Could you someone shed some light on the hashing function/algorithm Perl uses to map a string to an index ? Any relevant reading ?
[This answer predates the hashing function change made in 5.28. See "Default Hash Function Change" in the perldelta for 5.28.]
PERL_HASH_INTERNAL_
, defined in hv.h
, copied below:
/* 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 compiler)
* (b) catering for old code that uses a "char"
*
* The "hash seed" feature was added in Perl 5.8.1 to perturb the results
* 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()).
*/
#define PERL_HASH_INTERNAL_(hash,str,len,internal) \
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 = (internal ? PL_rehash_seed : 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
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