ToFixedT.H

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/* Copyright 2022-2026 The Regents of the University of California, through Lawrence
 *           Berkeley National Laboratory (subject to receipt of any required
 *           approvals from the U.S. Dept. of Energy). All rights reserved.
 *
 * This file is part of ImpactX.
 *
 * Authors: Chad Mitchell, Axel Huebl
 * License: BSD-3-Clause-LBNL
 */
#ifndef IMPACTX_TO_FIXED_T_H
#define IMPACTX_TO_FIXED_T_H
 
#include "particles/ImpactXParticleContainer.H"
 
#include <AMReX_GpuQualifiers.H>
#include <AMReX_Math.H>
#include <AMReX_REAL.H>
 
#include <cmath>
 
 
namespace impactx::transformation
{
    struct ToFixedT
    {
        using PType = ImpactXParticleContainer::ParticleType;

        ToFixedT (amrex::ParticleReal const ptd)
        : m_ptd(ptd)
        {
        }

        AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
        void operator() (
            amrex::ParticleReal & x,
            amrex::ParticleReal & y,
            amrex::ParticleReal & t,
            amrex::ParticleReal & px,
            amrex::ParticleReal & py,
            amrex::ParticleReal & pt) const
        {
            using namespace amrex::literals;
            using amrex::Math::powi;
 
            // small tolerance to avoid NaN for pz<0:
            constexpr amrex::ParticleReal tol =
#ifdef AMREX_SINGLE_PRECISION_PARTICLES
                1.0e-6_prt;
#else
                1.0e-12_prt;
#endif
 
            // compute value of reference pzd = beta*gamma
            amrex::ParticleReal const argd = -1.0_prt + powi<2>(m_ptd);
            // AMREX_ASSERT_WITH_MESSAGE(argd > 0.0_prt, "invalid pzd arg (<=0)");
            amrex::ParticleReal const pzdf = argd > 0.0_prt ? std::sqrt(argd) : tol;
 
            // transform momenta to dynamic units (eg, so that momenta are
            // normalized by mc):
            px = px * pzdf;
            py = py * pzdf;
            pt = pt * pzdf;
 
            // compute value of particle pz = beta*gamma
            amrex::ParticleReal const arg = -1.0_prt + powi<2>(m_ptd+pt) - powi<2>(px) - powi<2>(py);
            // AMREX_ASSERT_WITH_MESSAGE(arg > 0.0_prt, "invalid pz arg (<=0)");
            amrex::ParticleReal const pzf = arg > 0.0_prt ? std::sqrt(arg) : tol;
 
            // transform position and momentum (from fixed s to fixed t)
            x = x + px*t/(m_ptd+pt);
            // px = px;
            y = y + py*t/(m_ptd+pt);
            // py = py;
            auto & z = t;    // We store z in the same memory slot as t.
            z = pzf * t / (m_ptd + pt);
            auto & pz = pt;  // We store pz in the same memory slot as pt.
            pz = pzf - pzdf;
 
            // transform momenta to static units (eg, so that momenta are
            // normalized by pzdf):
            px = px / pzdf;
            py = py / pzdf;
            pz = pz / pzdf;
        }
 
    private:
        amrex::ParticleReal m_ptd;  
    };
 
} // namespace impactx::transformation
 
#endif // IMPACTX_TO_FIXED_T_H