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