Fix BigInteger.Rotate*

Author: kzrnm

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

@@ -1701,7 +1701,7 @@ private static BigInteger Add(ReadOnlySpan<uint> leftBits, int leftSign, ReadOnl
             }
 
             if (bitsFromPool != null)
-                    ArrayPool<uint>.Shared.Return(bitsFromPool);
+                ArrayPool<uint>.Shared.Return(bitsFromPool);
 
             return result;
         }
@@ -2636,7 +2636,7 @@ public static implicit operator BigInteger(nuint value)
 
             if (zdFromPool != null)
                 ArrayPool<uint>.Shared.Return(zdFromPool);
-        exit:
+            exit:
             if (xdFromPool != null)
                 ArrayPool<uint>.Shared.Return(xdFromPool);
 
@@ -3239,7 +3239,16 @@ public static BigInteger PopCount(BigInteger value)
         public static BigInteger RotateLeft(BigInteger value, int rotateAmount)
         {
             value.AssertValid();
-            int byteCount = (value._bits is null) ? sizeof(int) : (value._bits.Length * 4);
+
+            bool negx = value._sign < 0;
+            ReadOnlySpan<uint> bits = value._bits ?? stackalloc uint[1] { NumericsHelpers.Abs(value._sign) };
+            int xl = bits.Length;
+
+            if (negx && bits[^1] >= kuMaskHighBit
+                && !(bits.IndexOfAnyExcept(0u) == bits.Length - 1 && bits[^1] == kuMaskHighBit))
+                ++xl;
+
+            int byteCount = xl * 4;
 
             // Normalize the rotate amount to drop full rotations
             rotateAmount = (int)(rotateAmount % (byteCount * 8L));
@@ -3256,14 +3265,13 @@ public static BigInteger RotateLeft(BigInteger value, int rotateAmount)
             (int digitShift, int smallShift) = Math.DivRem(rotateAmount, kcbitUint);
 
             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);
             xd = xd.Slice(0, xl);
+            xd[^1] = 0;
 
-            bool negx = value.GetPartsForBitManipulation(xd);
+            bits.CopyTo(xd);
 
             int zl = xl;
             uint[]? zdFromPool = null;
@@ -3374,7 +3382,17 @@ public static BigInteger RotateLeft(BigInteger value, int rotateAmount)
         public static BigInteger RotateRight(BigInteger value, int rotateAmount)
         {
             value.AssertValid();
-            int byteCount = (value._bits is null) ? sizeof(int) : (value._bits.Length * 4);
+
+
+            bool negx = value._sign < 0;
+            ReadOnlySpan<uint> bits = value._bits ?? stackalloc uint[1] { NumericsHelpers.Abs(value._sign) };
+            int xl = bits.Length;
+
+            if (negx && bits[^1] >= kuMaskHighBit
+                && !(bits.IndexOfAnyExcept(0u) == bits.Length - 1 && bits[^1] == kuMaskHighBit))
+                ++xl;
+
+            int byteCount = xl * 4;
 
             // Normalize the rotate amount to drop full rotations
             rotateAmount = (int)(rotateAmount % (byteCount * 8L));
@@ -3391,14 +3409,13 @@ public static BigInteger RotateRight(BigInteger value, int rotateAmount)
             (int digitShift, int smallShift) = Math.DivRem(rotateAmount, kcbitUint);
 
             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);
             xd = xd.Slice(0, xl);
+            xd[^1] = 0;
 
-            bool negx = value.GetPartsForBitManipulation(xd);
+            bits.CopyTo(xd);
 
             int zl = xl;
             uint[]? zdFromPool = null;
@@ -3445,19 +3462,12 @@ public static BigInteger RotateRight(BigInteger value, int rotateAmount)
             {
                 int carryShift = kcbitUint - smallShift;
 
-                int dstIndex = 0;
-                int srcIndex = digitShift;
-
-                uint carry = 0;
+                int dstIndex = xd.Length - 1;
+                int srcIndex = digitShift == 0
+                    ? xd.Length - 1
+                    : digitShift - 1;
 
-                if (digitShift == 0)
-                {
-                    carry = xd[^1] << carryShift;
-                }
-                else
-                {
-                    carry = xd[srcIndex - 1] << carryShift;
-                }
+                uint carry = xd[digitShift] << carryShift;
 
                 do
                 {
@@ -3466,22 +3476,22 @@ public static BigInteger RotateRight(BigInteger value, int rotateAmount)
                     zd[dstIndex] = (part >> smallShift) | carry;
                     carry = part << carryShift;
 
-                    dstIndex++;
-                    srcIndex++;
+                    dstIndex--;
+                    srcIndex--;
                 }
-                while (srcIndex < xd.Length);
+                while ((uint)srcIndex < (uint)xd.Length);
 
-                srcIndex = 0;
+                srcIndex = xd.Length - 1;
 
-                while (dstIndex < zd.Length)
+                while ((uint)dstIndex < (uint)zd.Length)
                 {
                     uint part = xd[srcIndex];
 
                     zd[dstIndex] = (part >> smallShift) | carry;
                     carry = part << carryShift;
 
-                    dstIndex++;
-                    srcIndex++;
+                    dstIndex--;
+                    srcIndex--;
                 }
             }
 

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

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Globalization;
+using Microsoft.VisualStudio.CodeCoverage;
 using Xunit;
 
 namespace System.Numerics.Tests
@@ -173,9 +174,10 @@ public void RunRotateTests()
         [Fact]
         public void RunSmallTests()
         {
+            VerifyRotateString(Print(GetRandomSmallByteArray(-32)) + Print(GetRandomSmallByteArray(-2147483649)) + opstring);
             foreach (int i in new int[] {
-                    0,
-                    1,
+                0,
+                1,
                     16,
                     31,
                     32,
@@ -225,7 +227,13 @@ private static void VerifyRotateString(string opstring)
             StackCalc sc = new StackCalc(opstring);
             while (sc.DoNextOperation())
             {
+                Eq();
                 Assert.Equal(sc.snCalc.Peek().ToString(), sc.myCalc.Peek().ToString());
+                void Eq()
+                {
+                    if (sc.snCalc.Peek() != sc.myCalc.Peek())
+                        throw new Exception(opstring);
+                }
             }
         }