tan.h

#pragma once

#include <funcy/concepts.h>
#include <funcy/util/chainer.h>

#include <cmath>

namespace funcy
{
    struct Tan : Chainer< Tan >
    {
        explicit Tan( double x = 0. ) noexcept
        {
            update( x );
        }

        void update( double x ) noexcept
        {
            value = ::tan( x );
            firstDerivative = 1 + ( value * value );
        }

        [[nodiscard]] double d0() const noexcept
        {
            return value;
        }

        [[nodiscard]] double d1( double dx = 1. ) const noexcept
        {
            return firstDerivative * dx;
        }

        [[nodiscard]] double d2( double dx = 1., double dy = 1. ) const noexcept
        {
            return ( 2 * value * firstDerivative ) * dx * dy;
        }

        [[nodiscard]] double d3( double dx = 1., double dy = 1., double dz = 1. ) const noexcept
        {
            return 2 * firstDerivative * ( 1 + ( 3 * value * value ) ) * dx * dy * dz;
        }

    private:
        double value = 0., firstDerivative = 1.;
    };

    template < Function F >
    [[nodiscard]] auto tan( const F& f )
    {
        return Tan()( f );
    }
} // namespace funcy