/*
 * Copyright (C) 2023  Christopher J. Howard
 *
 * This file is part of Antkeeper source code.
 *
 * Antkeeper source code 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.
 *
 * Antkeeper source code 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 Antkeeper source code.  If not, see <http://www.gnu.org/licenses/>.
 */

#ifndef ANTKEEPER_COLOR_XYZ_HPP
#define ANTKEEPER_COLOR_XYZ_HPP

#include <engine/math/vector.hpp>

namespace color {

/// @name CIE XYZ color space
/// @{

/**
 * Returns the luminance of a CIE XYZ color.
 *
 * @param x CIE XYZ color.
 * @return return Luminance of @p x.
 */
template <class T>
[[nodiscard]] inline constexpr T xyz_to_luminance(const math::vec3<T>& x) noexcept
{
	return x[1];
}

/**
 * Transforms a CIE XYZ color into the CIE xyY color space.
 *
 * @param x CIE XYZ color.
 * @return CIE xyY color.
 */
template <class T>
[[nodiscard]] constexpr math::vec3<T> xyz_to_xyy(const math::vec3<T>& x) noexcept
{
	const T sum = x[0] + x[1] + x[2];
	return math::vec3<T>{x[0] / sum, x[1] / sum, x[1]};
}

/**
 * CIE 1931 standard observer color matching function for the X tristimulus value.
 *
 * @param lambda Wavelength of light, in nanometers.
 * @return Matching X tristimulus value.
 *
 * @see match(T)
 */
template <class T>
[[nodiscard]] T xyz_match_x(T lambda)
{
	const T t0 = (lambda - T{442.0}) * ((lambda < T{442.0}) ? T{0.0624} : T{0.0374});
	const T t1 = (lambda - T{599.8}) * ((lambda < T{599.8}) ? T{0.0264} : T{0.0323});
	const T t2 = (lambda - T{501.1}) * ((lambda < T{501.1}) ? T{0.0490} : T{0.0382});
	
	const T x0 = T{ 0.362} * std::exp(T{-0.5} * t0 * t0);
	const T x1 = T{ 1.056} * std::exp(T{-0.5} * t1 * t1);
	const T x2 = T{-0.065} * std::exp(T{-0.5} * t2 * t2);
	
	return x0 + x1 + x2;
}

/**
 * CIE 1931 standard observer color matching function for the Y tristimulus value.
 *
 * @param lambda Wavelength of light, in nanometers.
 * @return Matching Y tristimulus value.
 *
 * @see match(T)
 */
template <class T>
[[nodiscard]] T xyz_match_y(T lambda)
{
	const T t0 = (lambda - T{568.8}) * ((lambda < T{568.8}) ? T{0.0213} : T{0.0247});
	const T t1 = (lambda - T{530.9}) * ((lambda < T{530.9}) ? T{0.0613} : T{0.0322});
	
	const T y0 = T{0.821} * std::exp(T{-0.5} * t0 * t0);
	const T y1 = T{0.286} * std::exp(T{-0.5} * t1 * t1);
	
	return y0 + y1;
}

/**
 * CIE 1931 standard observer color matching function for the Z tristimulus value.
 *
 * @param lambda Wavelength of light, in nanometers.
 * @return Matching Z tristimulus value.
 *
 * @see match(T)
 */
template <class T>
[[nodiscard]] T xyz_match_z(T lambda)
{
	const T t0 = (lambda - T{437.0}) * ((lambda < T{437.0}) ? T{0.0845} : T{0.0278});
	const T t1 = (lambda - T{459.0}) * ((lambda < T{459.0}) ? T{0.0385} : T{0.0725});
	
	const T z0 = T{1.217} * std::exp(T{-0.5} * t0 * t0);
	const T z1 = T{0.681} * std::exp(T{-0.5} * t1 * t1);
	
	return z0 + z1;
}

/**
 * Fitted piecewise gaussian approximation to the CIE 1931 standard observer color matching function.
 *
 * @param lambda Wavelength of light, in nanometers.
 * @return Matching CIE XYZ color.
 *
 * @see match_x(T)
 * @see match_y(T)
 * @see match_z(T)
 *
 * @see Wyman, C., Sloan, P.J., & Shirley, P. (2013). Simple Analytic Approximations to the CIE XYZ Color Matching Functions.
 */
template <class T>
[[nodiscard]] math::vec3<T> xyz_match(T lambda)
{
	return math::vec3<T>
	{
		xyz_match_x<T>(lambda),
		xyz_match_y<T>(lambda),
		xyz_match_z<T>(lambda)
	};
}

/// @}

} // namespace color

#endif // ANTKEEPER_COLOR_XYZ_HPP