//
// 
// some of the code is rewritten from the AMD code:
// https://github.com/amd/aocl-libm-ose/blob/a43e092b03213eb0e129a1cc129900e6e4475778/src/ref/asinf.c


using System;
using System.Collections.Generic;
using System.Text;


namespace StgSharp.Math
{
    public static unsafe partial class Calc
    {

        private const float
        piby2_tail = 7.5497894159e-08f, /* 0x33a22168 */
        hpiby2_head = 7.8539812565e-01f, /* 0x3f490fda */
        piby2 = 1.5707963705e+00f; /* 0x3fc90fdb */


        /// <summary>
        /// Reurns the value of ArcSin(x)
        /// </summary>
        /// <param name="x">The sine value of the angle</param>
        /// <returns></returns>
        /// <exception cref="ArgumentException"></exception>
        public static float ASin(float x )
        {
            
            int sign = 1;
            float y ;

            if (x<0)
            {
                sign = -1;
                y = -x;
            }
            else
            {
                y = x;
            }

            if ( x>1)
            {
                throw new ArgumentException();
            }

            float r, s, u, v = 0.0f;
            bool transform = (y >= 0.5f);
            
            if (transform)
            {
                r = 0.5f * (1.0f - y);
                s = StgSharp.Math.Sqrt(r);
                y = s;
            }
            else
            {
                r = y * y;
            }
            u= r * (0.184161606965100694821398249421F +
                   (-0.0565298683201845211985026327361F +
                   (-0.0133819288943925804214011424456F -
                   0.00396137437848476485201154797087F * r) * r) * r) /
                  (1.10496961524520294485512696706F -
                   0.836411276854206731913362287293F * r);
            /**/

            if (transform)
            {
                float c, s1, p, q;
                uint us;

                us = *(uint*)&s;
                uint uss = us & 0xffff0000;
                s1 = *(float*)&uss;

                c = (r - s1 * s1) / (s + s1);
                p = 2.0F * s * u - (piby2_tail - 2.0F * c);
                q = hpiby2_head - 2.0F * s1;
                v = hpiby2_head - (p - q);
            }
            else
            {
                v = y + y * u;
            }/**/

            return sign * v;

        }


        /// <summary>
        /// Returns the angle whose sine is the quotient of two specified numbers.
        /// </summary>
        /// <param name="x">The x coordinate of a point.</param>
        /// <param name="y">The y coordinate of a point.</param>
        /// <returns></returns>
        /// <exception cref="ArgumentException"></exception>
        public static unsafe float ASin(float x, float y)
        {
            bool isXNeg = (x < 0);
            if (isXNeg)
            {
                x = -x;
            }
            float z = y /StgSharp.Math.Sqrt(x * x + y * y);



            int sign = 1;
            float t;

            if (z < 0)
            {
                sign = -1;
                t = -z;
            }
            else
            {
                t = z;
            }

            if (z > 1)
            {
                throw new ArgumentException();
            }

            float r, s, u, v = 0.0f;
            bool transform = (t >= 0.5f);

            if (transform)
            {
                r = 0.5f * (1.0f - t);
                s = StgSharp.Math.Sqrt(r);
                t = s;
            }
            else
            {
                r = t * t;
            }
            u = r * (0.184161606965100694821398249421F +
                   (-0.0565298683201845211985026327361F +
                   (-0.0133819288943925804214011424456F -
                   0.00396137437848476485201154797087F * r) * r) * r) /
                  (1.10496961524520294485512696706F -
                   0.836411276854206731913362287293F * r);
            /**/

            if (transform)
            {
                float c, s1, p, q;
                uint us;

                us = *(uint*)&s;
                uint uss = us & 0xffff0000;
                s1 = *(float*)&uss;

                c = (r - s1 * s1) / (s + s1);
                p = 2.0F * s * u - (piby2_tail - 2.0F * c);
                q = hpiby2_head - 2.0F * s1;
                v = hpiby2_head - (p - q);
            }
            else
            {
                v = t + t * u;
            }/**/

            if (isXNeg)
            {
                v = piby2 - x;
            }

            return sign * v;
        }
    }
}