ToFixedS.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_S_H
#define IMPACTX_TO_FIXED_S_H
 
#include "particles/ImpactXParticleContainer.H"
 
#include <AMReX_GpuQualifiers.H>
#include <AMReX_Math.H>
#include <AMReX_REAL.H>
 
#include <cmath>
 
 
namespace impactx::transformation
{
    struct ToFixedS
    {
        using PType = ImpactXParticleContainer::ParticleType;

        ToFixedS (amrex::ParticleReal const pzd)
        : m_pzd(pzd)
        {
        }

        AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
        void operator() (
            amrex::ParticleReal & x,
            amrex::ParticleReal & y,
            amrex::ParticleReal & z,
            amrex::ParticleReal & px,
            amrex::ParticleReal & py,
            amrex::ParticleReal & pz) const
        {
            using namespace amrex::literals;
            using amrex::Math::powi;
 
            // compute value of reference ptd = -gamma
            amrex::ParticleReal const argd = 1.0_prt + powi<2>(m_pzd);
            AMREX_ASSERT_WITH_MESSAGE(argd > 0.0_prt, "invalid ptd arg (<=0)");
            amrex::ParticleReal const ptdf = argd > 0.0_prt ? -std::sqrt(argd) : -1.0_prt;
 
            // transform momenta to dynamic units (e.g., so that momenta are
            // normalized by mc):
            px = px * m_pzd;
            py = py * m_pzd;
            pz = pz * m_pzd;
 
            // compute value of particle pt = -gamma
            amrex::ParticleReal const arg = 1.0_prt + powi<2>(m_pzd + pz) + powi<2>(px) + powi<2>(py);
            AMREX_ASSERT_WITH_MESSAGE(arg > 0.0_prt, "invalid pt arg (<=0)");
            amrex::ParticleReal const ptf = arg > 0.0_prt ? -std::sqrt(arg) : -1.0_prt;
 
            // transform position and momentum (from fixed t to fixed s)
            x = x - px * z / (m_pzd + pz);
            // px = px;
            y = y - py * z / (m_pzd + pz);
            // py = py;
            auto & t = z;    // We store t in the same memory slot as z.
            t = ptf * z / (m_pzd + pz);
            auto & pt = pz;  // We store pt in the same memory slot as pz.
            pt = ptf - ptdf;
 
            // transform momenta to static units (eg, so that momenta are
            // normalized by pzd):
            px = px / m_pzd;
            py = py / m_pzd;
            pt = pz / m_pzd;
        }
 
    private:
        amrex::ParticleReal m_pzd;  
    };
 
} // namespace impactx::transformation
 
#endif // IMPACTX_TO_FIXED_S_H