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125 lines
3.3 KiB
125 lines
3.3 KiB
3 years ago
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/*-
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* See the file LICENSE for redistribution information.
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*
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* Copyright (c) 1996, 1997, 1998, 1999
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* Sleepycat Software. All rights reserved.
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*/
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#include "db_config.h"
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#ifndef lint
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static const char sccsid[] = "@(#)db_shash.c 11.1 (Sleepycat) 7/25/99";
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#endif /* not lint */
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#ifndef NO_SYSTEM_INCLUDES
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#include <sys/types.h>
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#endif
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#include "db_int.h"
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/*
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* Table of good hash values. Up to ~250,000 buckets, we use powers of 2.
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* After that, we slow the rate of increase by half. For each choice, we
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* then use a nearby prime number as the hash value.
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*
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* If a terabyte is the maximum cache we'll see, and we assume there are
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* 10 1K buckets on each hash chain, then 107374182 is the maximum number
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* of buckets we'll ever need.
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*/
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static const struct {
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u_int32_t power;
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u_int32_t prime;
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} list[] = {
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{ 64, 67}, /* 2^6 */
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{ 128, 131}, /* 2^7 */
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{ 256, 257}, /* 2^8 */
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{ 512, 521}, /* 2^9 */
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{ 1024, 1031}, /* 2^10 */
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{ 2048, 2053}, /* 2^11 */
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{ 4096, 4099}, /* 2^12 */
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{ 8192, 8191}, /* 2^13 */
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{ 16384, 16381}, /* 2^14 */
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{ 32768, 32771}, /* 2^15 */
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{ 65536, 65537}, /* 2^16 */
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{ 131072, 131071}, /* 2^17 */
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{ 262144, 262147}, /* 2^18 */
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{ 393216, 393209}, /* 2^18 + 2^18/2 */
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{ 524288, 524287}, /* 2^19 */
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{ 786432, 786431}, /* 2^19 + 2^19/2 */
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{ 1048576, 1048573}, /* 2^20 */
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{ 1572864, 1572869}, /* 2^20 + 2^20/2 */
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{ 2097152, 2097169}, /* 2^21 */
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{ 3145728, 3145721}, /* 2^21 + 2^21/2 */
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{ 4194304, 4194301}, /* 2^22 */
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{ 6291456, 6291449}, /* 2^22 + 2^22/2 */
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{ 8388608, 8388617}, /* 2^23 */
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{ 12582912, 12582917}, /* 2^23 + 2^23/2 */
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{ 16777216, 16777213}, /* 2^24 */
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{ 25165824, 25165813}, /* 2^24 + 2^24/2 */
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{ 33554432, 33554393}, /* 2^25 */
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{ 50331648, 50331653}, /* 2^25 + 2^25/2 */
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{ 67108864, 67108859}, /* 2^26 */
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{ 100663296, 100663291}, /* 2^26 + 2^26/2 */
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{ 134217728, 134217757}, /* 2^27 */
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{ 201326592, 201326611}, /* 2^27 + 2^27/2 */
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{ 268435456, 268435459}, /* 2^28 */
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{ 402653184, 402653189}, /* 2^28 + 2^28/2 */
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{ 536870912, 536870909}, /* 2^29 */
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{ 805306368, 805306357}, /* 2^29 + 2^29/2 */
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{1073741824, 1073741827}, /* 2^30 */
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{0, 0}
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};
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/*
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* CDB___db_tablesize --
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* Choose a size for the hash table.
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*
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* PUBLIC: int CDB___db_tablesize __P((u_int32_t));
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*/
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int
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CDB___db_tablesize(n_buckets)
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u_int32_t n_buckets;
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{
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int i;
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/*
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* We try to be clever about how big we make the hash tables. Use a
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* prime number close to the "suggested" number of elements that will
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* be in the hash table. Use 64 as the minimum hash table size.
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*
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* Ref: Sedgewick, Algorithms in C, "Hash Functions"
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*/
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if (n_buckets < 64)
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n_buckets = 64;
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for (i = 0;; ++i) {
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if (list[i].power == 0) {
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--i;
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break;
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}
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if (list[i].power >= n_buckets)
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break;
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}
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return (list[i].prime);
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}
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/*
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* CDB___db_hashinit --
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* Initialize a hash table that resides in shared memory.
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*
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* PUBLIC: void CDB___db_hashinit __P((void *, u_int32_t));
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*/
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void
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CDB___db_hashinit(begin, nelements)
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void *begin;
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u_int32_t nelements;
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{
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u_int32_t i;
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SH_TAILQ_HEAD(hash_head) *headp;
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headp = (struct hash_head *)begin;
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for (i = 0; i < nelements; i++, headp++)
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SH_TAILQ_INIT(headp);
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}
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