Fix BitArray.CopyTo (#98846)

* Fix BitArray.CopyTo

* Revert refactors in fix BitArray.CopyTo

* Post merge fix

Author: lilinus

src/libraries/System.Collections/src/System/Collections/BitArray.cs

@@ -761,21 +761,14 @@ public unsafe void CopyTo(Array array, int index)
                     throw new ArgumentException(SR.Argument_InvalidOffLen);
                 }
 
-                Div32Rem(m_length, out int extraBits);
+                int quotient = Div32Rem(m_length, out int extraBits);
 
-                if (extraBits == 0)
-                {
-                    // we have perfect bit alignment, no need to sanitize, just copy
-                    Array.Copy(m_array, 0, intArray, index, m_array.Length);
-                }
-                else
-                {
-                    int last = (m_length - 1) >> BitShiftPerInt32;
-                    // do not copy the last int, as it is not completely used
-                    Array.Copy(m_array, 0, intArray, index, last);
+                Array.Copy(m_array, 0, intArray, index, quotient);
 
+                if (extraBits > 0)
+                {
                     // the last int needs to be masked
-                    intArray[index + last] = m_array[last] & unchecked((1 << extraBits) - 1);
+                    intArray[index + quotient] = m_array[quotient] & unchecked((1 << extraBits) - 1);
                 }
             }
             else if (array is byte[] byteArray)

src/libraries/System.Collections/tests/BitArray/BitArray_GetSetTests.cs

@@ -396,6 +396,18 @@ public static void CopyToIntArray()
             }
         }
 
+        // https://github.com/dotnet/runtime/issues/98813
+        [Fact]
+        public static void CopyToIntArray_Regression98813()
+        {
+            BitArray bitArray = new BitArray(256);
+            bitArray.Length = 32;
+            int[] expectedOutput = new int[] { 0 };
+            int[] actualOutput = new int[1];
+            bitArray.CopyTo(actualOutput, 0);
+            Assert.Equal(expectedOutput, actualOutput);
+        }
+
         // https://github.com/dotnet/runtime/issues/30440
         [Fact]
         public static void CopyToByteArray_Regression39929()