RawString append. about u8xmlparser

[ikorin24]

Hi, @juliolitwin

The easiest way to combine RawString is calling RawString.ToString() and treating them as string.

RawString foo = ...;
RawString bar = ...;

string combined = $"{foo} and {bar}";

If you also need to take care of performance, you can use RawString.AsSpan() to copy it to byte[] and treat it as utf-8.

However, I have not provided any classes or methods to do this. If you want to do this easily,
a library such as ZString might be suitable.

RawString foo = ...;
RawString bar = ...;

using (var sb = Cysharp.Text.ZString.CreateUtf8StringBuilder())
{
    sb.AppendLiteral(foo.AsSpan());
    sb.Append(" and ");
    sb.AppendLiteral(bar.AsSpan());
    string combined = sb.ToString();
}

[juliolitwin]

Hey @ikorin24, thanks for answering me.

I understand. The performance issue is really important for me, so the first suggestion is not so viable. I know ZString, but I wanted to be able to keep RawString, it's amazing and has a lot of power. I can make an append, but unfortunately the problem is freeing the memory after use.

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        internal RawString(int length)
        {
            Debug.Assert(length >= 0);
            _ptr = Marshal.AllocHGlobal(length);
            AllocationSafety.Add(length);
            _length = length;
        }

        public RawString Append(RawString value)
        {
            // Get the new length from the two strings.
            var totalLength = Length + value.Length;

            // Initialize the new buffer.
            var appendString = new RawString(totalLength);

            fixed (byte* ptr = this.AsSpan())
            fixed (byte* ptr2 = value.AsSpan())
            {
                // Copy the first string for the new buffer.
                SpanHelper.CreateReadOnlySpan<byte>(ptr, Length).CopyTo(SpanHelper.CreateSpan<byte>(appendString.GetPtr(), appendString.Length)[..Length]);
                
                // Copy the second string for the new buffer.
                SpanHelper.CreateReadOnlySpan<byte>(ptr2, value.Length).CopyTo(SpanHelper.CreateSpan<byte>(appendString.GetPtr(), appendString.Length).Slice(Length, value.Length));
            }

            return new RawString((byte*)appendString.Ptr, totalLength);
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public void Dispose()
        {
            Marshal.FreeHGlobal(_ptr);
            AllocationSafety.Remove(_length);
            Unsafe.AsRef(_ptr) = default;
            Unsafe.AsRef(_length) = 0;
        }

[ikorin24]

@juliolitwin OK, I'll tell you implementation of RawString.

RawString does not have ownership of its memory. It is similar to ReadOnlySpan<byte>.

All RawString memories are owned by the XmlObject, which is a pointer to a buffer of bytes read from xml.
This is released when XmlObject.Dispose() is called.
That means the only way to create a new RawString is slicing the buffer memory read from xml.
And I think that I will not provide any other way to create RawString instance.

So, for high performance combining RawString, we should write to Span<byte> you allocate somewhere.

public static bool TryCombine(RawString str1, RawString str2, Span<byte> destination)
{
    if (destination.Length < str1.Length + str2.Length)
    {
        return false;
    }
    str1.AsSpan().CopyTo(destination);
    str2.AsSpan().CopyTo(destination.Slice(str1.Length));
    return true;
}

Most methods in U8XmlParser provides the overload for RawString and ReadOnlySpan<byte>, so you can treat them in the same way.