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Vladimir Marangozov contributed updated comments.

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Guido van Rossum 1999-03-24 19:06:42 +00:00
parent cd037e7bed
commit 2bc137909d
1 changed files with 8 additions and 11 deletions
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19
Objects/dictobject.c
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@ -29,7 +29,7 @@ PERFORMANCE OF THIS SOFTWARE.
******************************************************************/ ******************************************************************/
/* Dictionaru object implementation using a hash table */ /* Dictionary object implementation using a hash table */
#include "Python.h" #include "Python.h"
@ -81,8 +81,8 @@ static long polys[] = {
static PyObject *dummy; /* Initialized by first call to newdictobject() */ static PyObject *dummy; /* Initialized by first call to newdictobject() */
/* /*
Invariant for entries: when in use, de_value is not NULL and de_key is Invariant for entries: when in use, me_value is not NULL and me_key is
not NULL and not dummy; when not in use, de_value is NULL and de_key not NULL and not dummy; when not in use, me_value is NULL and me_key
is either NULL or dummy. A dummy key value cannot be replaced by is either NULL or dummy. A dummy key value cannot be replaced by
NULL, since otherwise other keys may be lost. NULL, since otherwise other keys may be lost.
*/ */
@ -141,14 +141,11 @@ However, instead of going through the table at constant steps, we cycle
through the values of GF(2^n)-{0}. This avoids modulo computations, being through the values of GF(2^n)-{0}. This avoids modulo computations, being
much cheaper on RISC machines, without leading to clustering. much cheaper on RISC machines, without leading to clustering.
First a 32-bit hash value, 'sum', is computed from the key string. The initial probe index is computed as hash mod the table size.
The first character is added an extra time shifted by 8 to avoid hashing
single-character keys (often heavily used variables) too close together.
All arithmetic on sum should ignore overflow.
The initial probe index is then computed as sum mod the table size.
Subsequent probe indices use the values of x^i in GF(2^n) as an offset, Subsequent probe indices use the values of x^i in GF(2^n) as an offset,
where x is a root. The initial value is derived from sum, too. where x is a root. The initial value is derived from hash, too.
All arithmetic on hash should ignore overflow.
(This version is due to Reimer Behrends, some ideas are also due to (This version is due to Reimer Behrends, some ideas are also due to
Jyrki Alakuijala and Vladimir Marangozov.) Jyrki Alakuijala and Vladimir Marangozov.)
@ -186,7 +183,7 @@ lookdict(mp, key, hash)
freeslot = NULL; freeslot = NULL;
} }
/* XXX What if PyObject_Compare returned an exception? */ /* XXX What if PyObject_Compare returned an exception? */
/* Derive incr from sum, just to make it more arbitrary. Note that /* Derive incr from hash, just to make it more arbitrary. Note that
incr must not be 0, or we will get into an infinite loop.*/ incr must not be 0, or we will get into an infinite loop.*/
incr = (hash ^ ((unsigned long)hash >> 3)) & mask; incr = (hash ^ ((unsigned long)hash >> 3)) & mask;
if (!incr) if (!incr)