#region --- License ---
/*
Copyright (c) 2006 - 2008 The Open Toolkit library.

Permission is hereby granted, free of charge, to any person obtaining a copy of
this software and associated documentation files (the "Software"), to deal in
the Software without restriction, including without limitation the rights to
use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
of the Software, and to permit persons to whom the Software is furnished to do
so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
*/
#endregion

using System;
using System.Runtime.InteropServices;
using System.ComponentModel;
using System.Xml.Serialization;

namespace OpenTK.Math
{
    /// <summary>
    /// Represents a double-precision Quaternion.
    /// </summary>
    [Serializable]
    [StructLayout(LayoutKind.Sequential)]
    public struct Quaterniond : IEquatable<Quaterniond>
    {
        #region Fields

        Vector3d xyz;
        double w;

        #endregion

        #region Constructors

        /// <summary>
        /// Construct a new Quaterniond from vector and w components
        /// </summary>
        /// <param name="v">The vector part</param>
        /// <param name="w">The w part</param>
        public Quaterniond(Vector3d v, double w)
        {
            this.xyz = v;
            this.w = w;
        }

        /// <summary>
        /// Construct a new Quaterniond
        /// </summary>
        /// <param name="x">The x component</param>
        /// <param name="y">The y component</param>
        /// <param name="z">The z component</param>
        /// <param name="w">The w component</param>
        public Quaterniond(double x, double y, double z, double w)
            : this(new Vector3d(x, y, z), w)
        { }

        #endregion

        #region Public Members

        #region Properties

        /// <summary>
        /// Gets or sets an OpenTK.Math.Vector3d with the X, Y and Z components of this instance.
        /// </summary>
        [Obsolete("Use Xyz property instead.")]
        [CLSCompliant(false)]
        [EditorBrowsable(EditorBrowsableState.Never)]
        [XmlIgnore]
        public Vector3d XYZ { get { return Xyz; } set { Xyz = value; } }

        /// <summary>
        /// Gets or sets an OpenTK.Math.Vector3d with the X, Y and Z components of this instance.
        /// </summary>
        public Vector3d Xyz { get { return xyz; } set { xyz = value; } }

        /// <summary>
        /// Gets or sets the X component of this instance.
        /// </summary>
        [XmlIgnore]
        public double X { get { return xyz.X; } set { xyz.X = value; } }

        /// <summary>
        /// Gets or sets the Y component of this instance.
        /// </summary>
        [XmlIgnore]
        public double Y { get { return xyz.Y; } set { xyz.Y = value; } }

        /// <summary>
        /// Gets or sets the Z component of this instance.
        /// </summary>
        [XmlIgnore]
        public double Z { get { return xyz.Z; } set { xyz.Z = value; } }

        /// <summary>
        /// Gets or sets the W component of this instance.
        /// </summary>
        public double W { get { return w; } set { w = value; } }
        
        #endregion

        #region Instance

        #region ToAxisAngle

        /// <summary>
        /// Convert the current quaternion to axis angle representation
        /// </summary>
        /// <param name="axis">The resultant axis</param>
        /// <param name="angle">The resultant angle</param>
        public void ToAxisAngle(out Vector3d axis, out double angle)
        {
            Vector4d result = ToAxisAngle();
            axis = result.Xyz;
            angle = result.W;
        }

        /// <summary>
        /// Convert this instance to an axis-angle representation.
        /// </summary>
        /// <returns>A Vector4 that is the axis-angle representation of this quaternion.</returns>
        public Vector4d ToAxisAngle()
        {
            Quaterniond q = this;
            if (q.W > 1.0f)
                q.Normalize();

            Vector4d result = new Vector4d();

            result.W = 2.0f * (float)System.Math.Acos(q.W); // angle
            float den = (float)System.Math.Sqrt(1.0 - q.W * q.W);
            if (den > 0.0001f)
            {
                result.Xyz = q.Xyz / den;
            }
            else
            {
                // This occurs when the angle is zero. 
                // Not a problem: just set an arbitrary normalized axis.
                result.Xyz = Vector3d.UnitX;
            }

            return result;
        }

        #endregion

        #region public double Length

        /// <summary>
        /// Gets the length (magnitude) of the Quaterniond.
        /// </summary>
        /// <seealso cref="LengthSquared"/>
        public double Length
        {
            get
            {
                return (double)System.Math.Sqrt(W * W + Xyz.LengthSquared);
            }
        }

        #endregion

        #region public double LengthSquared

        /// <summary>
        /// Gets the square of the Quaterniond length (magnitude).
        /// </summary>
        public double LengthSquared
        {
            get
            {
                return W * W + Xyz.LengthSquared;
            }
        }

        #endregion

        #region public void Normalize()

        /// <summary>
        /// Scales the Quaterniond to unit length.
        /// </summary>
        public void Normalize()
        {
            double scale = 1.0f / this.Length;
            Xyz *= scale;
            W *= scale;
        }

        #endregion

        #region public void Conjugate()

        /// <summary>
        /// Convert this Quaterniond to its conjugate
        /// </summary>
        public void Conjugate()
        {
            Xyz = -Xyz;
        }

        #endregion

        #endregion

        #region Static

        #region Fields

        /// <summary>
        /// Defines the identity quaternion.
        /// </summary>
        public readonly static Quaterniond Identity = new Quaterniond(0, 0, 0, 1);

        #endregion

        #region Add

        /// <summary>
        /// Add two quaternions
        /// </summary>
        /// <param name="left">The first operand</param>
        /// <param name="right">The second operand</param>
        /// <returns>The result of the addition</returns>
        public static Quaterniond Add(Quaterniond left, Quaterniond right)
        {
            return new Quaterniond(
                left.Xyz + right.Xyz,
                left.W + right.W);
        }

        /// <summary>
        /// Add two quaternions
        /// </summary>
        /// <param name="left">The first operand</param>
        /// <param name="right">The second operand</param>
        /// <param name="result">The result of the addition</param>
        public static void Add(ref Quaterniond left, ref Quaterniond right, out Quaterniond result)
        {
            result = new Quaterniond(
                left.Xyz + right.Xyz,
                left.W + right.W);
        }

        #endregion

        #region Sub

        public static Quaterniond Sub(Quaterniond left, Quaterniond right)
        {
            return new Quaterniond(
                left.Xyz - right.Xyz,
                left.W - right.W);
        }

        public static void Sub(ref Quaterniond left, ref Quaterniond right, out Quaterniond result)
        {
            result = new Quaterniond(
                left.Xyz - right.Xyz,
                left.W - right.W);
        }

        #endregion

        #region Mult

        public static Quaterniond Mult(Quaterniond left, Quaterniond right)
        {
            return new Quaterniond(
                right.W * left.Xyz + left.W * right.Xyz + Vector3d.Cross(left.Xyz, right.Xyz),
                left.W * right.W - Vector3d.Dot(left.Xyz, right.Xyz));
        }

        public static void Mult(ref Quaterniond left, ref Quaterniond right, out Quaterniond result)
        {
            result = new Quaterniond(
                right.W * left.Xyz + left.W * right.Xyz + Vector3d.Cross(left.Xyz, right.Xyz),
                left.W * right.W - Vector3d.Dot(left.Xyz, right.Xyz));
        }

        #endregion

        #region Conjugate

        /// <summary>
        /// Get the conjugate of the given Quaterniond
        /// </summary>
        /// <param name="q">The Quaterniond</param>
        /// <returns>The conjugate of the given Quaterniond</returns>
        public static Quaterniond Conjugate(Quaterniond q)
        {
            return new Quaterniond(-q.Xyz, q.W);
        }

        /// <summary>
        /// Get the conjugate of the given Quaterniond
        /// </summary>
        /// <param name="q">The Quaterniond</param>
        /// <param name="result">The conjugate of the given Quaterniond</param>
        public static void Conjugate(ref Quaterniond q, out Quaterniond result)
        {
            result = new Quaterniond(-q.Xyz, q.W);
        }

        #endregion

        #region Invert

        /// <summary>
        /// Get the inverse of the given Quaterniond
        /// </summary>
        /// <param name="q">The Quaterniond to invert</param>
        /// <returns>The inverse of the given Quaterniond</returns>
        public static Quaterniond Invert(Quaterniond q)
        {
            Quaterniond result;
            Invert(ref q, out result);
            return result;
        }

        /// <summary>
        /// Get the inverse of the given Quaterniond
        /// </summary>
        /// <param name="q">The Quaterniond to invert</param>
        /// <param name="result">The inverse of the given Quaterniond</param>
        public static void Invert(ref Quaterniond q, out Quaterniond result)
        {
            double lengthSq = q.LengthSquared;
            if (lengthSq != 0.0)
            {
                double i = 1.0f / lengthSq;
                result = new Quaterniond(q.Xyz * -i, q.W * i);
            }
            else
            {
                result = q;
            }
        }

        #endregion

        #region Normalize

        /// <summary>
        /// Scale the given Quaterniond to unit length
        /// </summary>
        /// <param name="q">The Quaterniond to normalize</param>
        /// <returns>The normalized Quaterniond</returns>
        public static Quaterniond Normalize(Quaterniond q)
        {
            Quaterniond result;
            Normalize(ref q, out result);
            return result;
        }

        /// <summary>
        /// Scale the given Quaterniond to unit length
        /// </summary>
        /// <param name="q">The Quaterniond to normalize</param>
        /// <param name="result">The normalized Quaterniond</param>
        public static void Normalize(ref Quaterniond q, out Quaterniond result)
        {
            double scale = 1.0f / q.Length;
            result = new Quaterniond(q.Xyz * scale, q.W * scale);
        }

        #endregion

        #region FromAxisAngle

        /// <summary>
        /// Build a Quaterniond from the given axis and angle
        /// </summary>
        /// <param name="axis">The axis to rotate about</param>
        /// <param name="angle">The rotation angle in radians</param>
        /// <returns></returns>
        public static Quaterniond FromAxisAngle(Vector3d axis, double angle)
        {
            if (axis.LengthSquared == 0.0f)
                return Identity;

            Quaterniond result = Identity;

            angle *= 0.5f;
            axis.Normalize();
            result.Xyz = axis * (double)System.Math.Sin(angle);
            result.W = (double)System.Math.Cos(angle);

            return Normalize(result);
        }

        #endregion

        #region Slerp

        /// <summary>
        /// Do Spherical linear interpolation between two quaternions 
        /// </summary>
        /// <param name="q1">The first Quaterniond</param>
        /// <param name="q2">The second Quaterniond</param>
        /// <param name="blend">The blend factor</param>
        /// <returns>A smooth blend between the given quaternions</returns>
        public static Quaterniond Slerp(Quaterniond q1, Quaterniond q2, double blend)
        {
            // if either input is zero, return the other.
            if (q1.LengthSquared == 0.0f)
            {
                if (q2.LengthSquared == 0.0f)
                {
                    return Identity;
                }
                return q2;
            }
            else if (q2.LengthSquared == 0.0f)
            {
                return q1;
            }


            double cosHalfAngle = q1.W * q2.W + Vector3d.Dot(q1.Xyz, q2.Xyz);

            if (cosHalfAngle >= 1.0f || cosHalfAngle <= -1.0f)
            {
                // angle = 0.0f, so just return one input.
                return q1;
            }
            else if (cosHalfAngle < 0.0f)
            {
                q2.Xyz = -q2.Xyz;
                q2.W = -q2.W;
                cosHalfAngle = -cosHalfAngle;
            }

            double blendA;
            double blendB;
            if (cosHalfAngle < 0.99f)
            {
                // do proper slerp for big angles
                double halfAngle = (double)System.Math.Acos(cosHalfAngle);
                double sinHalfAngle = (double)System.Math.Sin(halfAngle);
                double oneOverSinHalfAngle = 1.0f / sinHalfAngle;
                blendA = (double)System.Math.Sin(halfAngle * (1.0f - blend)) * oneOverSinHalfAngle;
                blendB = (double)System.Math.Sin(halfAngle * blend) * oneOverSinHalfAngle;
            }
            else
            {
                // do lerp if angle is really small.
                blendA = 1.0f - blend;
                blendB = blend;
            }

            Quaterniond result = new Quaterniond(blendA * q1.Xyz + blendB * q2.Xyz, blendA * q1.W + blendB * q2.W);
            if (result.LengthSquared > 0.0f)
                return Normalize(result);
            else
                return Identity;
        }

        #endregion

        #endregion
                
        #region Operators

        public static Quaterniond operator +(Quaterniond left, Quaterniond right)
        {
            left.Xyz += right.Xyz;
            left.W += right.W;
            return left;
        }

        public static Quaterniond operator -(Quaterniond left, Quaterniond right)
        {
            left.Xyz -= right.Xyz;
            left.W -= right.W;
            return left;
        }

        public static Quaterniond operator *(Quaterniond left, Quaterniond right)
        {
            double w = left.W * right.W - Vector3d.Dot(left.Xyz, right.Xyz);
            left.Xyz = right.W * left.Xyz + left.W * right.Xyz + Vector3d.Cross(left.Xyz, right.Xyz);
            left.W = w;
            return left;
        }

        public static bool operator ==(Quaterniond left, Quaterniond right)
        {
            return left.Equals(right);
        }

        public static bool operator !=(Quaterniond left, Quaterniond right)
        {
            return !left.Equals(right);
        }

        #endregion

        #region Overrides

        #region public override string ToString()

        /// <summary>
        /// Returns a System.String that represents the current Quaterniond.
        /// </summary>
        /// <returns></returns>
        public override string ToString()
        {
            return String.Format("V: {0}, W: {1}", Xyz, W);
        }

        #endregion

        #region public override bool Equals (object o)

        /// <summary>
        /// Compares this object instance to another object for equality. 
        /// </summary>
        /// <param name="other">The other object to be used in the comparison.</param>
        /// <returns>True if both objects are Quaternions of equal value. Otherwise it returns false.</returns>
        public override bool Equals(object o)
        {
            if (o is Quaterniond == false) return false;
            return this == (Quaterniond)o;
        }

        #endregion

        #region public override int GetHashCode ()

        /// <summary>
        /// Provides the hash code for this object. 
        /// </summary>
        /// <returns>A hash code formed from the bitwise XOR of this objects members.</returns>
        public override int GetHashCode()
        {
            return Xyz.GetHashCode() ^ W.GetHashCode();
        }

        #endregion

        #endregion

        #endregion

#if false

        #region Fields

        /// <summary>The W component of the Quaterniond.</summary>
        public double W;

        /// <summary>The X component of the Quaterniond.</summary>
        public double X;

        /// <summary>The Y component of the Quaterniond.</summary>
        public double Y;

        /// <summary>The Z component of the Quaterniond.</summary>
        public double Z;

        #endregion
        
        #region Constructors

        /// <summary>Constructs left Quaterniond that is left copy of the given Quaterniond.</summary>
        /// <param name="quaterniond">The Quaterniond to copy.</param>
        public Quaterniond(ref Quaterniond Quaterniond) : this(Quaterniond.W, Quaterniond.X, Quaterniond.Y, Quaterniond.Z) { }

        /// <summary>Constructs left Quaterniond from the given components.</summary>
        /// <param name="w">The W component for the Quaterniond.</param>
        /// <param name="vector3d">A Vector representing the X, Y, and Z componets for the quaterion.</param>
        public Quaterniond(double w, ref Vector3d vector3d) : this(w, vector3d.X, vector3d.Y, vector3d.Z) { }

        /// <summary>Constructs left Quaterniond from the given axis and angle.</summary>
        /// <param name="axis">The axis for the Quaterniond.</param>
        /// <param name="angle">The angle for the quaternione.</param>
        public Quaterniond(ref Vector3d axis, double angle)
        {
            double halfAngle = Functions.DTOR * angle / 2;

            this.W = System.Math.Cos(halfAngle);

            double sin = System.Math.Sin(halfAngle);
            Vector3d axisNormalized;
            Vector3d.Normalize(ref axis, out axisNormalized);
            this.X = axisNormalized.X * sin;
            this.Y = axisNormalized.Y * sin;
            this.Z = axisNormalized.Z * sin;
        }

        /// <summary>Constructs left Quaterniond from the given components.</summary>
        /// <param name="w">The W component for the Quaterniond.</param>
        /// <param name="x">The X component for the Quaterniond.</param>
        /// <param name="y">The Y component for the Quaterniond.</param>
        /// <param name="z">The Z component for the Quaterniond.</param>
        public Quaterniond(double w, double x, double y, double z)
        {
            this.W = w;
            this.X = x;
            this.Y = y;
            this.Z = z;
        }

        /// <summary>Constructs left Quaterniond from the given array of double-precision floating point numbers.</summary>
        /// <param name="doubleArray">The array of doubles for the components of the Quaterniond.</param>
        public Quaterniond(double[] doubleArray)
        {
            if (doubleArray == null || doubleArray.GetLength(0) < 4) throw new MissingFieldException();

            this.W = doubleArray[0];
            this.X = doubleArray[1];
            this.Y = doubleArray[2];
            this.Z = doubleArray[3];
        }

        /// <summary>Constructs left Quaterniond from the given matrix.  Only contains rotation information.</summary>
        /// <param name="matrix">The matrix for the components of the Quaterniond.</param>
        public Quaterniond(ref Matrix4d matrix)
        {
            double scale = System.Math.Pow(matrix.Determinant, 1.0d/3.0d);
 
            W = System.Math.Sqrt(System.Math.Max(0, scale + matrix[0, 0] + matrix[1, 1] + matrix[2, 2])) / 2;
            X = System.Math.Sqrt(System.Math.Max(0, scale + matrix[0, 0] - matrix[1, 1] - matrix[2, 2])) / 2;
            Y = System.Math.Sqrt(System.Math.Max(0, scale - matrix[0, 0] + matrix[1, 1] - matrix[2, 2])) / 2;
            Z = System.Math.Sqrt(System.Math.Max(0, scale - matrix[0, 0] - matrix[1, 1] + matrix[2, 2])) / 2; 
            if( matrix[2,1] - matrix[1,2] < 0 ) X = -X;
            if( matrix[0,2] - matrix[2,0] < 0 ) Y = -Y;
            if( matrix[1,0] - matrix[0,1] < 0 ) Z = -Z;
        }

        public Quaterniond(ref Matrix3d matrix)
        {
            double scale = System.Math.Pow(matrix.Determinant, 1.0d / 3.0d);

            W = System.Math.Sqrt(System.Math.Max(0, scale + matrix[0, 0] + matrix[1, 1] + matrix[2, 2])) / 2;
            X = System.Math.Sqrt(System.Math.Max(0, scale + matrix[0, 0] - matrix[1, 1] - matrix[2, 2])) / 2;
            Y = System.Math.Sqrt(System.Math.Max(0, scale - matrix[0, 0] + matrix[1, 1] - matrix[2, 2])) / 2;
            Z = System.Math.Sqrt(System.Math.Max(0, scale - matrix[0, 0] - matrix[1, 1] + matrix[2, 2])) / 2;
            if (matrix[2, 1] - matrix[1, 2] < 0) X = -X;
            if (matrix[0, 2] - matrix[2, 0] < 0) Y = -Y;
            if (matrix[1, 0] - matrix[0, 1] < 0) Z = -Z;
        }

        #endregion

        #region Arithmetic Operators

        public void Add(ref Quaterniond Quaterniond)
        {
            W = W + Quaterniond.W;
            X = X + Quaterniond.X;
            Y = Y + Quaterniond.Y;
            Z = Z + Quaterniond.Z;
        }
        public void Add(ref Quaterniond Quaterniond, out Quaterniond result)
        {
            result.W = W + Quaterniond.W;
            result.X = X + Quaterniond.X;
            result.Y = Y + Quaterniond.Y;
            result.Z = Z + Quaterniond.Z;
        }
        public static void Add(ref Quaterniond left, ref Quaterniond right, out Quaterniond result)
        {
            result.W = left.W + right.W;
            result.X = left.X + right.X;
            result.Y = left.Y + right.Y;
            result.Z = left.Z + right.Z;
        }

        public void Subtract(ref Quaterniond Quaterniond)
        {
            W = W - Quaterniond.W;
            X = X - Quaterniond.X;
            Y = Y - Quaterniond.Y;
            Z = Z - Quaterniond.Z;
        }
        public void Subtract(ref Quaterniond Quaterniond, out Quaterniond result)
        {
            result.W = W - Quaterniond.W;
            result.X = X - Quaterniond.X;
            result.Y = Y - Quaterniond.Y;
            result.Z = Z - Quaterniond.Z;
        }
        public static void Subtract(ref Quaterniond left, ref Quaterniond right, out Quaterniond result)
        {
            result.W = left.W - right.W;
            result.X = left.X - right.X;
            result.Y = left.Y - right.Y;
            result.Z = left.Z - right.Z;
        }

        public void Multiply(ref Quaterniond Quaterniond)
        {
            double w = W * Quaterniond.W - X * Quaterniond.X - Y * Quaterniond.Y - Z * Quaterniond.Z;
            double x = W * Quaterniond.X + X * Quaterniond.W + Y * Quaterniond.Z - Z * Quaterniond.Y;
            double y = W * Quaterniond.Y + Y * Quaterniond.W + Z * Quaterniond.X - X * Quaterniond.Z;
            Z = W * Quaterniond.Z + Z * Quaterniond.W + X * Quaterniond.Y - Y * Quaterniond.X;
            W = w;
            X = x;
            Y = y;
        }
        public void Multiply(ref Quaterniond Quaterniond, out Quaterniond result)
        {
            result.W = W * Quaterniond.W - X * Quaterniond.X - Y * Quaterniond.Y - Z * Quaterniond.Z;
            result.X = W * Quaterniond.X + X * Quaterniond.W + Y * Quaterniond.Z - Z * Quaterniond.Y;
            result.Y = W * Quaterniond.Y + Y * Quaterniond.W + Z * Quaterniond.X - X * Quaterniond.Z;
            result.Z = W * Quaterniond.Z + Z * Quaterniond.W + X * Quaterniond.Y - Y * Quaterniond.X;
        }
        public static void Multiply(ref Quaterniond left, ref Quaterniond right, out Quaterniond result)
        {
            result.W = left.W * right.W - left.X * right.X - left.Y * right.Y - left.Z * right.Z;
            result.X = left.W * right.X + left.X * right.W + left.Y * right.Z - left.Z * right.Y;
            result.Y = left.W * right.Y + left.Y * right.W + left.Z * right.X - left.X * right.Z;
            result.Z = left.W * right.Z + left.Z * right.W + left.X * right.Y - left.Y * right.X;
        }

        public void Multiply(double scalar)
        {
            W = W * scalar;
            X = X * scalar;
            Y = Y * scalar;
            Z = Z * scalar;
        }
        public void Multiply(double scalar, out Quaterniond result)
        {
            result.W = W * scalar;
            result.X = X * scalar;
            result.Y = Y * scalar;
            result.Z = Z * scalar;
        }
        public static void Multiply(ref Quaterniond Quaterniond, double scalar, out Quaterniond result)
        {
            result.W = Quaterniond.W * scalar;
            result.X = Quaterniond.X * scalar;
            result.Y = Quaterniond.Y * scalar;
            result.Z = Quaterniond.Z * scalar;
        }

        public void Divide(double scalar)
        {
            if (scalar == 0) throw new DivideByZeroException();
            W = W / scalar;
            X = X / scalar;
            Y = Y / scalar;
            Z = Z / scalar;
        }
        public void Divide(double scalar, out Quaterniond result)
        {
            if (scalar == 0) throw new DivideByZeroException();
            result.W = W / scalar;
            result.X = X / scalar;
            result.Y = Y / scalar;
            result.Z = Z / scalar;
        }
        public static void Divide(ref Quaterniond Quaterniond, double scalar, out Quaterniond result)
        {
            if (scalar == 0) throw new DivideByZeroException();
            result.W = Quaterniond.W / scalar;
            result.X = Quaterniond.X / scalar;
            result.Y = Quaterniond.Y / scalar;
            result.Z = Quaterniond.Z / scalar;
        }

        #endregion
        
        #region Functions

        public double Modulus
        {
            get
            {
                return System.Math.Sqrt(W * W + X * X + Y * Y + Z * Z);
            }
        }
        public double ModulusSquared
        {
            get
            {
                return W * W + X * X + Y * Y + Z * Z;
            }
        }

        public static double DotProduct(Quaterniond left, Quaterniond right)
        {
            return left.W * right.W + left.X * right.X + left.Y * right.Y + left.Z * right.Z;
        }

        public void Normalize()
        {
            double modulus = System.Math.Sqrt(W * W + X * X + Y * Y + Z * Z);
            if (modulus == 0) throw new DivideByZeroException();
            W = W / modulus;
            X = X / modulus;
            Y = Y / modulus;
            Z = Z / modulus;
        }
        public void Normalize( out Quaterniond result )
        {
            double modulus = System.Math.Sqrt(W * W + X * X + Y * Y + Z * Z);
            if (modulus == 0) throw new DivideByZeroException();
            result.W = W / modulus;
            result.X = X / modulus;
            result.Y = Y / modulus;
            result.Z = Z / modulus;
        }
        public static void Normalize(ref Quaterniond Quaterniond, out Quaterniond result)
        {
            double modulus = System.Math.Sqrt(Quaterniond.W * Quaterniond.W + Quaterniond.X * Quaterniond.X + Quaterniond.Y * Quaterniond.Y + Quaterniond.Z * Quaterniond.Z);
            if (modulus == 0) throw new DivideByZeroException();
            result.W = Quaterniond.W / modulus;
            result.X = Quaterniond.X / modulus;
            result.Y = Quaterniond.Y / modulus;
            result.Z = Quaterniond.Z / modulus;
        }

        public void Conjugate()
        {
            X = -X;
            Y = -Y;
            Z = -Z;
        }
        public void Conjugate( out Quaterniond result )
        {
            result.W = W;
            result.X = -X;
            result.Y = -Y;
            result.Z = -Z;
        }
        public static void Conjugate(ref Quaterniond Quaterniond, out Quaterniond result)
        {
            result.W = Quaterniond.W;
            result.X = -Quaterniond.X;
            result.Y = -Quaterniond.Y;
            result.Z = -Quaterniond.Z;
        }

        public void Inverse()
        {
            double modulusSquared = W * W + X * X + Y * Y + Z * Z;
            if (modulusSquared <= 0) throw new InvalidOperationException();
            double inverseModulusSquared = 1.0 / modulusSquared;
            W = W * inverseModulusSquared;
            X = X * -inverseModulusSquared;
            Y = Y * -inverseModulusSquared;
            Z = Z * -inverseModulusSquared;
        }
        public void Inverse( out Quaterniond result )
        {
            double modulusSquared = W * W + X * X + Y * Y + Z * Z;
            if (modulusSquared <= 0) throw new InvalidOperationException();
            double inverseModulusSquared = 1.0 / modulusSquared;
            result.W = W * inverseModulusSquared;
            result.X = X * -inverseModulusSquared;
            result.Y = Y * -inverseModulusSquared;
            result.Z = Z * -inverseModulusSquared;
        }
        public static void Inverse(ref Quaterniond Quaterniond, out Quaterniond result)
        {
            double modulusSquared = Quaterniond.W * Quaterniond.W + Quaterniond.X * Quaterniond.X + Quaterniond.Y * Quaterniond.Y + Quaterniond.Z * Quaterniond.Z;
            if (modulusSquared <= 0) throw new InvalidOperationException();
            double inverseModulusSquared = 1.0 / modulusSquared;
            result.W = Quaterniond.W * inverseModulusSquared;
            result.X = Quaterniond.X * -inverseModulusSquared;
            result.Y = Quaterniond.Y * -inverseModulusSquared;
            result.Z = Quaterniond.Z * -inverseModulusSquared;
        }

        public void Log()
        {
            if (System.Math.Abs(W) < 1.0)
            {
                double angle = System.Math.Acos(W);
                double sin = System.Math.Sin(angle);

                if (System.Math.Abs(sin) >= 0)
                {
                    double coefficient = angle / sin;
                    X = X * coefficient;
                    Y = Y * coefficient;
                    Z = Z * coefficient;
                }
            }
            else
            {
                X = 0;
                Y = 0;
                Z = 0;
            }

            W = 0;
        }
        public void Log( out Quaterniond result )
        {
            if (System.Math.Abs(W) < 1.0)
            {
                double angle = System.Math.Acos(W);
                double sin = System.Math.Sin(angle);

                if (System.Math.Abs(sin) >= 0)
                {
                    double coefficient = angle / sin;
                    result.X = X * coefficient;
                    result.Y = Y * coefficient;
                    result.Z = Z * coefficient;
                }
                else
                {
                    result.X = X;
                    result.Y = Y;
                    result.Z = Z;
                }
            }
            else
            {
                result.X = 0;
                result.Y = 0;
                result.Z = 0;
            }

            result.W = 0;
        }
        public static void Log(ref Quaterniond Quaterniond, out Quaterniond result)
        {
            if (System.Math.Abs(Quaterniond.W) < 1.0)
            {
                double angle = System.Math.Acos(Quaterniond.W);
                double sin = System.Math.Sin(angle);

                if (System.Math.Abs(sin) >= 0)
                {
                    double coefficient = angle / sin;
                    result.X = Quaterniond.X * coefficient;
                    result.Y = Quaterniond.Y * coefficient;
                    result.Z = Quaterniond.Z * coefficient;
                }
                else
                {
                    result.X = Quaterniond.X;
                    result.Y = Quaterniond.Y;
                    result.Z = Quaterniond.Z;
                }
            }
            else
            {
                result.X = 0;
                result.Y = 0;
                result.Z = 0;
            }

            result.W = 0;
        }

        public void Exp()
        {
            double angle = System.Math.Sqrt(X * X + Y * Y + Z * Z);
            double sin = System.Math.Sin(angle);

            if (System.Math.Abs(sin) > 0)
            {
                double coefficient = angle / sin;
                W = 0;
                X = X * coefficient;
                Y = Y * coefficient;
                Z = Z * coefficient;
            }
            else
            {
                W = 0;
            }
        }
        public void Exp(out Quaterniond result)
        {
            double angle = System.Math.Sqrt(X * X + Y * Y + Z * Z);
            double sin = System.Math.Sin(angle);

            if (System.Math.Abs(sin) > 0)
            {
                double coefficient = angle / sin;
                result.W = 0;
                result.X = X * coefficient;
                result.Y = Y * coefficient;
                result.Z = Z * coefficient;
            }
            else
            {
                result.W = 0;
                result.X = X;
                result.Y = Y;
                result.Z = Z;
            }
        }
        public static void Exp(ref Quaterniond Quaterniond, out Quaterniond result)
        {
            double angle = System.Math.Sqrt(Quaterniond.X * Quaterniond.X + Quaterniond.Y * Quaterniond.Y + Quaterniond.Z * Quaterniond.Z);
            double sin = System.Math.Sin(angle);

            if (System.Math.Abs(sin) > 0)
            {
                double coefficient = angle / sin;
                result.W = 0;
                result.X = Quaterniond.X * coefficient;
                result.Y = Quaterniond.Y * coefficient;
                result.Z = Quaterniond.Z * coefficient;
            }
            else
            {
                result.W = 0;
                result.X = Quaterniond.X;
                result.Y = Quaterniond.Y;
                result.Z = Quaterniond.Z;
            }
        }

        /// <summary>Returns left matrix for this Quaterniond.</summary>
        public void Matrix4d(out Matrix4d result)
        {
            // TODO Expand
            result = new Matrix4d(ref this);
        }

        public void GetAxisAndAngle(out Vector3d axis, out double angle)
        {
            Quaterniond Quaterniond;
            Normalize(out Quaterniond);
            double cos = Quaterniond.W;
            angle = System.Math.Acos(cos) * 2 * Functions.RTOD;
            double sin = System.Math.Sqrt( 1.0d - cos * cos );
            if ( System.Math.Abs( sin ) < 0.0001 ) sin = 1;
            axis = new Vector3d(X / sin, Y / sin, Z / sin);
        }

        public static void Slerp(ref Quaterniond start, ref Quaterniond end, double blend, out Quaterniond result)
        {
            if (start.W == 0 && start.X == 0 && start.Y == 0 && start.Z == 0)
            {
                if (end.W == 0 && end.X == 0 && end.Y == 0 && end.Z == 0)
                {
                    result.W = 1;
                    result.X = 0;
                    result.Y = 0;
                    result.Z = 0;
                }
                else
                {
                    result = end;
                }
            }
            else if (end.W == 0 && end.X == 0 && end.Y == 0 && end.Z == 0)
            {
                result = start;
            }

            Vector3d startVector = new Vector3d(start.X, start.Y, start.Z);
            Vector3d endVector = new Vector3d(end.X, end.Y, end.Z);
            double cosHalfAngle = start.W * end.W + Vector3d.Dot(startVector, endVector);

            if (cosHalfAngle >= 1.0f || cosHalfAngle <= -1.0f)
            {
                // angle = 0.0f, so just return one input.
                result = start;
            }
            else if (cosHalfAngle < 0.0f)
            {
                end.W = -end.W;
                end.X = -end.X;
                end.Y = -end.Y;
                end.Z = -end.Z;
                cosHalfAngle = -cosHalfAngle;
            }

            double blendA;
            double blendB;
            if (cosHalfAngle < 0.99f)
            {
                // do proper slerp for big angles
                double halfAngle = (double)System.Math.Acos(cosHalfAngle);
                double sinHalfAngle = (double)System.Math.Sin(halfAngle);
                double oneOverSinHalfAngle = 1.0f / sinHalfAngle;
                blendA = (double)System.Math.Sin(halfAngle * (1.0f - blend)) * oneOverSinHalfAngle;
                blendB = (double)System.Math.Sin(halfAngle * blend) * oneOverSinHalfAngle;
            }
            else
            {
                // do lerp if angle is really small.
                blendA = 1.0f - blend;
                blendB = blend;
            }

            result.W = blendA * start.W + blendB * end.W;
            result.X = blendA * start.X + blendB * end.X;
            result.Y = blendA * start.Y + blendB * end.Y;
            result.Z = blendA * start.Z + blendB * end.Z;

            if (result.W != 0 || result.X != 0 || result.Y != 0 || result.Z != 0)
            {
                result.Normalize();
            }
            else
            {
                result.W = 1;
                result.X = 0;
                result.Y = 0;
                result.Z = 0;
            }
        }

        #endregion
        
        #region HashCode

        /// <summary>Returns the hash code for this instance.</summary>
        /// <returns>A 32-bit signed integer that is the hash code for this instance.</returns>
        public override int GetHashCode()
        {
            base.GetHashCode();
            return W.GetHashCode() ^ X.GetHashCode() ^ Y.GetHashCode() ^ Z.GetHashCode();
        }

        #endregion
        
        #region String and Parse

        /// <summary>Returns the fully qualified type name of this instance.</summary>
        /// <returns>A System.String containing left fully qualified type name.</returns>
        public override string ToString()
        {
            return string.Format("({0}, {1}, {2}, {3})", W, X, Y, Z);
        }

        /// <summary>Parses left string, converting it to left Quaterniond.</summary>
        /// <param name="str">The string to parse.</param>
        /// <returns>The Quaterniond represented by the string.</returns>
        public static void Parse(string str, out Quaterniond result)
        {
            Match match = new Regex(@"\((?<w>.*),(?<x>.*),(?<y>.*),(?<z>.*)\)", RegexOptions.None).Match(str);
            if (!match.Success) throw new Exception("Parse failed!");

            result.W = double.Parse(match.Result("${w}"));
            result.X = double.Parse(match.Result("${x}"));
            result.Y = double.Parse(match.Result("${y}"));
            result.Z = double.Parse(match.Result("${z}"));
        }

        #endregion
        
        #region Constants

        /// <summary>A quaterion with all zero components.</summary>
        public static readonly Quaterniond Zero = new Quaterniond(0, 0, 0, 0);

        /// <summary>A quaterion representing an identity.</summary>
        public static readonly Quaterniond Identity = new Quaterniond(1, 0, 0, 0);

        /// <summary>A quaterion representing the W axis.</summary>
        public static readonly Quaterniond WAxis = new Quaterniond(1, 0, 0, 0);

        /// <summary>A quaterion representing the X axis.</summary>
        public static readonly Quaterniond XAxis = new Quaterniond(0, 1, 0, 0);

        /// <summary>A quaterion representing the Y axis.</summary>
        public static readonly Quaterniond YAxis = new Quaterniond(0, 0, 1, 0);

        /// <summary>A quaterion representing the Z axis.</summary>
        public static readonly Quaterniond ZAxis = new Quaterniond(0, 0, 0, 1);

        #endregion

#endif

        #region IEquatable<Quaterniond> Members

        /// <summary>
        /// Compares this Quaterniond instance to another Quaterniond for equality. 
        /// </summary>
        /// <param name="other">The other Quaterniond to be used in the comparison.</param>
        /// <returns>True if both instances are equal; false otherwise.</returns>
        public bool Equals(Quaterniond other)
        {
            return Xyz == other.Xyz && W == other.W;
        }

        #endregion
    }
}