G4Exp.hh

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//
//
// $Id:$
//
// 
// --------------------------------------------------------------------
//
// Class Description:
//
// The basic idea is to exploit Pade polynomials.
// A lot of ideas were inspired by the cephes math library
// (by Stephen L. Moshier moshier@na-net.ornl.gov) as well as actual code. 
// The Cephes library can be found here:  http://www.netlib.org/cephes/
// Code and algorithms for G4Exp have been extracted and adapted for Geant4
// from the original implementation in the VDT mathematical library
// (https://svnweb.cern.ch/trac/vdt), version 0.3.7.

// Original implementation created on: Jun 23, 2012
//      Author: Danilo Piparo, Thomas Hauth, Vincenzo Innocente
//
// --------------------------------------------------------------------
/* 
 * VDT is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Lesser Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 * 
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Lesser Public License for more details.
 * 
 * You should have received a copy of the GNU Lesser Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
// --------------------------------------------------------------------
#ifndef G4Exp_h
#define G4Exp_h 1

#ifdef WIN32

  #define G4Exp std::exp

#else

#include <limits>
#include <stdint.h>
#include "G4Types.hh"

namespace G4ExpConsts
{  
  const G4double EXP_LIMIT = 708;

  const G4double PX1exp = 1.26177193074810590878E-4;
  const G4double PX2exp = 3.02994407707441961300E-2;
  const G4double PX3exp = 9.99999999999999999910E-1;
  const G4double QX1exp = 3.00198505138664455042E-6;
  const G4double QX2exp = 2.52448340349684104192E-3;
  const G4double QX3exp = 2.27265548208155028766E-1;
  const G4double QX4exp = 2.00000000000000000009E0;

  const G4double LOG2E = 1.4426950408889634073599; // 1/log(2)

  const G4float MAXLOGF = 88.72283905206835f;
  const G4float MINLOGF = -88.f;

  const G4float C1F =   0.693359375f;
  const G4float C2F =  -2.12194440e-4f;

  const G4float PX1expf = 1.9875691500E-4f;
  const G4float PX2expf =1.3981999507E-3f;
  const G4float PX3expf =8.3334519073E-3f;
  const G4float PX4expf =4.1665795894E-2f;
  const G4float PX5expf =1.6666665459E-1f;
  const G4float PX6expf =5.0000001201E-1f;

  const G4float LOG2EF = 1.44269504088896341f;

  //----------------------------------------------------------------------------
  // Used to switch between different type of interpretations of the data
  // (64 bits)
  //
  union ieee754
  {
    ieee754 () {};
    ieee754 (G4double thed) {d=thed;};
    ieee754 (uint64_t thell) {ll=thell;};
    ieee754 (G4float thef) {f[0]=thef;};
    ieee754 (uint32_t thei) {i[0]=thei;};
    G4double d;
    G4float f[2];
    uint32_t i[2];
    uint64_t ll;
    uint16_t s[4];
  };

  //----------------------------------------------------------------------------
  // Converts an unsigned long long to a double
  //
  inline G4double uint642dp(uint64_t ll)
  {
    ieee754 tmp;
    tmp.ll=ll;
    return tmp.d;
  }

  //----------------------------------------------------------------------------
  // Converts an int to a float
  //
  inline G4float uint322sp(G4int x)
  {
    ieee754 tmp;
    tmp.i[0]=x;
    return tmp.f[0];
  }

  //----------------------------------------------------------------------------
  // Converts a float to an int
  //
  inline uint32_t sp2uint32(G4float x)
  {
    ieee754 tmp;
    tmp.f[0]=x;
    return tmp.i[0];
  }

  //----------------------------------------------------------------------------
  inline G4double fpfloor(const G4double x)
  {
    // no problem since exp is defined between -708 and 708. Int is enough for it!
    int32_t ret = int32_t (x);
    ret-=(sp2uint32(x)>>31);  
    return ret;
  }

  //----------------------------------------------------------------------------
  inline G4float fpfloor(const G4float x)
  {
    int32_t ret = int32_t (x);
    ret-=(sp2uint32(x)>>31);  
    return ret;
  }
}

// Exp double precision --------------------------------------------------------


inline G4double G4Exp(G4double initial_x)
{
    G4double x = initial_x;
    G4double px=G4ExpConsts::fpfloor(G4ExpConsts::LOG2E * x +0.5);
 
    const int32_t n = int32_t(px);

    x -= px * 6.93145751953125E-1;
    x -= px * 1.42860682030941723212E-6;

    const G4double xx = x * x;

    // px = x * P(x**2).
    px = G4ExpConsts::PX1exp;
    px *= xx;
    px += G4ExpConsts::PX2exp;
    px *= xx;
    px += G4ExpConsts::PX3exp;
    px *= x;

    // Evaluate Q(x**2).
    G4double qx = G4ExpConsts::QX1exp;
    qx *= xx;
    qx += G4ExpConsts::QX2exp;
    qx *= xx;
    qx += G4ExpConsts::QX3exp;
    qx *= xx;
    qx += G4ExpConsts::QX4exp;

    // e**x = 1 + 2x P(x**2)/( Q(x**2) - P(x**2) )
    x = px / (qx - px);
    x = 1.0 + 2.0 * x;

    // Build 2^n in double.
    x *= G4ExpConsts::uint642dp(( ((uint64_t)n) +1023)<<52);

    if (initial_x > G4ExpConsts::EXP_LIMIT)
            x = std::numeric_limits<G4double>::infinity();
    if (initial_x < -G4ExpConsts::EXP_LIMIT)
            x = 0.;

    return x;
}

// Exp single precision --------------------------------------------------------

inline G4float G4Expf(G4float initial_x)
{
    G4float x = initial_x;

    G4float z = G4ExpConsts::fpfloor( G4ExpConsts::LOG2EF * x +0.5f ); /* std::floor() truncates toward -infinity. */

    x -= z * G4ExpConsts::C1F;
    x -= z * G4ExpConsts::C2F;
    const int32_t n = int32_t ( z );

    const G4float x2 = x * x;

    z = x*G4ExpConsts::PX1expf;
    z += G4ExpConsts::PX2expf;
    z *= x;
    z += G4ExpConsts::PX3expf;
    z *= x;
    z += G4ExpConsts::PX4expf;
    z *= x;
    z += G4ExpConsts::PX5expf;
    z *= x;
    z += G4ExpConsts::PX6expf;
    z *= x2;
    z += x + 1.0f;

    /* multiply by power of 2 */
    z *=  G4ExpConsts::uint322sp((n+0x7f)<<23);

    if (initial_x > G4ExpConsts::MAXLOGF) z=std::numeric_limits<G4float>::infinity();
    if (initial_x < G4ExpConsts::MINLOGF) z=0.f;

    return z;
}

//------------------------------------------------------------------------------

void expv(const uint32_t size, G4double const * __restrict__ iarray, G4double* __restrict__ oarray);
void G4Expv(const uint32_t size, G4double const * __restrict__ iarray, G4double* __restrict__ oarray);
void expfv(const uint32_t size, G4float const * __restrict__ iarray, G4float* __restrict__ oarray);
void G4Expfv(const uint32_t size, G4float const * __restrict__ iarray, G4float* __restrict__ oarray);

#endif /* WIN32 */

#endif