8fe3c82b |
1 | /* -*-c-*- |
2 | * |
3 | * $Id: unihash.c,v 1.1 2003/10/12 14:43:24 mdw Exp $ |
4 | * |
5 | * Simple and efficient universal hashing for hashtables |
6 | * |
7 | * (c) 2003 Straylight/Edgeware |
8 | */ |
9 | |
10 | /*----- Licensing notice --------------------------------------------------* |
11 | * |
12 | * This file is part of the mLib utilities library. |
13 | * |
14 | * mLib is free software; you can redistribute it and/or modify |
15 | * it under the terms of the GNU Library General Public License as |
16 | * published by the Free Software Foundation; either version 2 of the |
17 | * License, or (at your option) any later version. |
18 | * |
19 | * mLib is distributed in the hope that it will be useful, |
20 | * but WITHOUT ANY WARRANTY; without even the implied warranty of |
21 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
22 | * GNU Library General Public License for more details. |
23 | * |
24 | * You should have received a copy of the GNU Library General Public |
25 | * License along with mLib; if not, write to the Free |
26 | * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, |
27 | * MA 02111-1307, USA. |
28 | */ |
29 | |
30 | /*----- Revision history --------------------------------------------------* |
31 | * |
32 | * $Log: unihash.c,v $ |
33 | * Revision 1.1 2003/10/12 14:43:24 mdw |
34 | * Universal hashing. |
35 | * |
36 | */ |
37 | |
38 | /*----- Header files ------------------------------------------------------*/ |
39 | |
40 | #include <assert.h> |
41 | #include <stdlib.h> |
42 | |
43 | #include "unihash.h" |
44 | |
45 | /*----- Main code ---------------------------------------------------------*/ |
46 | |
47 | /* --- @unihash_setkey@ --- * |
48 | * |
49 | * Arguments: @unihash_info *i@ = where to store the precomputed tables |
50 | * @uint32 k@ = the key to set, randomly chosen |
51 | * |
52 | * Returns: --- |
53 | * |
54 | * Use: Calculates the tables required for efficient hashing. |
55 | */ |
56 | |
57 | static uint32 mul(uint32 x, uint32 y) |
58 | { |
59 | uint32 z = 0; |
60 | while (y) { |
61 | if (y & 1) z ^= x; |
62 | if (x & (1 << 31)) |
63 | x = U32(x << 1) ^ UNIHASH_POLY; |
64 | else |
65 | x = U32(x << 1); |
66 | y = U32(y >> 1); |
67 | } |
68 | return (z); |
69 | } |
70 | |
71 | void unihash_setkey(unihash_info *i, uint32 k) |
72 | { |
73 | size_t a; |
74 | size_t b; |
75 | uint32 x = 1; |
76 | |
77 | for (a = 0; a < UNIHASH_NBATCH; a++) { |
78 | x = mul(x, k); |
79 | for (b = 0; b < 256; b++) { |
80 | i->s[a][0][b] = mul(x, b << 0); |
81 | i->s[a][1][b] = mul(x, b << 8); |
82 | i->s[a][2][b] = mul(x, b << 16); |
83 | i->s[a][3][b] = mul(x, b << 24); |
84 | } |
85 | } |
86 | } |
87 | |
88 | /* --- @unihash_hash@ --- * |
89 | * |
90 | * Arguments: @const unihash_info *i@ = pointer to precomputed table |
91 | * @uint32 a@ = @i->[0][0][1]@ or value from previous call |
92 | * @const void *p@ = pointer to data to hash |
93 | * @size_t sz@ = size of the data |
94 | * |
95 | * Returns: --- |
96 | * |
97 | * Use: Hashes data. Call this as many times as needed. |
98 | */ |
99 | |
100 | uint32 unihash_hash(const unihash_info *i, uint32 a, |
101 | const void *p, size_t sz) |
102 | { |
103 | const octet *pp = p; |
104 | |
105 | assert(UNIHASH_NBATCH == 4); |
106 | |
107 | #define FULLMULT(u, x) \ |
108 | (i->s[u][0][U8((x) >> 0)] ^ i->s[u][1][U8((x) >> 8)] ^ \ |
109 | i->s[u][2][U8((x) >> 16)] ^ i->s[u][3][U8((x) >> 24)]); |
110 | |
111 | #define BYTEMULT(u, x) i->s[u][0][x] |
112 | |
113 | /* --- Do the main bulk in batches of %$n$% bytes --- * |
114 | * |
115 | * We have %$a$% and %$m_{n-1}, \ldots, m_1, m_0$%; we want |
116 | * |
117 | * %$a' = (a + m_{n-1}) k^n + m_{n-2} k^{n-1} + \cdots + m_1 k^2 + m_0 k$% |
118 | */ |
119 | |
120 | while (sz >= UNIHASH_NBATCH) { |
121 | a ^= *pp++; |
122 | a = FULLMULT(3, a); |
123 | a ^= BYTEMULT(2, *pp++); |
124 | a ^= BYTEMULT(1, *pp++); |
125 | a ^= BYTEMULT(0, *pp++); |
126 | } |
127 | |
128 | /* --- The tail end is a smaller batch --- */ |
129 | |
130 | switch (sz) { |
131 | case 3: a ^= *pp++; a = FULLMULT(2, a); goto batch_2; |
132 | case 2: a ^= *pp++; a = FULLMULT(1, a); goto batch_1; |
133 | case 1: a ^= *pp++; a = FULLMULT(0, a); goto batch_0; |
134 | batch_2: a ^= BYTEMULT(1, *pp++); |
135 | batch_1: a ^= BYTEMULT(0, *pp++); |
136 | batch_0: break; |
137 | } |
138 | |
139 | return (a); |
140 | } |
141 | |
142 | /* --- @unihash@ --- * |
143 | * |
144 | * Arguments: @const unihash_info *i@ = precomputed tables |
145 | * @const void *p@ = pointer to data to hash |
146 | * @size_t sz@ = size of the data |
147 | * |
148 | * Returns: The hash value computed. |
149 | * |
150 | * Use: All-in-one hashing function. No faster than using the |
151 | * separate calls, but more convenient. |
152 | */ |
153 | |
154 | uint32 unihash(const unihash_info *i, const void *p, size_t sz) |
155 | { |
156 | return (UNIHASH(i, p, sz)); |
157 | } |
158 | |
159 | /*----- That's all, folks -------------------------------------------------*/ |