From c31cfcef316de035d3ee5a7bceff64438826fabe Mon Sep 17 00:00:00 2001
From: Qujak <jasper.gerigk@gmx.net>
Date: Sun, 7 Jan 2018 20:06:02 +0100
Subject: [PATCH] Changed Math.Acos and added tests

---
 .../System/MathTest.cs                        | 19 +++++
 .../Cosmos.System2_Plugs/System/MathImpl.cs   | 70 ++++++++++++++++++-
 2 files changed, 86 insertions(+), 3 deletions(-)

diff --git a/Tests/Cosmos.Compiler.Tests.Bcl/System/MathTest.cs b/Tests/Cosmos.Compiler.Tests.Bcl/System/MathTest.cs
index 2c92be2bc..e04c1d8eb 100644
--- a/Tests/Cosmos.Compiler.Tests.Bcl/System/MathTest.cs
+++ b/Tests/Cosmos.Compiler.Tests.Bcl/System/MathTest.cs
@@ -31,6 +31,25 @@ namespace Cosmos.Compiler.Tests.Bcl.System
 
             #endregion Math.Asin
 
+            #region Math.Acos
+
+            result = Math.Acos(1.1);
+            Assert.IsTrue(double.IsNaN(result), "Math.Acos returns NaN for values larger than 1");
+
+            result = Math.Acos(-1.1);
+            Assert.IsTrue(double.IsNaN(result), "Math.Acos returns NaN for values smaller than -1");
+
+            result = Math.Acos(1);
+            Assert.IsTrue(EqualityHelper.DoublesAreEqual(result, 0), "Acos returns correct value for 1");
+
+            result = Math.Acos(-1);
+            Assert.IsTrue(EqualityHelper.DoublesAreEqual(result, Math.PI), "Acos returns correct value for -1");
+
+            result = Math.Acos(0);
+            Assert.IsTrue(EqualityHelper.DoublesAreEqual(result, 1.5707963267949), "Acos returns correct value for 9");
+
+            #endregion Math.Acos
+
             #region Math.Cos
 
             result = Math.Cos(4);
diff --git a/source/Cosmos.System2_Plugs/System/MathImpl.cs b/source/Cosmos.System2_Plugs/System/MathImpl.cs
index 47077a189..c77884abc 100644
--- a/source/Cosmos.System2_Plugs/System/MathImpl.cs
+++ b/source/Cosmos.System2_Plugs/System/MathImpl.cs
@@ -67,9 +67,73 @@ namespace Cosmos.System_Plugs.System
 
         public static double Acos(double x)
         {
-            if ((x > 1.0) || (x < -1.0))
-                throw new ArgumentOutOfRangeException("Domain error");
-            return (pio2 - Asin(x));
+            double pio2_hi = 1.57079632679489655800e+00, /* 0x3FF921FB, 0x54442D18 */
+                pio2_lo = 6.12323399573676603587e-17, /* 0x3C91A626, 0x33145C07 */
+                pS0 = 1.66666666666666657415e-01, /* 0x3FC55555, 0x55555555 */
+                pS1 = -3.25565818622400915405e-01, /* 0xBFD4D612, 0x03EB6F7D */
+                pS2 = 2.01212532134862925881e-01, /* 0x3FC9C155, 0x0E884455 */
+                pS3 = -4.00555345006794114027e-02, /* 0xBFA48228, 0xB5688F3B */
+                pS4 = 7.91534994289814532176e-04, /* 0x3F49EFE0, 0x7501B288 */
+                pS5 = 3.47933107596021167570e-05, /* 0x3F023DE1, 0x0DFDF709 */
+                qS1 = -2.40339491173441421878e+00, /* 0xC0033A27, 0x1C8A2D4B */
+                qS2 = 2.02094576023350569471e+00, /* 0x40002AE5, 0x9C598AC8 */
+                qS3 = -6.88283971605453293030e-01, /* 0xBFE6066C, 0x1B8D0159 */
+                qS4 = 7.70381505559019352791e-02; /* 0x3FB3B8C5, 0xB12E9282 */
+
+            double z, p, q, r, w, s, c, df;
+            int hx, ix;
+            hx = HighWord(x);
+            ix = hx & 0x7fffffff;
+            if (ix >= 0x3ff00000)
+            {   /* |x| >= 1 */
+                if (((ix - 0x3ff00000) | LowWord(x)) == 0)
+                {   /* |x|==1 */
+                    if (hx > 0)
+                        return 0.0;     /* acos(1) = 0  */
+                    else
+                        return Math.PI + 2.0 * pio2_lo; /* acos(-1)= pi */
+                }
+                return (x - x) / (x - x);       /* acos(|x|>1) is NaN */
+            }
+            if (ix < 0x3fe00000)
+            {   /* |x| < 0.5 */
+                if (ix <= 0x3c600000)
+                    return pio2_hi + pio2_lo;/*if|x|<2**-57*/
+                z = x * x;
+                p = z * (pS0 + z * (pS1 + z * (pS2 + z * (pS3 + z * (pS4 + z * pS5)))));
+                q = 1 + z * (qS1 + z * (qS2 + z * (qS3 + z * qS4)));
+                r = p / q;
+                return pio2_hi - (x - (pio2_lo - x * r));
+            }
+            else if (hx < 0)
+            {       /* x < -0.5 */
+                z = (1 + x) * 0.5;
+                p = z * (pS0 + z * (pS1 + z * (pS2 + z * (pS3 + z * (pS4 + z * pS5)))));
+                q = 1 + z * (qS1 + z * (qS2 + z * (qS3 + z * qS4)));
+                s = Math.Sqrt(z);
+                r = p / q;
+                w = r * s - pio2_lo;
+                return Math.PI - 2.0 * (s + w);
+            }
+            else
+            {           /* x > 0.5 */
+                z = (1 - x) * 0.5;
+                s = Math.Sqrt(z);
+                df = s;
+                //__LO(df) = 0;
+                Byte[] bdf = BitConverter.GetBytes(BitConverter.DoubleToInt64Bits(df));
+                for (int i = 0; i < 4; i++)
+                {
+                    bdf[i + (BitConverter.IsLittleEndian ? 4 : 0)] = 0;
+                }
+                df = BitConverter.ToDouble(bdf, 0);
+                c = (z - df * df) / (s + df);
+                p = z * (pS0 + z * (pS1 + z * (pS2 + z * (pS3 + z * (pS4 + z * pS5)))));
+                q = 1 + z * (qS1 + z * (qS2 + z * (qS3 + z * qS4)));
+                r = p / q;
+                w = r * s + c;
+                return 2.0 * (df + w);
+            }
         }
 
         #endregion Acos