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/* -*- c++ -*- ----------------------------------------------------------
LAMMPS - Large-scale Atomic/Molecular Massively Parallel Simulator
http://lammps.sandia.gov, Sandia National Laboratories
Steve Plimpton, sjplimp@sandia.gov
Copyright (2003) Sandia Corporation. Under the terms of Contract
DE-AC04-94AL85000 with Sandia Corporation, the U.S. Government retains
certain rights in this software. This software is distributed under
the GNU General Public License.
See the README file in the top-level LAMMPS directory.
------------------------------------------------------------------------- */
#ifndef LMP_MATH_SPECIAL_H
#define LMP_MATH_SPECIAL_H
#include <cmath>
namespace LAMMPS_NS {
namespace MathSpecial {
// support function for scaled error function complement
extern double erfcx_y100(const double y100);
// fast 2**x function without argument checks for little endian CPUs
extern double exp2_x86(double x);
// fast e**x function for little endian CPUs, falls back to libc on other platforms
extern double fm_exp(double x);
// scaled error function complement exp(x*x)*erfc(x) for coul/long styles
static inline double my_erfcx(const double x)
{
if (x >= 0.0) return erfcx_y100(400.0/(4.0+x));
else return 2.0*exp(x*x) - erfcx_y100(400.0/(4.0-x));
}
// exp(-x*x) for coul/long styles
static inline double expmsq(double x)
{
x *= x;
x *= 1.4426950408889634074; // log_2(e)
#if defined(__BYTE_ORDER__) && __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
return (x < 1023.0) ? exp2_x86(-x) : 0.0;
#else
return (x < 1023.0) ? exp2(-x) : 0.0;
#endif
}
// x**2, use instead of pow(x,2.0)
static inline double square(const double &x) { return x*x; }
// x**3, use instead of pow(x,3.0)
static inline double cube(const double &x) { return x*x*x; }
// return -1.0 for odd n, 1.0 for even n, like pow(-1.0,n)
static inline double powsign(const int n) { return (n & 1) ? -1.0 : 1.0; }
// optimized version of pow(x,n) with n being integer
// up to 10x faster than pow(x,y)
static inline double powint(const double &x, const int n) {
double yy,ww;
if (x == 0.0) return 0.0;
int nn = (n > 0) ? n : -n;
ww = x;
for (yy = 1.0; nn != 0; nn >>= 1, ww *=ww)
if (nn & 1) yy *= ww;
return (n > 0) ? yy : 1.0/yy;
}
// optimized version of (sin(x)/x)**n with n being a _positive_ integer
static inline double powsinxx(const double &x, int n) {
double yy,ww;
if (x == 0.0) return 1.0;
ww = sin(x)/x;
for (yy = 1.0; n != 0; n >>= 1, ww *=ww)
if (n & 1) yy *= ww;
return yy;
}
}
}
#endif