Improve Random (performance, APIs, ...)

src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems

@@ -483,6 +483,10 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\PlatformNotSupportedException.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Progress.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Random.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Random.ImplBase.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Random.LegacyImpl.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Random.Xoshiro128StarStarImpl.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Random.Xoshiro256StarStarImpl.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Range.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\RankException.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\ReadOnlyMemory.cs" />

src/libraries/System.Private.CoreLib/src/System/Numerics/BitOperations.cs

@@ -215,6 +215,32 @@ ref MemoryMarshal.GetReference(Log2DeBruijn),
                 (IntPtr)(int)((value * 0x07C4ACDDu) >> 27));
         }
 
+        /// <summary>Returns the integer (ceiling) log of the specified value, base 2.</summary>
+        /// <param name="value">The value.</param>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static int Log2Ceiling(uint value)
+        {
+            int result = Log2(value);
+            if (PopCount(value) != 1)
+            {
+                result++;
+            }
+            return result;
+        }
+
+        /// <summary>Returns the integer (ceiling) log of the specified value, base 2.</summary>
+        /// <param name="value">The value.</param>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static int Log2Ceiling(ulong value)
+        {
+            int result = Log2(value);
+            if (PopCount(value) != 1)
+            {
+                result++;
+            }
+            return result;
+        }
+
         /// <summary>
         /// Returns the population count (number of bits set) of a mask.
         /// Similar in behavior to the x86 instruction POPCNT.

src/libraries/System.Private.CoreLib/src/System/Random.ImplBase.cs

@@ -0,0 +1,34 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System
+{
+    public partial class Random
+    {
+        /// <summary>Base type for all generator implementations that plug into the base Random.</summary>
+        internal abstract class ImplBase
+        {
+            public abstract double Sample();
+
+            public abstract int Next();
+
+            public abstract int Next(int maxValue);
+
+            public abstract int Next(int minValue, int maxValue);
+
+            public abstract long NextInt64();
+
+            public abstract long NextInt64(long maxValue);
+
+            public abstract long NextInt64(long minValue, long maxValue);
+
+            public abstract float NextSingle();
+
+            public abstract double NextDouble();
+
+            public abstract void NextBytes(byte[] buffer);
+
+            public abstract void NextBytes(Span<byte> buffer);
+        }
+    }
+}

src/libraries/System.Private.CoreLib/src/System/Random.LegacyImpl.cs

@@ -0,0 +1,224 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Diagnostics;
+using System.Numerics;
+
+namespace System
+{
+    public partial class Random
+    {
+        /// <summary>
+        /// Provides an implementation used for compatibility with cases where either a) the
+        /// sequence of numbers could be predicted based on the algorithm employed historically and
+        /// thus expected (e.g. a specific seed used in tests) or b) where a derived type may
+        /// reasonably expect its overrides to be called.  The algorithm is based on a modified version
+        /// of Knuth's subtractive random number generator algorithm.  See https://github.com/dotnet/runtime/issues/23198
+        /// for a discussion of some of the modifications / discrepancies.
+        /// </summary>
+        private sealed class LegacyImpl : ImplBase
+        {
+            /// <summary>Thread-static instance used to seed any legacy implementations created with the default ctor.</summary>
+            [ThreadStatic]
+            private static ImplBase? t_seedGenerator;
+
+            /// <summary>Reference to the <see cref="Random"/> containing this implementation instance.</summary>
+            /// <remarks>Used to ensure that any calls to other virtual members are performed using the Random-derived instance, if one exists.</remarks>
+            private readonly Random _parent;
+            private readonly int[] _seedArray;
+            private int _inext;
+            private int _inextp;
+
+            public LegacyImpl(Random parent) : this(parent, (t_seedGenerator ??= CreateDefaultImpl()).Next())
+            {
+            }
+
+            public LegacyImpl(Random parent, int Seed)
+            {
+                _parent = parent;
+
+                // Initialize seed array.
+                int[] seedArray = _seedArray = new int[56];
+
+                int subtraction = (Seed == int.MinValue) ? int.MaxValue : Math.Abs(Seed);
+                int mj = 161803398 - subtraction; // magic number based on Phi (golden ratio)
+                seedArray[55] = mj;
+                int mk = 1;
+
+                int ii = 0;
+                for (int i = 1; i < 55; i++)
+                {
+                    // The range [1..55] is special (Knuth) and so we're wasting the 0'th position.
+                    if ((ii += 21) >= 55)
+                    {
+                        ii -= 55;
+                    }
+
+                    seedArray[ii] = mk;
+                    mk = mj - mk;
+                    if (mk < 0)
+                    {
+                        mk += int.MaxValue;
+                    }
+
+                    mj = seedArray[ii];
+                }
+
+                for (int k = 1; k < 5; k++)
+                {
+                    for (int i = 1; i < 56; i++)
+                    {
+                        int n = i + 30;
+                        if (n >= 55)
+                        {
+                            n -= 55;
+                        }
+
+                        seedArray[i] -= seedArray[1 + n];
+                        if (seedArray[i] < 0)
+                        {
+                            seedArray[i] += int.MaxValue;
+                        }
+                    }
+                }
+
+                _inextp = 21;
+            }
+
+            public override double Sample() =>
+                // Including the division at the end gives us significantly improved random number distribution.
+                InternalSample() * (1.0 / int.MaxValue);
+
+            public override int Next() => InternalSample();
+
+            public override int Next(int maxValue) => (int)(_parent.Sample() * maxValue);
+
+            public override int Next(int minValue, int maxValue)
+            {
+                long range = (long)maxValue - minValue;
+                return range <= int.MaxValue ?
+                    (int)(_parent.Sample() * range) + minValue :
+                    (int)((long)(GetSampleForLargeRange() * range) + minValue);
+            }
+
+            public override long NextInt64()
+            {
+                while (true)
+                {
+                    // Get top 63 bits to get a value in the range [0, long.MaxValue], but try again
+                    // if the value is actually long.MaxValue, as the method is defined to return a value
+                    // in the range [0, long.MaxValue).
+                    ulong result = NextUInt64() >> 1;
+                    if (result != long.MaxValue)
+                    {
+                        return (long)result;
+                    }
+                }
+            }
+
+            public override long NextInt64(long maxValue) => NextInt64(0, maxValue);
+
+            public override long NextInt64(long minValue, long maxValue)
+            {
+                ulong exclusiveRange = (ulong)(maxValue - minValue);
+
+                if (exclusiveRange > 1)
+                {
+                    // Narrow down to the smallest range [0, 2^bits] that contains maxValue - minValue
+                    // Then repeatedly generate a value in that outer range until we get one within the inner range.
+                    int bits = BitOperations.Log2Ceiling(exclusiveRange);
+                    while (true)
+                    {
+                        ulong result = NextUInt64() >> (sizeof(long) * 8 - bits);
+                        if (result < exclusiveRange)
+                        {
+                            return (long)result + minValue;
+                        }
+                    }
+                }
+
+                Debug.Assert(minValue == maxValue || minValue + 1 == maxValue);
+                return minValue;
+            }
+
+            /// <summary>Produces a value in the range [0, ulong.MaxValue].</summary>
+            private unsafe ulong NextUInt64()
+            {
+                Span<byte> resultBytes = stackalloc byte[8];
+                NextBytes(resultBytes);
+                return BitConverter.ToUInt64(resultBytes);
+            }
+
+            public override double NextDouble() => _parent.Sample();
+
+            public override float NextSingle() => (float)_parent.Sample();
+
+            public override void NextBytes(byte[] buffer)
+            {
+                for (int i = 0; i < buffer.Length; i++)
+                {
+                    buffer[i] = (byte)InternalSample();
+                }
+            }
+
+            public override void NextBytes(Span<byte> buffer)
+            {
+                for (int i = 0; i < buffer.Length; i++)
+                {
+                    buffer[i] = (byte)_parent.Next();
+                }
+            }
+
+            private int InternalSample()
+            {
+                int locINext = _inext;
+                if (++locINext >= 56)
+                {
+                    locINext = 1;
+                }
+
+                int locINextp = _inextp;
+                if (++locINextp >= 56)
+                {
+                    locINextp = 1;
+                }
+
+                int[] seedArray = _seedArray;
+                int retVal = seedArray[locINext] - seedArray[locINextp];
+
+                if (retVal == int.MaxValue)
+                {
+                    retVal--;
+                }
+                if (retVal < 0)
+                {
+                    retVal += int.MaxValue;
+                }
+
+                seedArray[locINext] = retVal;
+                _inext = locINext;
+                _inextp = locINextp;
+
+                return retVal;
+            }
+
+            private double GetSampleForLargeRange()
+            {
+                // The distribution of the double returned by Sample is not good enough for a large range.
+                // If we use Sample for a range [int.MinValue..int.MaxValue), we will end up getting even numbers only.
+                int result = InternalSample();
+
+                // We can't use addition here: the distribution will be bad if we do that.
+                if (InternalSample() % 2 == 0) // decide the sign based on second sample
+                {
+                    result = -result;
+                }
+
+                double d = result;
+                d += int.MaxValue - 1; // get a number in range [0..2*int.MaxValue-1)
+                d /= 2u * int.MaxValue - 1;
+                return d;
+            }
+        }
+    }
+}