/*

* This file is part of the GreasePad distribution (https://github.com/FraunhoferIOSB/GreasePad).

* Copyright (c) 2022-2026 Jochen Meidow, Fraunhofer IOSB

*

* This program is free software: you can redistribute it and/or modify

* it under the terms of the GNU General Public License as published by

* the Free Software Foundation, either version 3 of the License, or

* (at your option) any later version.

*

* This program is distributed in the hope that it will be useful,

* but WITHOUT ANY WARRANTY; without even the implied warranty of

* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the

* GNU General Public License for more details.

*

* You should have received a copy of the GNU General Public License

* along with this program. If not, see <https://www.gnu.org/licenses/>.

*/

#ifndef ACUTE_H

#define ACUTE_H

#include <Eigen/Core>

#include <cassert>

#include <cmath>

#include "skew.h"

namespace Geometry {

using Eigen::Matrix3d;

using Eigen::Vector3d;

//! acute angle between two straight lines in radians

[[nodiscard,maybe_unused]] static inline double acute( const Vector3d & l,

const Vector3d & m)

{

constexpr double pi = 3.141592653589793;

static const Matrix3d S3 = skew( Vector3d(0,0,1) );

static const Matrix3d G3 = Vector3d(1,1,0).asDiagonal();

// PCV (7.34); std::atan2(0,0) := 0

const double gamma = std::atan2( l.dot(S3*m), l.dot(G3*m) );

const double beta = std::fmod( gamma+2*pi, pi );

const double alpha = std::fmin( beta, pi-beta );

assert( alpha >= 0.0 && alpha <= pi/2 );

return alpha;

}

} // namespace Geometry

#endif // ACUTE_H