sebastienros.jint/Jint/Native/String/StringInlHelper.cs

using System.Text;

namespace Jint.Native.String
{
    /// <summary>
    /// Some internacionalization logic that is special or specific to determined culture.
    /// </summary>
    internal class StringInlHelper
    {
        private static List<int> GetLithuaninanReplaceableCharIdx(string input)
        {
            List<int> replaceableCharsIdx = new List<int>();
            for (int i = 0; i < input.Length; i++)
            {
                if (input[i].Equals('\u0307'))
                {
                    replaceableCharsIdx.Add(i);
                }
            }

            // For capital I and J we do not replace the dot above (\u3017).
            replaceableCharsIdx
                .RemoveAll(idx => (idx > 0) && input[idx - 1] == 'I' || input[idx - 1] == 'J');

            return replaceableCharsIdx;
        }

        /// <summary>
        /// Lithuanian case is a bit special. For more info see:
        /// https://github.com/tc39/test262/blob/main/test/intl402/String/prototype/toLocaleUpperCase/special_casing_Lithuanian.js
        /// </summary>
        public static string LithuanianStringProcessor(string input)
        {
            var replaceableCharsIdx = GetLithuaninanReplaceableCharIdx(input);
            if (replaceableCharsIdx.Count > 0)
            {
                StringBuilder stringBuilder = new StringBuilder(input);

                // Remove characters in reverse order to avoid index shifting
                for (int i = replaceableCharsIdx.Count - 1; i >= 0; i--)
                {
                    int index = replaceableCharsIdx[i];
                    if (index >= 0 && index < stringBuilder.Length)
                    {
                        stringBuilder.Remove(index, 1);
                    }
                }

                return stringBuilder.ToString();
            }

            return input;
        }
    }
}