lib/hash.h

   1 /*
   2  * Copyright (c) 2008, 2009, 2010, 2012, 2013 Nicira, Inc.
   3  *
   4  * Licensed under the Apache License, Version 2.0 (the "License");
   5  * you may not use this file except in compliance with the License.
   6  * You may obtain a copy of the License at:
   7  *
   8  *     http://www.apache.org/licenses/LICENSE-2.0
   9  *
  10  * Unless required by applicable law or agreed to in writing, software
  11  * distributed under the License is distributed on an "AS IS" BASIS,
  12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  13  * See the License for the specific language governing permissions and
  14  * limitations under the License.
  15  */
  16 #ifndef HASH_H
  17 #define HASH_H 1
  18
  19 #include <stdbool.h>
  20 #include <stddef.h>
  21 #include <stdint.h>
  22 #include <string.h>
  23 #include "util.h"
  24
  25 #ifdef __cplusplus
  26 extern "C" {
  27 #endif
  28
  29 static inline uint32_t
  30 hash_rot(uint32_t x, int k)
  31 {
  32     return (x << k) | (x >> (32 - k));
  33 }
  34
  35 uint32_t hash_words(const uint32_t data[], size_t n_words, uint32_t basis);
  36 uint32_t hash_bytes(const void *, size_t n_bytes, uint32_t basis);
  37
  38 static inline uint32_t hash_int(uint32_t x, uint32_t basis);
  39 static inline uint32_t hash_2words(uint32_t, uint32_t);
  40 uint32_t hash_3words(uint32_t, uint32_t, uint32_t);
  41
  42 static inline uint32_t hash_boolean(bool x, uint32_t basis);
  43 uint32_t hash_double(double, uint32_t basis);
  44
  45 static inline uint32_t hash_pointer(const void *, uint32_t basis);
  46 static inline uint32_t hash_string(const char *, uint32_t basis);
  47
  48 /* Murmurhash by Austin Appleby,
  49  * from http://code.google.com/p/smhasher/source/browse/trunk/MurmurHash3.cpp.
  50  *
  51  * The upstream license there says:
  52  *
  53  * // MurmurHash3 was written by Austin Appleby, and is placed in the public
  54  * // domain. The author hereby disclaims copyright to this source code.
  55  *
  56  * See hash_words() for sample usage. */
  57
  58 static inline uint32_t mhash_add__(uint32_t hash, uint32_t data)
  59 {
  60     data *= 0xcc9e2d51;
  61     data = hash_rot(data, 15);
  62     data *= 0x1b873593;
  63     return hash ^ data;
  64 }
  65
  66 static inline uint32_t mhash_add(uint32_t hash, uint32_t data)
  67 {
  68     hash = mhash_add__(hash, data);
  69     hash = hash_rot(hash, 13);
  70     return hash * 5 + 0xe6546b64;
  71 }
  72
  73 static inline uint32_t mhash_finish(uint32_t hash, size_t n_bytes)
  74 {
  75     hash ^= n_bytes;
  76     hash ^= hash >> 16;
  77     hash *= 0x85ebca6b;
  78     hash ^= hash >> 13;
  79     hash *= 0xc2b2ae35;
  80     hash ^= hash >> 16;
  81     return hash;
  82 }
  83
  84 static inline uint32_t hash_string(const char *s, uint32_t basis)
  85 {
  86     return hash_bytes(s, strlen(s), basis);
  87 }
  88
  89 static inline uint32_t hash_int(uint32_t x, uint32_t basis)
  90 {
  91     return hash_2words(x, basis);
  92 }
  93
  94 /* An attempt at a useful 1-bit hash function.  Has not been analyzed for
  95  * quality. */
  96 static inline uint32_t hash_boolean(bool x, uint32_t basis)
  97 {
  98     const uint32_t P0 = 0xc2b73583;   /* This is hash_int(1, 0). */
  99     const uint32_t P1 = 0xe90f1258;   /* This is hash_int(2, 0). */
 100     return (x ? P0 : P1) ^ hash_rot(basis, 1);
 101 }
 102
 103 static inline uint32_t hash_pointer(const void *p, uint32_t basis)
 104 {
 105     /* Often pointers are hashed simply by casting to integer type, but that
 106      * has pitfalls since the lower bits of a pointer are often all 0 for
 107      * alignment reasons.  It's hard to guess where the entropy really is, so
 108      * we give up here and just use a high-quality hash function.
 109      *
 110      * The double cast suppresses a warning on 64-bit systems about casting to
 111      * an integer to different size.  That's OK in this case, since most of the
 112      * entropy in the pointer is almost certainly in the lower 32 bits. */
 113     return hash_int((uint32_t) (uintptr_t) p, basis);
 114 }
 115
 116 static inline uint32_t hash_2words(uint32_t x, uint32_t y)
 117 {
 118     return mhash_finish(mhash_add(mhash_add(x, 0), y), 4);
 119 }
 120
 121 #ifdef __cplusplus
 122 }
 123 #endif
 124
 125 #endif /* hash.h */