154 lines
4.0 KiB
C++
154 lines
4.0 KiB
C++
/**
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* @file llrand.cpp
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* @brief Global random generator.
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*
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* $LicenseInfo:firstyear=2000&license=viewerlgpl$
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* Second Life Viewer Source Code
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* Copyright (C) 2010, Linden Research, Inc.
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*
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* This library is free software; you can redistribute it and/or
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* modify it under the terms of the GNU Lesser General Public
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* License as published by the Free Software Foundation;
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* version 2.1 of the License only.
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*
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* This library is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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* Lesser General Public License for more details.
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*
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* You should have received a copy of the GNU Lesser General Public
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* License along with this library; if not, write to the Free Software
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* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
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*
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* Linden Research, Inc., 945 Battery Street, San Francisco, CA 94111 USA
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* $/LicenseInfo$
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*/
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#include "linden_common.h"
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#include "llrand.h"
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#include "lluuid.h"
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/**
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* Through analysis, we have decided that we want to take values which
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* are close enough to 1.0 to map back to 0.0. We came to this
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* conclusion from noting that:
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*
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* [0.0, 1.0)
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*
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* when scaled to the integer set:
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*
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* [0, 4)
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*
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* there is some value close enough to 1.0 that when multiplying by 4,
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* gets truncated to 4. Therefore:
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*
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* [0,1-eps] => 0
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* [1,2-eps] => 1
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* [2,3-eps] => 2
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* [3,4-eps] => 3
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*
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* So 0 gets uneven distribution if we simply clamp. The actual
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* clamp utilized in this file is to map values out of range back
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* to 0 to restore uniform distribution.
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*
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* Also, for clamping floats when asking for a distribution from
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* [0.0,g) we have determined that for values of g < 0.5, then
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* rand*g=g, which is not the desired result. As above, we clamp to 0
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* to restore uniform distribution.
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*/
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// gRandomGenerator is a stateful static object, which is therefore not
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// inherently thread-safe.
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static thread_local LLRandLagFib2281 gRandomGenerator(LLUUID::getRandomSeed());
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// no default implementation, only specific F64 and F32 specializations
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template <typename REAL>
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inline REAL ll_internal_random();
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template <>
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inline F64 ll_internal_random<F64>()
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{
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// *HACK: Through experimentation, we have found that dual core
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// CPUs (or at least multi-threaded processes) seem to
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// occasionally give an obviously incorrect random number -- like
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// 5^15 or something. Sooooo, clamp it as described above.
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F64 rv{ gRandomGenerator() };
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if(!((rv >= 0.0) && (rv < 1.0))) return fmod(rv, 1.0);
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return rv;
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}
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template <>
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inline F32 ll_internal_random<F32>()
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{
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// *HACK: clamp the result as described above.
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// Per Monty, it's important to clamp using the correct fmodf() rather
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// than expanding to F64 for fmod() and then truncating back to F32. Prior
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// to this change, we were getting sporadic ll_frand() == 1.0 results.
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F32 rv{ narrow(gRandomGenerator()) };
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if(!((rv >= 0.0f) && (rv < 1.0f))) return fmodf(rv, 1.0f);
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return rv;
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}
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/*------------------------------ F64 aliases -------------------------------*/
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inline F64 ll_internal_random_double()
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{
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return ll_internal_random<F64>();
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}
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F64 ll_drand()
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{
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return ll_internal_random_double();
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}
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/*------------------------------ F32 aliases -------------------------------*/
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inline F32 ll_internal_random_float()
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{
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return ll_internal_random<F32>();
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}
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F32 ll_frand()
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{
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return ll_internal_random_float();
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}
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/*-------------------------- clamped random range --------------------------*/
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S32 ll_rand()
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{
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return ll_rand(RAND_MAX);
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}
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S32 ll_rand(S32 val)
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{
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// The clamping rules are described above.
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S32 rv = (S32)(ll_internal_random_double() * val);
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if(rv == val) return 0;
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return rv;
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}
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template <typename REAL>
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REAL ll_grand(REAL val)
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{
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// The clamping rules are described above.
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REAL rv = ll_internal_random<REAL>() * val;
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if(val > 0)
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{
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if(rv >= val) return REAL();
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}
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else
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{
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if(rv <= val) return REAL();
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}
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return rv;
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}
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F32 ll_frand(F32 val)
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{
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return ll_grand<F32>(val);
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}
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F64 ll_drand(F64 val)
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{
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return ll_grand<F64>(val);
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}
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