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https://github.com/openvswitch/ovs
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e519ea28d6
Until now this library has based its random number upon those returned by libc's rand() function. This has always bugged me--it is not a good solution since rand() varies in quality so much. This commit changes the random library to use a simple but high-quality PRNG.
110 lines
2.3 KiB
C
110 lines
2.3 KiB
C
/*
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* Copyright (c) 2008, 2009, 2010 Nicira Networks.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at:
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#include <config.h>
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#include "random.h"
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#include <errno.h>
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#include <stdlib.h>
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#include <sys/time.h>
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#include "util.h"
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/* This is the 32-bit PRNG recommended in G. Marsaglia, "Xorshift RNGs",
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* _Journal of Statistical Software_ 8:14 (July 2003). According to the paper,
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* it has a period of 2**32 - 1 and passes almost all tests of randomness.
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*
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* We use this PRNG instead of libc's rand() because rand() varies in quality
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* and because its maximum value also varies between 32767 and INT_MAX, whereas
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* we often want random numbers in the full range of uint32_t. */
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/* Current random state. */
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static uint32_t seed;
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static uint32_t random_next(void);
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void
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random_init(void)
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{
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if (!seed) {
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struct timeval tv;
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if (gettimeofday(&tv, NULL) < 0) {
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ovs_fatal(errno, "gettimeofday");
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}
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seed = tv.tv_sec ^ tv.tv_usec;
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if (!seed) {
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/* A 'seed' of 0 is fatal to randomness--the random value will
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* always be 0--so use the initial seed mentioned by Marsaglia. */
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seed = UINT32_C(2463534242);
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}
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}
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}
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void
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random_bytes(void *p_, size_t n)
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{
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uint8_t *p = p_;
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random_init();
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for (; n > 4; p += 4, n -= 4) {
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uint32_t x = random_next();
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memcpy(p, &x, 4);
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}
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if (n) {
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uint32_t x = random_next();
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memcpy(p, &x, n);
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}
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}
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uint8_t
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random_uint8(void)
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{
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return random_uint32();
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}
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uint16_t
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random_uint16(void)
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{
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return random_uint32();
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}
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uint32_t
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random_uint32(void)
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{
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random_init();
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return random_next();
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}
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int
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random_range(int max)
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{
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return random_uint32() % max;
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}
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static uint32_t
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random_next(void)
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{
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seed ^= seed << 13;
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seed >>= 17;
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seed ^= seed << 5;
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return seed;
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}
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