Fix the unsigned right shift operator of BigInteger (#112879)

* Add tests for the shift operator of BigInteger

* Fix the unsigned right shift operator of BigInteger

* avoid stackalloc

* external sign element

Author: kzrnm

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

@@ -5284,13 +5284,32 @@ static bool INumberBase<BigInteger>.TryConvertToTruncating<TOther>(BigInteger va
 
             BigInteger result;
 
+            bool negx = value._sign < 0;
+            uint smallBits = NumericsHelpers.Abs(value._sign);
+            scoped ReadOnlySpan<uint> bits = value._bits;
+            if (bits.IsEmpty)
+            {
+                bits = new ReadOnlySpan<uint>(in smallBits);
+            }
+
+            int xl = bits.Length;
+            if (negx && (bits[^1] >= kuMaskHighBit) && ((bits[^1] != kuMaskHighBit) || bits.IndexOfAnyExcept(0u) != (bits.Length - 1)))
+            {
+                // For a shift of N x 32 bit,
+                // We check for a special case where its sign bit could be outside the uint array after 2's complement conversion.
+                // For example given [0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF], its 2's complement is [0x01, 0x00, 0x00]
+                // After a 32 bit right shift, it becomes [0x00, 0x00] which is [0x00, 0x00] when converted back.
+                // The expected result is [0x00, 0x00, 0xFFFFFFFF] (2's complement) or [0x00, 0x00, 0x01] when converted back
+                // If the 2's component's last element is a 0, we will track the sign externally
+                ++xl;
+            }
+
             uint[]? xdFromPool = null;
-            int xl = value._bits?.Length ?? 1;
             Span<uint> xd = (xl <= BigIntegerCalculator.StackAllocThreshold
                           ? stackalloc uint[BigIntegerCalculator.StackAllocThreshold]
                           : xdFromPool = ArrayPool<uint>.Shared.Rent(xl)).Slice(0, xl);
-
-            bool negx = value.GetPartsForBitManipulation(xd);
+            xd[^1] = 0;
+            bits.CopyTo(xd);
 
             if (negx)
             {

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

@@ -108,6 +108,8 @@ public static BigInteger DoBinaryOperatorMine(BigInteger num1, BigInteger num2,
                     return new BigInteger(Max(bytes1, bytes2).ToArray());
                 case "b>>":
                     return new BigInteger(ShiftLeft(bytes1, Negate(bytes2)).ToArray());
+                case "b>>>":
+                    return new BigInteger(ShiftRightUnsigned(bytes1, bytes2).ToArray());
                 case "b<<":
                     return new BigInteger(ShiftLeft(bytes1, bytes2).ToArray());
                 case "bRotateLeft":
@@ -641,11 +643,68 @@ public static List<byte> Not(List<byte> bytes)
             return bnew;
         }
 
+        public static List<byte> ShiftRightUnsigned(List<byte> bytes1, List<byte> bytes2)
+        {
+            int byteShift = (int)new BigInteger(Divide(Copy(bytes2), new List<byte>(new byte[] { 8 })).ToArray());
+            sbyte bitShift = (sbyte)new BigInteger(Remainder(Copy(bytes2), new List<byte>(new byte[] { 8 })).ToArray());
+
+            if (byteShift == 0 && bitShift == 0)
+                return bytes1;
+
+            if (byteShift < 0 || bitShift < 0)
+                return ShiftLeft(bytes1, Negate(bytes2));
+
+            Trim(bytes1);
+
+            byte fill = (bytes1[bytes1.Count - 1] & 0x80) != 0 ? byte.MaxValue : (byte)0;
+
+            if (fill == byte.MaxValue)
+            {
+                while (bytes1.Count % 4 != 0)
+                {
+                    bytes1.Add(fill);
+                }
+            }
+
+            if (byteShift >= bytes1.Count)
+            {
+                return [fill];
+            }
+
+            if (fill == byte.MaxValue)
+            {
+                bytes1.Add(0);
+            }
+
+            for (int i = 0; i < bitShift; i++)
+            {
+                bytes1 = ShiftRight(bytes1);
+            }
+
+            List<byte> temp = new List<byte>();
+            for (int i = byteShift; i < bytes1.Count; i++)
+            {
+                temp.Add(bytes1[i]);
+            }
+            bytes1 = temp;
+
+            if (fill == byte.MaxValue && bytes1.Count % 4 == 1)
+            {
+                bytes1.RemoveAt(bytes1.Count - 1);
+            }
+
+            Trim(bytes1);
+
+            return bytes1;
+        }
+
         public static List<byte> ShiftLeft(List<byte> bytes1, List<byte> bytes2)
         {
             int byteShift = (int)new BigInteger(Divide(Copy(bytes2), new List<byte>(new byte[] { 8 })).ToArray());
             sbyte bitShift = (sbyte)new BigInteger(Remainder(bytes2, new List<byte>(new byte[] { 8 })).ToArray());
 
+            Trim(bytes1);
+
             for (int i = 0; i < Math.Abs(bitShift); i++)
             {
                 if (bitShift < 0)

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

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Collections.Generic;
 using Xunit;
 
 namespace System.Numerics.Tests
@@ -151,6 +152,56 @@ public static void RunLeftShiftTests()
             }
         }
 
+        [Fact]
+        public void RunSmallTests()
+        {
+            foreach (int i in new int[] {
+                    0,
+                    1,
+                    16,
+                    31,
+                    32,
+                    33,
+                    63,
+                    64,
+                    65,
+                    100,
+                    127,
+                    128,
+            })
+            {
+                foreach (int shift in new int[] {
+                    0,
+                    -1, 1,
+                    -16, 16,
+                    -31, 31,
+                    -32, 32,
+                    -33, 33,
+                    -63, 63,
+                    -64, 64,
+                    -65, 65,
+                    -100, 100,
+                    -127, 127,
+                    -128, 128,
+                })
+                {
+                    var num = Int128.One << i;
+                    for (int k = -1; k <= 1; k++)
+                    {
+                        foreach (int sign in new int[] { -1, +1 })
+                        {
+                            Int128 value128 = sign * (num + k);
+
+                            byte[] tempByteArray1 = GetRandomSmallByteArray(value128);
+                            byte[] tempByteArray2 = GetRandomSmallByteArray(shift);
+
+                            VerifyLeftShiftString(Print(tempByteArray2) + Print(tempByteArray1) + "b<<");
+                        }
+                    }
+                }
+            }
+        }
+
         private static void VerifyLeftShiftString(string opstring)
         {
             StackCalc sc = new StackCalc(opstring);
@@ -160,6 +211,19 @@ private static void VerifyLeftShiftString(string opstring)
             }
         }
 
+        private static byte[] GetRandomSmallByteArray(Int128 num)
+        {
+            byte[] value = new byte[16];
+
+            for (int i = 0; i < value.Length; i++)
+            {
+                value[i] = (byte)num;
+                num >>= 8;
+            }
+
+            return value;
+        }
+
         private static byte[] GetRandomByteArray(Random random)
         {
             return GetRandomByteArray(random, random.Next(0, 1024));