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// ************************************************************************************************
//
// libformfactor: efficient and accurate computation of scattering form factors
//
//! @file ff/Box.cpp
//! @brief Implements class Box.
//!
//! @homepage https://jugit.fz-juelich.de/mlz/libformfactor
//! @license GNU General Public License v3 or higher (see COPYING)
//! @copyright Forschungszentrum Jülich GmbH 2022
//! @authors Scientific Computing Group at MLZ (see CITATION, AUTHORS)
//
// ************************************************************************************************
//! The mathematics implemented here is described in full detail in a paper
//! by Joachim Wuttke, entitled
//! "Form factor (Fourier shape transform) of polygon and polyhedron."
#include "ff/Box.h"
#include "ff/Edge.h"
#include "ff/Face.h"
#include "ff/Polyhedron.h"
#include "ff/Topology.h"
#include <algorithm>
#include <numeric>
#include <stdexcept>
namespace {
complex_t sinc(const complex_t z) // cardinal sine function, sin(x)/x
{
if (z == complex_t(0., 0.))
return 1.0;
return std::sin(z) / z;
}
std::vector<R3> base_vertices(double length, double width)
{
const double a = length / 2;
const double b = width / 2;
return {{+a, +b, 0}, {+a, -b, 0}, {-a, -b, 0}, {-a, +b, 0}};
}
} // namespace
ff::Box::Box(double length, double width, double height, const R3& location)
: Prism(true, height, base_vertices(length, width), location), m_length(length), m_width(width)
{
}
ff::Box::~Box() = default;
complex_t ff::Box::formfactor_at_center(const C3& q) const
{
complex_t qzHdiv2 = m_height / 2 * q.z();
return m_length * m_width * m_height * sinc(m_length / 2 * q.x()) * sinc(m_width / 2 * q.y())
* sinc(qzHdiv2);
}
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