Added more comments, minor code refactoring

Author: Sergio0694

Microsoft.Toolkit.HighPerformance/Extensions/ReadOnlySpanExtensions.cs

@@ -221,34 +221,20 @@ public static ReadOnlySpan2D<T> AsSpan2D<T>(this ReadOnlySpan<T> span, int offse
         /// <exception cref="ArgumentOutOfRangeException">Thrown if <paramref name="value"/> does not belong to <paramref name="span"/>.</exception>
         [Pure]
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static unsafe int IndexOf<T>(this ReadOnlySpan<T> span, in T value)
+        public static int IndexOf<T>(this ReadOnlySpan<T> span, in T value)
         {
             ref T r0 = ref MemoryMarshal.GetReference(span);
             ref T r1 = ref Unsafe.AsRef(value);
             IntPtr byteOffset = Unsafe.ByteOffset(ref r0, ref r1);
 
-            if (sizeof(IntPtr) == sizeof(long))
-            {
-                long elementOffset = (long)byteOffset / Unsafe.SizeOf<T>();
-
-                if ((ulong)elementOffset >= (ulong)span.Length)
-                {
-                    SpanExtensions.ThrowArgumentOutOfRangeExceptionForInvalidReference();
-                }
+            nint elementOffset = byteOffset / (nint)(uint)Unsafe.SizeOf<T>();
 
-                return unchecked((int)elementOffset);
-            }
-            else
+            if ((nuint)elementOffset >= (uint)span.Length)
             {
-                int elementOffset = (int)byteOffset / Unsafe.SizeOf<T>();
-
-                if ((uint)elementOffset >= (uint)span.Length)
-                {
-                    SpanExtensions.ThrowArgumentOutOfRangeExceptionForInvalidReference();
-                }
-
-                return elementOffset;
+                SpanExtensions.ThrowArgumentOutOfRangeExceptionForInvalidReference();
             }
+
+            return (int)elementOffset;
         }
 
         /// <summary>

Microsoft.Toolkit.HighPerformance/Extensions/SpanExtensions.cs

@@ -167,33 +167,19 @@ public static Span<TTo> Cast<TFrom, TTo>(this Span<TFrom> span)
         /// <exception cref="ArgumentOutOfRangeException">Thrown if <paramref name="value"/> does not belong to <paramref name="span"/>.</exception>
         [Pure]
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static unsafe int IndexOf<T>(this Span<T> span, ref T value)
+        public static int IndexOf<T>(this Span<T> span, ref T value)
         {
             ref T r0 = ref MemoryMarshal.GetReference(span);
             IntPtr byteOffset = Unsafe.ByteOffset(ref r0, ref value);
 
-            if (sizeof(IntPtr) == sizeof(long))
-            {
-                long elementOffset = (long)byteOffset / Unsafe.SizeOf<T>();
-
-                if ((ulong)elementOffset >= (ulong)span.Length)
-                {
-                    ThrowArgumentOutOfRangeExceptionForInvalidReference();
-                }
+            nint elementOffset = byteOffset / (nint)(uint)Unsafe.SizeOf<T>();
 
-                return unchecked((int)elementOffset);
-            }
-            else
+            if ((nuint)elementOffset >= (uint)span.Length)
             {
-                int elementOffset = (int)byteOffset / Unsafe.SizeOf<T>();
-
-                if ((uint)elementOffset >= (uint)span.Length)
-                {
-                    ThrowArgumentOutOfRangeExceptionForInvalidReference();
-                }
-
-                return elementOffset;
+                ThrowArgumentOutOfRangeExceptionForInvalidReference();
             }
+
+            return (int)elementOffset;
         }
 
         /// <summary>

Microsoft.Toolkit.HighPerformance/Memory/ReadOnlyMemory2D{T}.cs

@@ -830,6 +830,13 @@ public bool TryGetMemory(out ReadOnlyMemory<T> memory)
                 }
                 else if (typeof(T) == typeof(char) && this.instance.GetType() == typeof(string))
                 {
+                    // Here we need to create a Memory<char> from the wrapped string, and to do so we need to do an inverse
+                    // lookup to find the initial index of the string with respect to the byte offset we're currently using,
+                    // which refers to the raw string object data. This can include variable padding or other additional
+                    // fields on different runtimes. The lookup operation is still O(1) and just computes the byte offset
+                    // difference between the start of the Span<char> (which directly wraps just the actual character data
+                    // within the string), and the input reference, which we can get from the byte offset in use. The result
+                    // is the character index which we can use to create the final Memory<char> instance.
                     string text = Unsafe.As<string>(this.instance);
                     int index = text.AsSpan().IndexOf(in text.DangerousGetObjectDataReferenceAt<char>(this.offset));
                     ReadOnlyMemory<char> temp = text.AsMemory(index, (int)Length);