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1 | /* -*-c-*- |
2 | * | |
3 | * $Id: lcrand.c,v 1.1 2000/05/21 11:28:30 mdw Exp $ | |
4 | * | |
5 | * Simple linear congruential generator | |
6 | * | |
7 | * (c) 1999 Straylight/Edgeware | |
8 | * (c) 2000 Mark Wooding | |
9 | */ | |
10 | ||
11 | /*----- Licensing notice --------------------------------------------------* | |
12 | * | |
13 | * Copyright (c) 2000 Mark Wooding | |
14 | * All rights reserved. | |
15 | * | |
16 | * Redistribution and use in source and binary forms, with or without | |
17 | * modification, are permitted provided that the following conditions are | |
18 | * met: | |
19 | * | |
20 | * 1. Redistributions of source code must retain the above copyright | |
21 | * notice, this list of conditions and the following disclaimer. | |
22 | * | |
23 | * 2, Redistributions in binary form must reproduce the above copyright | |
24 | * notice, this list of conditions and the following disclaimer in the | |
25 | * documentation and/or other materials provided with the distribution. | |
26 | * | |
27 | * 3. The name of the authors may not be used to endorse or promote | |
28 | * products derived from this software without specific prior written | |
29 | * permission. | |
30 | * | |
31 | * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED | |
32 | * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF | |
33 | * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN | |
34 | * NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, | |
35 | * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES | |
36 | * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR | |
37 | * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) | |
38 | * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, | |
39 | * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN | |
40 | * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE | |
41 | * POSSIBILITY OF SUCH DAMAGE. | |
42 | * | |
43 | * Instead of accepting the above terms, you may redistribute and/or modify | |
44 | * this software under the terms of either the GNU General Public License, | |
45 | * or the GNU Library General Public License, published by the Free | |
46 | * Software Foundation; either version 2 of the License, or (at your | |
47 | * option) any later version. | |
48 | */ | |
49 | ||
50 | /*----- Revision history --------------------------------------------------* | |
51 | * | |
52 | * $Log: lcrand.c,v $ | |
53 | * Revision 1.1 2000/05/21 11:28:30 mdw | |
54 | * Initial check-in. | |
55 | * | |
56 | * --- Past lives (Catacomb) --- * | |
57 | * | |
58 | * Revision 1.2 1999/12/13 15:34:01 mdw | |
59 | * Add support for seeding from a generic pseudorandom source. | |
60 | * | |
61 | * Revision 1.1 1999/12/10 23:15:27 mdw | |
62 | * Noncryptographic random number generator. | |
63 | * | |
64 | */ | |
65 | ||
66 | /*----- Header files ------------------------------------------------------*/ | |
67 | ||
68 | #include <stdio.h> | |
69 | #include <stdlib.h> | |
70 | #include <string.h> | |
71 | ||
72 | #include "bits.h" | |
73 | #include "lcrand.h" | |
74 | ||
75 | /*----- Magic numbers -----------------------------------------------------*/ | |
76 | ||
77 | /* --- The generator parameters --- */ | |
78 | ||
79 | #define P LCRAND_P /* Modulus */ | |
80 | #define A LCRAND_A /* Multiplier (primitive mod @p@) */ | |
81 | #define C LCRAND_C /* Additive constant */ | |
82 | ||
83 | /* --- Precomputed values for modular reduction --- */ | |
84 | ||
85 | #define D 5 /* %$p = 2^{32} - d$% */ | |
86 | ||
87 | /* --- Other useful bits --- */ | |
88 | ||
89 | #define P256 4294967040u /* Highest multiple of 256 < %$p$% */ | |
90 | ||
91 | /*----- Main code ---------------------------------------------------------*/ | |
92 | ||
93 | /* --- @lcrand@ --- * | |
94 | * | |
95 | * Arguments: @uint32 x@ = seed value | |
96 | * | |
97 | * Returns: New state of the generator. | |
98 | * | |
99 | * Use: Steps the generator. Returns %$ax + c \bmod p$%. | |
100 | */ | |
101 | ||
102 | uint32 lcrand(uint32 x) | |
103 | { | |
104 | uint32 a[2], xx[2]; | |
105 | uint32 yy[2]; | |
106 | ||
107 | /* --- Unpack things into the arrays --- */ | |
108 | ||
109 | a[0] = U16(A); a[1] = U16(A >> 16); | |
110 | xx[0] = U16(x); xx[1] = U16(x >> 16); | |
111 | ||
112 | /* --- Multiply everything together --- * | |
113 | * | |
114 | * This is plain old long multiplication, although it looks a bit strange. | |
115 | * I set up the top and bottom partial products directly where they're | |
116 | * supposed to be. The cross terms I add together, with the low 16 bits in | |
117 | * @q@ and the high 32 bits in @p@. These I then add into the product. | |
118 | */ | |
119 | ||
120 | { | |
121 | uint32 p, q; | |
122 | ||
123 | yy[0] = a[0] * xx[0]; | |
124 | yy[1] = a[1] * xx[1]; | |
125 | ||
126 | p = a[0] * xx[1]; | |
127 | q = p + a[1] * xx[0]; | |
128 | p = ((q < p) << 16) + (q >> 16); | |
129 | q = U16(q) << 16; | |
130 | ||
131 | q += yy[0]; | |
132 | if (q < yy[0]) | |
133 | p++; | |
134 | else | |
135 | p += (q >> 16) >> 16; | |
136 | yy[0] = q; | |
137 | ||
138 | yy[1] += p; | |
139 | } | |
140 | ||
141 | /* --- Now reduce mod p --- * | |
142 | * | |
143 | * I'm using shifts and adds to do the multiply step here. This needs to | |
144 | * be changed if @D@ ever becomes something other than 5. | |
145 | */ | |
146 | ||
147 | #if D != 5 | |
148 | # error "Change shift sequence!" | |
149 | #endif | |
150 | ||
151 | { | |
152 | uint32 q; | |
153 | ||
154 | q = yy[1]; | |
155 | x = yy[0]; | |
156 | ||
157 | while (q) { | |
158 | uint32 y, z; | |
159 | y = q >> 30; | |
160 | z = q << 2; | |
161 | z += q; | |
162 | if (z < q) | |
163 | y++; | |
164 | else | |
165 | y += (q >> 16) >> 16; | |
166 | q = y; | |
167 | x += z; | |
168 | if (x < z || x > P) | |
169 | x -= P; | |
170 | } | |
171 | } | |
172 | ||
173 | /* --- Now add on the constant --- */ | |
174 | ||
175 | x += C; | |
176 | if (x < C || x >= P) | |
177 | x -= P; | |
178 | ||
179 | /* --- Done --- */ | |
180 | ||
181 | return (x); | |
182 | } | |
183 | ||
184 | /* --- @lcrand_range@ --- * | |
185 | * | |
186 | * Arguments: @uint32 *x@ = pointer to seed value (updated) | |
187 | * @uint32 m@ = limit allowable | |
188 | * | |
189 | * Returns: A uniformly distributed pseudorandom integer in the interval | |
190 | * %$[0, m)$%. | |
191 | */ | |
192 | ||
193 | uint32 lcrand_range(uint32 *x, uint32 m) | |
194 | { | |
195 | uint32 xx = *x; | |
196 | uint32 r = P - P % m; | |
197 | do xx = lcrand(xx); while (xx >= r); | |
198 | *x = xx; | |
199 | return (xx / (r / m)); | |
200 | } | |
201 | ||
202 | /*----- That's all, folks -------------------------------------------------*/ |