Add spaces to string based upon the position of Uppercase letters

ToSplitTitleString.cs

/// <summary>
/// Returns the input string with spaces inserted at each uppercase character.
/// </summary>
/// <param name="input">The input string to split.</param>
/// <returns>The <see cref="string"/>.</returns>
public static string ToSplitTitleString(string input)
{
    int spaces = 0;
    Rune whitespace = new(0x0020);
    Rune previous = default;
    foreach (Rune rune in input.EnumerateRunes())
    {
        if (previous != default && Rune.IsUpper(rune) && !Rune.IsWhiteSpace(previous))
        {
            spaces++;
        }

        previous = rune;
    }

    if (spaces == 0)
    {
        return input;
    }

    unsafe
    {
        // Create the new string in an efficient manner by padding out a new span and copying the runes across.
        fixed (char* inputPtr = input)
        {
            return string.Create(input.Length + (spaces * whitespace.Utf16SequenceLength), (Ptr: (IntPtr)inputPtr, input.Length), (chars, args) =>
            {
                ReadOnlySpan<char> source = new((char*)args.Ptr, args.Length);
                Span<char> destination = chars;
                previous = default;
                int i = 0;
                foreach (Rune rune in source.EnumerateRunes())
                {
                    if (previous != default && Rune.IsUpper(rune) && !Rune.IsWhiteSpace(previous))
                    {
                        // Insert a space before the uppercase rune.
                        whitespace.EncodeToUtf16(chars.Slice(i));
                        i += whitespace.Utf16SequenceLength;
                    }

                    rune.EncodeToUtf16(chars.Slice(i));
                    i += rune.Utf16SequenceLength;
                    previous = rune;
                }
            });
        }
    }
}