#pragma once

#include <hpr/math/vector.hpp>
#include <hpr/exception.hpp>


namespace hpr
{
    // forward declarations

    class Quaternion;

    inline
    Quaternion inverse(const Quaternion& q);

    inline
    Quaternion operator*(const Quaternion& lhs, const Quaternion& rhs);

    // aliases

    using quat = Quaternion;

    // class declaration
    class Quaternion
    {

    public:

        enum RotationSequence
        {
            ZYX, ZYZ, ZXY, ZXZ, YXZ, YXY, YZX, YZY, XYZ, XYX, XZY, XZX
        };

    protected:

        scalar p_real;
        vec3 p_imag;

    public:

        inline
        Quaternion() :
            p_real{},
            p_imag{}
        {}

        inline
        Quaternion(const scalar real, const vec3& imag) :
            p_real {real},
            p_imag {imag}
        {}

        inline explicit
        Quaternion(const scalar real) :
            p_real {real},
            p_imag {}
        {}

        inline explicit
        Quaternion(const vec3& imag) :
            p_real {},
            p_imag {imag}
        {}

        inline
        Quaternion(const vec3& vs, const scalar& theta) :
            p_real {cos(0.5 * theta)},
            p_imag {sin(0.5 * theta) * vs / mag(vs)}
        {}

        static inline
        Quaternion unit(const vec3& vs)
        {
            return Quaternion(sqrt(1 - norm(vs)), vs);
        }

        inline
        Quaternion(const RotationSequence rs, const vec3& angles)
        {
            switch (rs)
            {
                case XYZ:
                    *this = Quaternion(vec3(0, 1, 0), angles[0]) *
                            Quaternion(vec3(0, 1, 0), angles[1]) *
                            Quaternion(vec3(0, 0, 1), angles[2]);
                    break;

                default:
                    throw std::runtime_error("Unknown rotation sequence");
            }
        }

        inline
        scalar real() const
        {
            return p_real;
        }

        inline
        scalar& real()
        {
            return p_real;
        }

        inline
        vec3 imag() const
        {
            return p_imag;
        }

        inline
        vec3& imag()
        {
            return p_imag;
        }

        inline
        scalar& operator[](Size n)
        {
            if (n > 3)
                throw hpr::OutOfRange();
            if (n == 0)
                return p_real;
            else
                return p_imag[n - 1];
        }

        inline
        scalar operator[](Size n) const
        {
            if (n > 3)
                throw hpr::OutOfRange();
            if (n == 0)
                return p_real;
            else
                return p_imag[n - 1];
        }

        inline
        void operator+=(const Quaternion& q)
        {
            p_real += q.p_real;
            p_imag += q.p_imag;
        }

        inline
        void operator-=(const Quaternion& q)
        {
            p_real -= q.p_real;
            p_imag -= q.p_imag;
        }

        inline
        void operator*=(const Quaternion& q)
        {
            scalar temp = p_real;
            p_real = p_real * q.p_real - dot(p_imag, q.p_imag);
            p_imag = temp * q.p_imag + q.p_real * p_imag + cross(p_imag, q.p_imag);
        }

        inline
        void operator/=(const Quaternion& q)
        {
            operator*=(inverse(q));
        }

        inline
        void operator*=(const scalar s)
        {
            p_real *= s;
            p_imag *= s;
        }

        inline
        void operator/=(const scalar s)
        {
            p_real /= s;
            p_imag /= s;
        }
    };

    inline
    bool equal(const Quaternion& lhs, const Quaternion& rhs)
    {
        return lhs.real() == rhs.real() && lhs.imag() == rhs.imag();
    }

    inline
    bool operator==(const Quaternion& lhs, const Quaternion& rhs)
    {
        return equal(lhs, rhs);
    }

    inline
    bool operator!=(const Quaternion& lhs, const Quaternion& rhs)
    {
        return !equal(lhs, rhs);
    }

    inline
    Quaternion operator+(const Quaternion& lhs, const Quaternion& rhs)
    {
        return {lhs.real() + rhs.real(), lhs.imag() + rhs.imag()};
    }

    inline
    Quaternion operator-(const Quaternion& q)
    {
        return {q.real(), q.imag()};
    }

    inline
    Quaternion operator-(const Quaternion& lhs, const Quaternion& rhs)
    {
        return {lhs.real() - rhs.real(), lhs.imag() - rhs.imag()};
    }

    inline
    Quaternion operator*(const Quaternion& lhs, const Quaternion& rhs)
    {
        return {
            lhs.real() * rhs.real() - dot(lhs.imag(), rhs.imag()),
            lhs.real() * rhs.imag() + rhs.real() * lhs.imag() + cross(lhs.imag(), rhs.imag())
        };
    }

    inline
    Quaternion operator/(const Quaternion& lhs, const Quaternion& rhs)
    {
        return lhs * inverse(rhs);
    }

    inline
    Quaternion operator*(const scalar s, const Quaternion& q)
    {
        return {s * q.real(), s * q.imag()};
    }

    inline
    Quaternion operator*(const Quaternion& q, const scalar s)
    {
        return {q.real() * s, q.imag() * s};
    }

    inline
    Quaternion operator/(const Quaternion& q, const scalar s)
    {
        return {q.real() / s, q.imag() / s};
    }

    inline
    scalar norm(const Quaternion& q)
    {
        return sqrt(pow(q.real(), 2) + dot(q.imag(), q.imag()));
    }

    inline
    Quaternion conjugate(const Quaternion& q)
    {
        return {q.real(), -q.imag()};
    }

    inline
    Quaternion inverse(const Quaternion& q)
    {
        return conjugate(q) / pow(norm(q), 2);
    }

    inline
    Quaternion normalize(const Quaternion& q)
    {
        return q / norm(q);
    }

    inline
    vec3 rotate(const vec3& point, const vec3& axis, const scalar& angle)
    {
        Quaternion p {point};
        Quaternion q {normalize(axis), angle};
        return (q * p * inverse(q)).imag();
    }

    void decompose(const Quaternion& q, vec3& axis, scalar& angle)
    {
        const scalar qnorm = norm(q.imag());
        axis = q.imag() / qnorm;
        angle = 2 * atan2(qnorm, q.real());
    }

} // end namespace hpr