SpecularMagneticTanhStrategy.cpp

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//  ************************************************************************************************
//
//  BornAgain: simulate and fit reflection and scattering
//
//! @file      Sample/Specular/SpecularMagneticTanhStrategy.cpp
//! @brief     Implements class SpecularMagneticTanhStrategy.
//!
//! @homepage  http://www.bornagainproject.org
//! @license   GNU General Public License v3 or higher (see COPYING)
//! @copyright Forschungszentrum Jülich GmbH 2020
//! @authors   Scientific Computing Group at MLZ (see CITATION, AUTHORS)
//
//  ************************************************************************************************
 
#include "Sample/Specular/SpecularMagneticTanhStrategy.h"
#include "Base/Math/Constants.h"
#include "Base/Math/Functions.h"
 
namespace {
const double pi2_15 = std::pow(M_PI_2, 1.5);
} // namespace
 
Eigen::Matrix2cd
SpecularMagneticTanhStrategy::computeRoughnessMatrix(const MatrixRTCoefficients& coeff,
                                                     double sigma, bool inverse) const
{
    if (sigma < 10 * std::numeric_limits<double>::epsilon())
        return Eigen::Matrix2cd{Eigen::Matrix2cd::Identity()};
 
    const double sigeff = pi2_15 * sigma;
    const auto b = coeff.m_b;
 
    if (std::abs(b.mag() - 1.) < std::numeric_limits<double>::epsilon() * 10.) {
        Eigen::Matrix2cd Q;
        const double factor1 = 2. * (1. + b.z());
        Q << (1. + b.z()), (I * b.y() - b.x()), (b.x() + I * b.y()), (b.z() + 1.);
 
        complex_t l1 = std::sqrt(Math::tanhc(sigeff * coeff.m_lambda(1)));
        complex_t l2 = std::sqrt(Math::tanhc(sigeff * coeff.m_lambda(0)));
 
        if (inverse) {
            l1 = 1. / l1;
            l2 = 1. / l2;
        }
 
        const Eigen::Matrix2cd lambda = Eigen::DiagonalMatrix<complex_t, 2>({l1, l2});
 
        return Q * lambda * Q.adjoint() / factor1;
 
    } else if (b.mag() < 10 * std::numeric_limits<double>::epsilon()) {
        complex_t alpha =
            std::sqrt(Math::tanhc(0.5 * sigeff * (coeff.m_lambda(1) + coeff.m_lambda(0))));
        if (inverse)
            alpha = 1. / alpha;
        const Eigen::Matrix2cd lambda = Eigen::DiagonalMatrix<complex_t, 2>({alpha, alpha});
 
        return lambda;
    }
 
    throw std::runtime_error("Broken magnetic field vector");
}
 
std::pair<Eigen::Matrix2cd, Eigen::Matrix2cd>
SpecularMagneticTanhStrategy::computeBackwardsSubmatrices(const MatrixRTCoefficients& coeff_i,
                                                          const MatrixRTCoefficients& coeff_i1,
                                                          double sigma) const
{
    Eigen::Matrix2cd R{Eigen::Matrix2cd::Identity()};
    Eigen::Matrix2cd RInv{Eigen::Matrix2cd::Identity()};
 
    if (sigma != 0.) {
        R = computeRoughnessMatrix(coeff_i1, sigma, false)
            * computeRoughnessMatrix(coeff_i, sigma, true);
 
        RInv = computeRoughnessMatrix(coeff_i, sigma, false)
               * computeRoughnessMatrix(coeff_i1, sigma, true);
    }
 
    const Eigen::Matrix2cd mproduct = coeff_i.computeInverseP() * coeff_i1.computeP();
    const Eigen::Matrix2cd mp = 0.5 * (RInv + mproduct * R);
    const Eigen::Matrix2cd mm = 0.5 * (RInv - mproduct * R);
 
    return {mp, mm};
}