7 /* Stolen mostly from: lookup3.c, by Bob Jenkins, May 2006, Public Domain.
9 * http://burtleburtle.net/bob/c/lookup3.c
13 * hash - fast hash of an array for internal use
14 * @p: the array or pointer to first element
15 * @num: the number of elements to hash
16 * @base: the base number to roll into the hash (usually 0)
18 * The memory region pointed to by p is combined with the base to form
21 * This hash will have different results on different machines, so is
22 * only useful for internal hashes (ie. not hashes sent across the
23 * network or saved to disk).
25 * It may also change with future versions: it could even detect at runtime
26 * what the fastest hash to use is.
28 * See also: hash_stable.
31 * #include "hash/hash.h"
35 * // Simple demonstration: idential strings will have the same hash, but
36 * // two different strings will probably not.
37 * int main(int argc, char *argv[])
39 * uint32_t hash1, hash2;
42 * err(1, "Usage: %s <string1> <string2>", argv[0]);
44 * hash1 = hash(argv[1], strlen(argv[1]), 0);
45 * hash2 = hash(argv[2], strlen(argv[2]), 0);
46 * printf("Hash is %s\n", hash1 == hash2 ? "same" : "different");
50 #define hash(p, num, base) hash_any((p), (num)*sizeof(*(p)), (base))
53 * hash_stable - hash of an array for external use
54 * @p: the array or pointer to first element
55 * @num: the number of elements to hash
56 * @base: the base number to roll into the hash (usually 0)
58 * The memory region pointed to by p is combined with the base to form
61 * This hash will have the same results on different machines, so can
62 * be used for external hashes (ie. hashes sent across the network or
63 * saved to disk). The results will not change in future versions of
67 * #include "hash/hash.h"
71 * int main(int argc, char *argv[])
74 * err(1, "Usage: %s <string-to-hash>", argv[0]);
76 * printf("Hash stable result is %u\n",
77 * hash_stable(argv[1], strlen(argv[1]), 0));
81 #define hash_stable(p, num, base) \
82 hash_any_stable((p), (num)*sizeof(*(p)), (base))
85 * hash_u32 - fast hash an array of 32-bit values for internal use
86 * @key: the array of uint32_t
87 * @num: the number of elements to hash
88 * @base: the base number to roll into the hash (usually 0)
90 * The array of uint32_t pointed to by @key is combined with the base
91 * to form a 32-bit hash. This is 2-3 times faster than hash() on small
92 * arrays, but the advantage vanishes over large hashes.
94 * This hash will have different results on different machines, so is
95 * only useful for internal hashes (ie. not hashes sent across the
96 * network or saved to disk).
98 uint32_t hash_u32(const uint32_t *key, size_t num, uint32_t base);
101 * hash_string - very fast hash of an ascii string
102 * @str: the nul-terminated string
104 * The string is hashed, using a hash function optimized for ASCII and
105 * similar strings. It's weaker than the other hash functions.
107 * This hash may have different results on different machines, so is
108 * only useful for internal hashes (ie. not hashes sent across the
109 * network or saved to disk). The results will be different from the
110 * other hash functions in this module, too.
112 static inline uint32_t hash_string(const char *string)
114 /* This is Karl Nelson <kenelson@ece.ucdavis.edu>'s X31 hash.
115 * It's a little faster than the (much better) lookup3 hash(): 56ns vs
116 * 84ns on my 2GHz Intel Core Duo 2 laptop for a 10 char string. */
119 for (ret = 0; *string; string++)
120 ret = (ret << 5) - ret + *string;
125 /* Our underlying operations. */
126 uint32_t hash_any(const void *key, size_t length, uint32_t base);
127 uint32_t hash_any_stable(const void *key, size_t length, uint32_t base);
130 * hash_pointer - hash a pointer for internal use
131 * @p: the pointer value to hash
132 * @base: the base number to roll into the hash (usually 0)
134 * The pointer p (not what p points to!) is combined with the base to form
137 * This hash will have different results on different machines, so is
138 * only useful for internal hashes (ie. not hashes sent across the
139 * network or saved to disk).
142 * #include "hash/hash.h"
144 * // Code to keep track of memory regions.
146 * struct region *chain;
150 * // We keep a simple hash table.
151 * static struct region *region_hash[128];
153 * static void add_region(struct region *r)
155 * unsigned int h = hash_pointer(r->start);
157 * r->chain = region_hash[h];
158 * region_hash[h] = r->chain;
161 * static void find_region(const void *start)
165 * for (r = region_hash[hash_pointer(start)]; r; r = r->chain)
166 * if (r->start == start)
171 static inline uint32_t hash_pointer(const void *p, uint32_t base)
173 if (sizeof(p) % sizeof(uint32_t) == 0) {
174 /* This convoluted union is the right way of aliasing. */
176 uint32_t u32[sizeof(p) / sizeof(uint32_t)];
180 return hash_u32(u.u32, sizeof(p) / sizeof(uint32_t), base);
182 return hash(&p, 1, base);