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/*-
* Public Domain 2014-2015 MongoDB, Inc.
* Public Domain 2008-2014 WiredTiger, Inc.
*
* This is free and unencumbered software released into the public domain.
*
* Anyone is free to copy, modify, publish, use, compile, sell, or
* distribute this software, either in source code form or as a compiled
* binary, for any purpose, commercial or non-commercial, and by any
* means.
*
* In jurisdictions that recognize copyright laws, the author or authors
* of this software dedicate any and all copyright interest in the
* software to the public domain. We make this dedication for the benefit
* of the public at large and to the detriment of our heirs and
* successors. We intend this dedication to be an overt act of
* relinquishment in perpetuity of all present and future rights to this
* software under copyright law.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
* IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
* OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
* ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
* OTHER DEALINGS IN THE SOFTWARE.
*/
#include "wt_internal.h"
#undef M_W
#define M_W(r) r.x.w
#undef M_Z
#define M_Z(r) r.x.z
/*
* __wt_random_init --
* Initialize return of a 32-bit pseudo-random number.
*/
void
__wt_random_init(WT_RAND_STATE volatile * rnd_state)
{
WT_RAND_STATE rnd;
M_W(rnd) = 521288629;
M_Z(rnd) = 362436069;
*rnd_state = rnd;
}
/*
* __wt_random --
* Return a 32-bit pseudo-random number.
*
* This is an implementation of George Marsaglia's multiply-with-carry pseudo-
* random number generator. Computationally fast, with reasonable randomness
* properties.
*
* We have to be very careful about races here. Multiple threads can call
* __wt_random concurrently, and it is okay if those concurrent calls get the
* same return value. What is *not* okay is if reading the shared state races
* with an update and uses two different values for m_w or m_z. That could
* result in a value of zero, in which case they would be stuck on zero
* forever. Take local copies of the shared values to avoid this.
*/
uint32_t
__wt_random(WT_RAND_STATE volatile * rnd_state)
{
WT_RAND_STATE rnd;
uint32_t w, z;
/*
* Take a copy of the random state so we can ensure that the
* calculation operates on the state consistently regardless of
* concurrent calls with the same random state.
*/
rnd = *rnd_state;
w = M_W(rnd);
z = M_Z(rnd);
/*
* Check if the value goes to 0 (from which we won't recover), and reset
* to the initial state. This has additional benefits if a caller fails
* to initialize the state, or initializes with a seed that results in a
* short period.
*/
if (z == 0 || w == 0) {
__wt_random_init(&rnd);
w = M_W(rnd);
z = M_Z(rnd);
}
M_Z(rnd) = z = 36969 * (z & 65535) + (z >> 16);
M_W(rnd) = w = 18000 * (w & 65535) + (w >> 16);
*rnd_state = rnd;
return ((z << 16) + (w & 65535));
}
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