88 *
99 *
1010 * IDENTIFICATION
11- * $Header: /cvsroot/pgsql/src/backend/access/hash/hashfunc.c,v 1.31 2002/02/25 04:06:47 momjian Exp $
11+ * $Header: /cvsroot/pgsql/src/backend/access/hash/hashfunc.c,v 1.32 2002/03/06 20:49:38 momjian Exp $
1212 *
1313 * NOTES
1414 * These functions are stored in pg_amproc. For each operator class
@@ -95,6 +95,8 @@ hashname(PG_FUNCTION_ARGS)
9595{
9696 char * key = NameStr (* PG_GETARG_NAME (0 ));
9797
98+ Assert (strlen (key ) <= NAMEDATALEN );
99+
98100 return hash_any (key , strlen (key ));
99101}
100102
@@ -116,61 +118,89 @@ hashvarlena(PG_FUNCTION_ARGS)
116118 return result ;
117119}
118120
121+ /* This hash function was written by Bob Jenkins
122+ * (bob_jenkins@burtleburtle.net), and superficially adapted
123+ * for PostgreSQL by Neil Conway. For more information on this
124+ * hash function, see http://burtleburtle.net/bob/hash/doobs.html
125+ */
119126
120127/*
121- * hash_any --- compute a hash function for any specified chunk of memory
122- *
123- * This can be used as the underlying hash function for any pass-by-reference
124- * data type in which there are no non-significant bits.
125- *
126- * (Comment from the original db3 hashing code: )
127- *
128- * This is INCREDIBLY ugly, but fast. We break the string up into 8 byte
129- * units. On the first time through the loop we get the 'leftover bytes'
130- * (strlen % 8). On every later iteration, we perform 8 HASHC's so we handle
131- * all 8 bytes. Essentially, this saves us 7 cmp & branch instructions. If
132- * this routine is heavily used enough, it's worth the ugly coding.
133- *
134- * "OZ's original sdbm hash"
128+ * mix -- mix 3 32-bit values reversibly.
129+ * For every delta with one or two bits set, and the deltas of all three
130+ * high bits or all three low bits, whether the original value of a,b,c
131+ * is almost all zero or is uniformly distributed,
132+ * - If mix() is run forward or backward, at least 32 bits in a,b,c
133+ * have at least 1/4 probability of changing.
134+ * - If mix() is run forward, every bit of c will change between 1/3 and
135+ * 2/3 of the time. (Well, 22/100 and 78/100 for some 2-bit deltas.)
136+ */
137+ #define mix (a ,b ,c ) \
138+ { \
139+ a -= b; a -= c; a ^= (c>>13); \
140+ b -= c; b -= a; b ^= (a<<8); \
141+ c -= a; c -= b; c ^= (b>>13); \
142+ a -= b; a -= c; a ^= (c>>12); \
143+ b -= c; b -= a; b ^= (a<<16); \
144+ c -= a; c -= b; c ^= (b>>5); \
145+ a -= b; a -= c; a ^= (c>>3); \
146+ b -= c; b -= a; b ^= (a<<10); \
147+ c -= a; c -= b; c ^= (b>>15); \
148+ }
149+
150+ /*
151+ * hash_any() -- hash a variable-length key into a 32-bit value
152+ * k : the key (the unaligned variable-length array of bytes)
153+ * len : the length of the key, counting by bytes
154+ * Returns a 32-bit value. Every bit of the key affects every bit of
155+ * the return value. Every 1-bit and 2-bit delta achieves avalanche.
156+ * About 6*len+35 instructions. The best hash table sizes are powers
157+ * of 2. There is no need to do mod a prime (mod is sooo slow!).
158+ * If you need less than 32 bits, use a bitmask.
135159 */
136160Datum
137- hash_any (const char * keydata , int keylen )
161+ hash_any (register const char * k , register int keylen )
138162{
139- uint32 n ;
140- int loop ;
141-
142- #define HASHC n = *keydata++ + 65599 * n
143-
144- n = 0 ;
145- if (keylen > 0 )
146- {
147- loop = (keylen + 8 - 1 ) >> 3 ;
148-
149- switch (keylen & (8 - 1 ))
150- {
151- case 0 :
152- do
153- { /* All fall throughs */
154- HASHC ;
155- case 7 :
156- HASHC ;
157- case 6 :
158- HASHC ;
159- case 5 :
160- HASHC ;
161- case 4 :
162- HASHC ;
163- case 3 :
164- HASHC ;
165- case 2 :
166- HASHC ;
167- case 1 :
168- HASHC ;
169- } while (-- loop );
170- }
171- }
172-
173- #undef HASHC
174-
175- PG_RETURN_UINT32 (n );
163+ register Datum a ,b ,c ,len ;
164+
165+ /* Set up the internal state */
166+ len = keylen ;
167+ a = b = 0x9e3779b9 ; /* the golden ratio; an arbitrary value */
168+ /* Another arbitrary value. If the hash function is called
169+ * multiple times, this could be the previously generated
170+ * hash value; however, the interface currently doesn't allow
171+ * this. AFAIK this isn't a big deal.
172+ */
173+ c = 3923095 ;
174+
175+ /* handle most of the key */
176+ while (len >= 12 )
177+ {
178+ a += (k [0 ] + ((Datum )k [1 ]<<8 ) + ((Datum )k [2 ]<<16 ) + ((Datum )k [3 ]<<24 ));
179+ b += (k [4 ] + ((Datum )k [5 ]<<8 ) + ((Datum )k [6 ]<<16 ) + ((Datum )k [7 ]<<24 ));
180+ c += (k [8 ] + ((Datum )k [9 ]<<8 ) + ((Datum )k [10 ]<<16 )+ ((Datum )k [11 ]<<24 ));
181+ mix (a ,b ,c );
182+ k += 12 ; len -= 12 ;
183+ }
184+
185+ /* handle the last 11 bytes */
186+ c += keylen ;
187+ switch (len ) /* all the case statements fall through */
188+ {
189+ case 11 : c += ((Datum )k [10 ]<<24 );
190+ case 10 : c += ((Datum )k [9 ]<<16 );
191+ case 9 : c += ((Datum )k [8 ]<<8 );
192+ /* the first byte of c is reserved for the length */
193+ case 8 : b += ((Datum )k [7 ]<<24 );
194+ case 7 : b += ((Datum )k [6 ]<<16 );
195+ case 6 : b += ((Datum )k [5 ]<<8 );
196+ case 5 : b += k [4 ];
197+ case 4 : a += ((Datum )k [3 ]<<24 );
198+ case 3 : a += ((Datum )k [2 ]<<16 );
199+ case 2 : a += ((Datum )k [1 ]<<8 );
200+ case 1 : a += k [0 ];
201+ /* case 0: nothing left to add */
202+ }
203+ mix (a ,b ,c );
204+ /* report the result */
205+ return c ;
176206}
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