Redo minor cleanups in System.Runtime.Numerics (#71274)

* Replace union with BitConverter

* Cleanup complex

* Use HashCode

* Update src/libraries/System.Runtime.Numerics/src/System/Numerics/NumericsHelpers.cs

* Update src/libraries/System.Runtime.Numerics/src/System/Numerics/BigInteger.cs
Co-authored-by: NTanner Gooding <tagoo@outlook.com>
Co-authored-by: NStephen Toub <stoub@microsoft.com>
Co-authored-by: NTanner Gooding <tagoo@outlook.com>

src/libraries/System.Runtime.Numerics/src/System/Numerics/BigInteger.cs

@@ -1084,12 +1084,10 @@ public override int GetHashCode()
             if (_bits is null)
                 return _sign;
 
-            int hash = _sign;
-            for (int iv = _bits.Length; --iv >= 0;)
-                hash = unchecked((int)CombineHash((uint)hash, _bits[iv]));
-            return hash;
-
-            static uint CombineHash(uint u1, uint u2) => ((u1 << 7) | (u1 >> 25)) ^ u2;
+            HashCode hash = default;
+            hash.AddBytes(MemoryMarshal.AsBytes(_bits.AsSpan()));
+            hash.Add(_sign);
+            return hash.ToHashCode();
         }
 
         public override bool Equals([NotNullWhen(true)] object? obj)

src/libraries/System.Runtime.Numerics/src/System/Numerics/Complex.cs

@@ -383,8 +383,7 @@ public static Complex Reciprocal(Complex value)
 
         public override bool Equals([NotNullWhen(true)] object? obj)
         {
-            if (!(obj is Complex)) return false;
-            return Equals((Complex)obj);
+            return obj is Complex other && Equals(other);
         }
 
         public bool Equals(Complex value)
@@ -392,14 +391,7 @@ public bool Equals(Complex value)
             return m_real.Equals(value.m_real) && m_imaginary.Equals(value.m_imaginary);
         }
 
-        public override int GetHashCode()
-        {
-            int n1 = 99999997;
-            int realHash = m_real.GetHashCode() % n1;
-            int imaginaryHash = m_imaginary.GetHashCode();
-            int finalHash = realHash ^ imaginaryHash;
-            return finalHash;
-        }
+        public override int GetHashCode() => HashCode.Combine(m_real, m_imaginary);
 
         public override string ToString() => $"<{m_real}; {m_imaginary}>";
 

src/libraries/System.Runtime.Numerics/src/System/Numerics/NumericsHelpers.cs

@@ -2,32 +2,20 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Diagnostics;
-using System.Runtime.InteropServices;
 
 namespace System.Numerics
 {
-    [StructLayout(LayoutKind.Explicit)]
-    internal struct DoubleUlong
-    {
-        [FieldOffset(0)]
-        public double dbl;
-        [FieldOffset(0)]
-        public ulong uu;
-    }
-
     internal static class NumericsHelpers
     {
         private const int kcbitUint = 32;
 
         public static void GetDoubleParts(double dbl, out int sign, out int exp, out ulong man, out bool fFinite)
         {
-            DoubleUlong du;
-            du.uu = 0;
-            du.dbl = dbl;
+            ulong bits = BitConverter.DoubleToUInt64Bits(dbl);
 
-            sign = 1 - ((int)(du.uu >> 62) & 2);
-            man = du.uu & 0x000FFFFFFFFFFFFF;
-            exp = (int)(du.uu >> 52) & 0x7FF;
+            sign = 1 - ((int)(bits >> 62) & 2);
+            man = bits & 0x000FFFFFFFFFFFFF;
+            exp = (int)(bits >> 52) & 0x7FF;
             if (exp == 0)
             {
                 // Denormalized number.
@@ -51,11 +39,12 @@ public static void GetDoubleParts(double dbl, out int sign, out int exp, out ulo
 
         public static double GetDoubleFromParts(int sign, int exp, ulong man)
         {
-            DoubleUlong du;
-            du.dbl = 0;
+            ulong bits;
 
             if (man == 0)
-                du.uu = 0;
+            {
+                bits = 0;
+            }
             else
             {
                 // Normalize so that 0x0010 0000 0000 0000 is the highest bit set.
@@ -74,7 +63,7 @@ public static double GetDoubleFromParts(int sign, int exp, ulong man)
                 if (exp >= 0x7FF)
                 {
                     // Infinity.
-                    du.uu = 0x7FF0000000000000;
+                    bits = 0x7FF0000000000000;
                 }
                 else if (exp <= 0)
                 {
@@ -83,25 +72,25 @@ public static double GetDoubleFromParts(int sign, int exp, ulong man)
                     if (exp < -52)
                     {
                         // Underflow to zero.
-                        du.uu = 0;
+                        bits = 0;
                     }
                     else
                     {
-                        du.uu = man >> -exp;
-                        Debug.Assert(du.uu != 0);
+                        bits = man >> -exp;
+                        Debug.Assert(bits != 0);
                     }
                 }
                 else
                 {
                     // Mask off the implicit high bit.
-                    du.uu = (man & 0x000FFFFFFFFFFFFF) | ((ulong)exp << 52);
+                    bits = (man & 0x000FFFFFFFFFFFFF) | ((ulong)exp << 52);
                 }
             }
 
             if (sign < 0)
-                du.uu |= 0x8000000000000000;
+                bits |= 0x8000000000000000;
 
-            return du.dbl;
+            return BitConverter.UInt64BitsToDouble(bits);
         }
 
         // Do an in-place two's complement. "Dangerous" because it causes