hash_PeRlHaSh += (hash_PeRlHaSh << 3); \
hash_PeRlHaSh ^= (hash_PeRlHaSh >> 11); \
(hash) = (hash_PeRlHaSh + (hash_PeRlHaSh << 15)); \
} STMT_END
