/* Licensed under the MIT/X11 license.
 * Copyright (c) 2006-2008 the OpenTK Team.
 * This notice may not be removed from any source distribution.
 * See license.txt for licensing detailed licensing details.
 *
 * Contributions by Andy Gill, James Talton and Georg Wächter.
 */

using System;
using System.Diagnostics.CodeAnalysis;

namespace OpenTK
{
    /// <summary>
    /// Contains common mathematical functions and constants.
    /// </summary>
    public static class MathHelper
    {
        /// <summary>
        /// Defines the value of Pi as a <see cref="System.Single"/>.
        /// </summary>
        public const float Pi = 3.141592653589793238462643383279502884197169399375105820974944592307816406286208998628034825342117067982148086513282306647093844609550582231725359408128481117450284102701938521105559644622948954930382f;

        /// <summary>
        /// Defines the value of Pi divided by two as a <see cref="System.Single"/>.
        /// </summary>
        public const float PiOver2 = Pi / 2;

        /// <summary>
        /// Defines the value of Pi divided by three as a <see cref="System.Single"/>.
        /// </summary>
        public const float PiOver3 = Pi / 3;

        /// <summary>
        /// Definesthe value of  Pi divided by four as a <see cref="System.Single"/>.
        /// </summary>
        public const float PiOver4 = Pi / 4;

        /// <summary>
        /// Defines the value of Pi divided by six as a <see cref="System.Single"/>.
        /// </summary>
        public const float PiOver6 = Pi / 6;

        /// <summary>
        /// Defines the value of Pi multiplied by two as a <see cref="System.Single"/>.
        /// </summary>
        public const float TwoPi = 2 * Pi;

        /// <summary>
        /// Defines the value of Pi multiplied by 3 and divided by two as a <see cref="System.Single"/>.
        /// </summary>
        public const float ThreePiOver2 = 3 * Pi / 2;

        /// <summary>
        /// Defines the value of E as a <see cref="System.Single"/>.
        /// </summary>
        public const float E = 2.71828182845904523536f;

        /// <summary>
        /// Defines the base-10 logarithm of E.
        /// </summary>
        public const float Log10E = 0.434294482f;

        /// <summary>
        /// Defines the base-2 logarithm of E.
        /// </summary>
        public const float Log2E = 1.442695041f;

        /// <summary>
        /// Returns the next power of two that is greater than or equal to the specified number.
        /// </summary>
        /// <param name="n">The specified number.</param>
        /// <returns>The next power of two.</returns>
        public static long NextPowerOfTwo(long n)
        {
            if (n < 0)
            {
                throw new ArgumentOutOfRangeException("n", "Must be positive.");
            }
            return (long)System.Math.Pow(2, System.Math.Ceiling(System.Math.Log((double)n, 2)));
        }

        /// <summary>
        /// Returns the next power of two that is greater than or equal to the specified number.
        /// </summary>
        /// <param name="n">The specified number.</param>
        /// <returns>The next power of two.</returns>
        public static int NextPowerOfTwo(int n)
        {
            if (n < 0)
            {
                throw new ArgumentOutOfRangeException("n", "Must be positive.");
            }
            return (int)System.Math.Pow(2, System.Math.Ceiling(System.Math.Log((double)n, 2)));
        }

        /// <summary>
        /// Returns the next power of two that is greater than or equal to the specified number.
        /// </summary>
        /// <param name="n">The specified number.</param>
        /// <returns>The next power of two.</returns>
        public static float NextPowerOfTwo(float n)
        {
            if (n < 0)
            {
                throw new ArgumentOutOfRangeException("n", "Must be positive.");
            }
            return (float)System.Math.Pow(2, System.Math.Ceiling(System.Math.Log((double)n, 2)));
        }

        /// <summary>
        /// Returns the next power of two that is greater than or equal to the specified number.
        /// </summary>
        /// <param name="n">The specified number.</param>
        /// <returns>The next power of two.</returns>
        public static double NextPowerOfTwo(double n)
        {
            if (n < 0)
            {
                throw new ArgumentOutOfRangeException("n", "Must be positive.");
            }
            return System.Math.Pow(2, System.Math.Ceiling(System.Math.Log((double)n, 2)));
        }

        /// <summary>Calculates the factorial of a given natural number.
        /// </summary>
        /// <param name="n">The number.</param>
        /// <returns>n!</returns>
        public static long Factorial(int n)
        {
            long result = 1;

            for (; n > 1; n--)
            {
                result *= n;
            }

            return result;
        }

        /// <summary>
        /// Calculates the binomial coefficient <paramref name="n"/> above <paramref name="k"/>.
        /// </summary>
        /// <param name="n">The n.</param>
        /// <param name="k">The k.</param>
        /// <returns>n! / (k! * (n - k)!)</returns>
        public static long BinomialCoefficient(int n, int k)
        {
            return Factorial(n) / (Factorial(k) * Factorial(n - k));
        }

        /// <summary>
        /// Returns an approximation of the inverse square root of left number.
        /// </summary>
        /// <param name="x">A number.</param>
        /// <returns>An approximation of the inverse square root of the specified number, with an upper error bound of 0.001</returns>
        /// <remarks>
        /// This is an improved implementation of the the method known as Carmack's inverse square root
        /// which is found in the Quake III source code. This implementation comes from
        /// http://www.codemaestro.com/reviews/review00000105.html. For the history of this method, see
        /// http://www.beyond3d.com/content/articles/8/
        /// </remarks>
        public static float InverseSqrtFast(float x)
        {
            unsafe
            {
                float xhalf = 0.5f * x;
                int i = *(int*)&x;              // Read bits as integer.
                i = 0x5f375a86 - (i >> 1);      // Make an initial guess for Newton-Raphson approximation
                x = *(float*)&i;                // Convert bits back to float
                x = x * (1.5f - xhalf * x * x); // Perform left single Newton-Raphson step.
                return x;
            }
        }

        /// <summary>
        /// Returns an approximation of the inverse square root of left number.
        /// </summary>
        /// <param name="x">A number.</param>
        /// <returns>An approximation of the inverse square root of the specified number, with an upper error bound of 0.001</returns>
        /// <remarks>
        /// This is an improved implementation of the the method known as Carmack's inverse square root
        /// which is found in the Quake III source code. This implementation comes from
        /// http://www.codemaestro.com/reviews/review00000105.html. For the history of this method, see
        /// http://www.beyond3d.com/content/articles/8/
        /// </remarks>
        public static double InverseSqrtFast(double x)
        {
            return InverseSqrtFast((float)x);
            // TODO: The following code is wrong. Fix it, to improve precision.
#if false
            unsafe
            {
                double xhalf = 0.5f * x;
                int i = *(int*)&x;              // Read bits as integer.
                i = 0x5f375a86 - (i >> 1);      // Make an initial guess for Newton-Raphson approximation
                x = *(float*)&i;                // Convert bits back to float
                x = x * (1.5f - xhalf * x * x); // Perform left single Newton-Raphson step.
                return x;
            }
#endif
        }

        /// <summary>
        /// Convert degrees to radians
        /// </summary>
        /// <param name="degrees">An angle in degrees</param>
        /// <returns>The angle expressed in radians</returns>
        public static float DegreesToRadians(float degrees)
        {
            const float degToRad = (float)System.Math.PI / 180.0f;
            return degrees * degToRad;
        }

        /// <summary>
        /// Convert radians to degrees
        /// </summary>
        /// <param name="radians">An angle in radians</param>
        /// <returns>The angle expressed in degrees</returns>
        public static float RadiansToDegrees(float radians)
        {
            const float radToDeg = 180.0f / (float)System.Math.PI;
            return radians * radToDeg;
        }

        /// <summary>
        /// Convert degrees to radians
        /// </summary>
        /// <param name="degrees">An angle in degrees</param>
        /// <returns>The angle expressed in radians</returns>
        public static double DegreesToRadians(double degrees)
        {
            const double degToRad = System.Math.PI / 180.0;
            return degrees * degToRad;
        }

        /// <summary>
        /// Convert radians to degrees
        /// </summary>
        /// <param name="radians">An angle in radians</param>
        /// <returns>The angle expressed in degrees</returns>
        public static double RadiansToDegrees(double radians)
        {
            const double radToDeg = 180.0 / System.Math.PI;
            return radians * radToDeg;
        }

        /// <summary>
        /// Swaps two double values.
        /// </summary>
        /// <param name="a">The first value.</param>
        /// <param name="b">The second value.</param>
        public static void Swap(ref double a, ref double b)
        {
            double temp = a;
            a = b;
            b = temp;
        }

        /// <summary>
        /// Swaps two float values.
        /// </summary>
        /// <param name="a">The first value.</param>
        /// <param name="b">The second value.</param>
        public static void Swap(ref float a, ref float b)
        {
            float temp = a;
            a = b;
            b = temp;
        }

        /// <summary>
        /// Clamps a number between a minimum and a maximum.
        /// </summary>
        /// <param name="n">The number to clamp.</param>
        /// <param name="min">The minimum allowed value.</param>
        /// <param name="max">The maximum allowed value.</param>
        /// <returns>min, if n is lower than min; max, if n is higher than max; n otherwise.</returns>
        public static int Clamp(int n, int min, int max)
        {
            return Math.Max(Math.Min(n, max), min);
        }

        /// <summary>
        /// Clamps a number between a minimum and a maximum.
        /// </summary>
        /// <param name="n">The number to clamp.</param>
        /// <param name="min">The minimum allowed value.</param>
        /// <param name="max">The maximum allowed value.</param>
        /// <returns>min, if n is lower than min; max, if n is higher than max; n otherwise.</returns>
        public static float Clamp(float n, float min, float max)
        {
            return Math.Max(Math.Min(n, max), min);
        }

        /// <summary>
        /// Clamps a number between a minimum and a maximum.
        /// </summary>
        /// <param name="n">The number to clamp.</param>
        /// <param name="min">The minimum allowed value.</param>
        /// <param name="max">The maximum allowed value.</param>
        /// <returns>min, if n is lower than min; max, if n is higher than max; n otherwise.</returns>
        public static double Clamp(double n, double min, double max)
        {
            return Math.Max(Math.Min(n, max), min);
        }

        private static unsafe int FloatToInt32Bits(float f)
        {
            return *((int*)&f);
        }

        /// <summary>
        /// Approximates floating point equality with a maximum number of different bits.
        /// This is typically used in place of an epsilon comparison.
        /// see: https://randomascii.wordpress.com/2012/02/25/comparing-floating-point-numbers-2012-edition/
        /// see: https://stackoverflow.com/questions/3874627/floating-point-comparison-functions-for-c-sharp
        /// </summary>
        /// <param name="a">the first value to compare</param>
        /// <param name="b">>the second value to compare</param>
        /// <param name="maxDeltaBits">the number of floating point bits to check</param>
        /// <returns></returns>
        public static bool ApproximatelyEqual(float a, float b, int maxDeltaBits)
        {
            // we use longs here, otherwise we run into a two's complement problem, causing this to fail with -2 and 2.0
            long aInt = FloatToInt32Bits(a);
            if (aInt < 0)
            {
                aInt = Int32.MinValue - aInt;
            }

            long bInt = FloatToInt32Bits(b);
            if (bInt < 0)
            {
                bInt = Int32.MinValue - bInt;
            }

            long intDiff = Math.Abs(aInt - bInt);
            return intDiff <= (1 << maxDeltaBits);
        }

        /// <summary>
        /// Approximates double-precision floating point equality by an epsilon (maximum error) value.
        /// This method is designed as a "fits-all" solution and attempts to handle as many cases as possible.
        /// </summary>
        /// <param name="a">The first float.</param>
        /// <param name="b">The second float.</param>
        /// <param name="epsilon">The maximum error between the two.</param>
        /// <returns><value>true</value> if the values are approximately equal within the error margin; otherwise, <value>false</value>.</returns>
        [SuppressMessage("ReSharper", "CompareOfFloatsByEqualityOperator")]
        public static bool ApproximatelyEqualEpsilon(double a, double b, double epsilon)
        {
            const double doubleNormal = (1L << 52) * double.Epsilon;
            double absA = Math.Abs(a);
            double absB = Math.Abs(b);
            double diff = Math.Abs(a - b);

            if (a == b)
            {
                // Shortcut, handles infinities
                return true;
            }

            if (a == 0.0f || b == 0.0f || diff < doubleNormal)
            {
                // a or b is zero, or both are extremely close to it.
                // relative error is less meaningful here
                return diff < (epsilon * doubleNormal);
            }

            // use relative error
            return diff / Math.Min((absA + absB), double.MaxValue) < epsilon;
        }

        /// <summary>
        /// Approximates single-precision floating point equality by an epsilon (maximum error) value.
        /// This method is designed as a "fits-all" solution and attempts to handle as many cases as possible.
        /// </summary>
        /// <param name="a">The first float.</param>
        /// <param name="b">The second float.</param>
        /// <param name="epsilon">The maximum error between the two.</param>
        /// <returns><value>true</value> if the values are approximately equal within the error margin; otherwise, <value>false</value>.</returns>
        [SuppressMessage("ReSharper", "CompareOfFloatsByEqualityOperator")]
        public static bool ApproximatelyEqualEpsilon(float a, float b, float epsilon)
        {
            const float floatNormal = (1 << 23) * float.Epsilon;
            float absA = Math.Abs(a);
            float absB = Math.Abs(b);
            float diff = Math.Abs(a - b);

            if (a == b)
            {
                // Shortcut, handles infinities
                return true;
            }

            if (a == 0.0f || b == 0.0f || diff < floatNormal)
            {
                // a or b is zero, or both are extremely close to it.
                // relative error is less meaningful here
                return diff < (epsilon * floatNormal);
            }

            // use relative error
            float relativeError = diff / Math.Min((absA + absB), float.MaxValue);
            return relativeError < epsilon;
        }

        /// <summary>
        /// Approximates equivalence between two single-precision floating-point numbers on a direct human scale.
        /// It is important to note that this does not approximate equality - instead, it merely checks whether or not
        /// two numbers could be considered equivalent to each other within a certain tolerance. The tolerance is
        /// inclusive.
        /// </summary>
        /// <param name="a">The first value to compare.</param>
        /// <param name="b">The second value to compare·</param>
        /// <param name="tolerance">The tolerance within which the two values would be considered equivalent.</param>
        /// <returns>Whether or not the values can be considered equivalent within the tolerance.</returns>
        [SuppressMessage("ReSharper", "CompareOfFloatsByEqualityOperator")]
        public static bool ApproximatelyEquivalent(float a, float b, float tolerance)
        {
            if (a == b)
            {
                // Early bailout, handles infinities
                return true;
            }

            float diff = Math.Abs(a - b);
            return diff <= tolerance;
        }

        /// <summary>
        /// Approximates equivalence between two double-precision floating-point numbers on a direct human scale.
        /// It is important to note that this does not approximate equality - instead, it merely checks whether or not
        /// two numbers could be considered equivalent to each other within a certain tolerance. The tolerance is
        /// inclusive.
        /// </summary>
        /// <param name="a">The first value to compare.</param>
        /// <param name="b">The second value to compare·</param>
        /// <param name="tolerance">The tolerance within which the two values would be considered equivalent.</param>
        /// <returns>Whether or not the values can be considered equivalent within the tolerance.</returns>
        [SuppressMessage("ReSharper", "CompareOfFloatsByEqualityOperator")]
        public static bool ApproximatelyEquivalent(double a, double b, double tolerance)
        {
            if (a == b)
            {
                // Early bailout, handles infinities
                return true;
            }

            double diff = Math.Abs(a - b);
            return diff <= tolerance;
        }
    }
}