| 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 -------------------------------------------------*/ |