Utf16ValueStringBuilder.Insert runs into System.ArgumentException: Destination is too short.

[JohannesDeml]

I wanted to benchmark the library and ran into an issue for the StringBuilder.Insert equivalent method. Here is a simple program that replicated the issue:

using System;
using Cysharp.Text;

public static class Program
{
	private static int Main(string[] args)
	{
		string result = ZStringBuilderInsert();
		return 0;
	}

	private static string ZStringBuilderInsert()
	{
		var stringArray = new string[100];
		for (int i = 0; i < stringArray.Length; i++)
		{
			stringArray[i] = new string('A', 1000);
		}

		using (Utf16ValueStringBuilder sb = ZString.CreateStringBuilder())
		{
			for (var i = stringArray.Length - 1; i >= 0; i--)
			{
				var part = stringArray[i];
				try
				{
					sb.Insert(0, part);
				}
				catch (Exception e)
				{
					Console.WriteLine($"Error during string concatenation #{i}: " + e.Message);
					throw;
				}
			}

			return sb.ToString();
		}
	}
}

This results in the following error:

MicroBenchmarks/bin/Release/net8.0/MicroBenchmarks
Error during string concatenation #67: Destination is too short. (Parameter 'destination')
Unhandled exception. System.ArgumentException: Destination is too short. (Parameter 'destination')
   at Cysharp.Text.Utf16ValueStringBuilder.Insert(Int32 index, ReadOnlySpan`1 value, Int32 count)
   at Cysharp.Text.Utf16ValueStringBuilder.Insert(Int32 index, String value)
   at MicroBenchmarks.Program.ZStringBuilderInsert() in /Users/johannes.deml/Documents/CSharp/MicroBenchmarksDotNet/MicroBenchmarks/Program.cs:line 39
   at MicroBenchmarks.Program.Main(String[] args) in /Users/johannes.deml/Documents/CSharp/MicroBenchmarksDotNet/MicroBenchmarks/Program.cs:line 20

Process finished with exit code 134.

I guess there is an issue with resizing the buffer? I didn't look too deep into the issue, but I didn't find it tracked here, so I thought it is a good idea to open up an issue for the exception.

[TheCodingOwl]

Is there an older package that doesn't contain this error?

I was going to use this but if it crashes for something as straightforward as a repeated insert I don't think it's usable. It seems the creator has abandoned this project.

[JohannesDeml]

To be fair, it is only happening at 67K characters with the insert. The append (which I guess is the more common way to do string concatenation) method does not have the issue. But yeah, a bit disappointing to hit an error with such a simple setup.

[roponator]

This error happens if the buffer contains more than Utf16ValueStringBuilder.DefaultBufferSize = 32768; characters.

Fix:

In file Utf16ValueStringBuilder.cs in function public void Insert(int index, ReadOnlySpan value, int count) fix this line:

     var newBuffer = ArrayPool<char>.Shared.Rent(Math.Max(currentLength + value.Length * count, newSize)); // Fix
    // var newBuffer = ArrayPool<char>.Shared.Rent(Math.Max(currentLength, newSize)); // Old, wrong

I did some manual test cases and it seems to work.

Simple repro example that crashes the original:

        // ok, no crash.
        {
            Cysharp.Text.Utf16ValueStringBuilder Sb = ZString.CreateStringBuilder();

            int SmallerThanDefaultBufferSize = 20000;   // Must be SMALLER than Utf16ValueStringBuilder.DefaultBufferSize = 32768; 
          
            for (int i = 0; i < SmallerThanDefaultBufferSize ; ++i)
            {
                Sb.Append("a");
            }

            Sb.Insert(0, "b");
        }

        // fail, crashes
        {
            Cysharp.Text.Utf16ValueStringBuilder Sb = ZString.CreateStringBuilder();

            int LargerThanDefaultBufferSize = 33000; // Crashes if LARGER than Utf16ValueStringBuilder.DefaultBufferSize = 32768; 


            for (int i = 0; i < LargerThanDefaultBufferSize; ++i)
            {
                Sb.Append("a");
            }

            Sb.Insert(1, "b"); // CRASH HERE!
        }