ERF_NumericalConstants.H

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/*! @file ERF_NumericalConstants.H
 *  \brief Dimensionless numeric literals and pure mathematical constants.
 *
 *  Nothing in this file is physics.  These are spelling aids that let the code
 *  write `myhalf` instead of `amrex::Real(0.5)` and so stay precision-agnostic
 *  between double and single (AMREX_USE_FLOAT) builds, plus the mathematical
 *  constant pi and the two combinations of it that recur often enough to be
 *  worth naming.
 *
 *  GPU note: these are host constexpr variables, so device code may use their
 *  *value* freely but must never odr-use them (bind a reference to one or take
 *  its address).  In particular, do not pass a bare constant to std::min/std::max
 *  or amrex::min/amrex::max -- those take const T&, and nvcc then reports
 *  'identifier "zero" is undefined in device code'.  Write amrex::Real(zero)
 *  instead, which passes a temporary and costs nothing.
 *
 *  Kept separate from ERF_Constants.H so that a file needing only numeric
 *  literals does not have to pull in the thermodynamic constants, and so that
 *  ERF_Constants.H reads as a list of physical constants.  ERF_Constants.H
 *  includes this header, so every existing consumer keeps compiling unchanged.
 */
 
#ifndef ERF_NUMERICAL_CONSTANTS_H_
#define ERF_NUMERICAL_CONSTANTS_H_
 
#include <limits>
 
#include <AMReX_REAL.H>
 
//! Roundoff floor of the working precision: 2.22e-16 in a double build, 1.19e-07
//! under AMREX_USE_FLOAT.  Write a tolerance as a multiple of this rather than as
//! a hard-wired literal such as amrex::Real(1.e-16), which in a single-precision
//! build sits far below the noise floor and so never fires.
constexpr amrex::Real real_eps     = std::numeric_limits<amrex::Real>::epsilon();
 
constexpr amrex::Real zero         = amrex::Real(0.0);
constexpr amrex::Real one          = amrex::Real(1.0);
constexpr amrex::Real two          = amrex::Real(2.0);
constexpr amrex::Real three        = amrex::Real(3.0);
constexpr amrex::Real four         = amrex::Real(4.0);
constexpr amrex::Real myhalf       = amrex::Real(0.5);
constexpr amrex::Real fourth       = amrex::Real(0.25);
constexpr amrex::Real third        = amrex::Real(1.0/3.0);
constexpr amrex::Real eightthirds  = amrex::Real(8.0/3.0);
 
constexpr amrex::Real PI           = amrex::Real(3.14159265358979323846264338327950288);
constexpr amrex::Real PIoTwo       = PI/two;
constexpr amrex::Real four_thirds_pi = four/three*PI;
 
#endif