Lua-CSharp/src/Lua/Internal/StringHelper.cs

using System.Diagnostics.CodeAnalysis;
using System.Runtime.CompilerServices;
using System.Text;

namespace Lua.Internal;

internal static class StringHelper
{
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static ReadOnlySpan<char> Slice(string s, int i, int j)
    {
        if (i < 0) i = s.Length + i + 1;
        if (j < 0) j = s.Length + j + 1;

        if (i < 1) i = 1;
        if (j > s.Length) j = s.Length;

        return i > j ? "" : s.AsSpan()[(i - 1)..j];
    }

    public static bool TryFromStringLiteral(ReadOnlySpan<char> literal, [NotNullWhen(true)] out string? result)
    {
        var builder = new ValueStringBuilder(literal.Length);
        for (int i = 0; i < literal.Length; i++)
        {
            var c = literal[i];
            if (c is '\\' && i < literal.Length - 1)
            {
                i++;
                c = literal[i];

                switch (c)
                {
                    case 'a':
                        builder.Append('\a');
                        break;
                    case 'b':
                        builder.Append('\b');
                        break;
                    case 'f':
                        builder.Append('\f');
                        break;
                    case 'n':
                        builder.Append('\n');
                        break;
                    case 'r':
                        builder.Append('\r');
                        break;
                    case 't':
                        builder.Append('\t');
                        break;
                    case 'v':
                        builder.Append('\v');
                        break;
                    case '\\':
                        builder.Append('\\');
                        break;
                    case '\"':
                        builder.Append('\"');
                        break;
                    case '\'':
                        builder.Append('\'');
                        break;
                    case '[':
                        builder.Append('[');
                        break;
                    case ']':
                        builder.Append(']');
                        break;
                    default:
                        if (char.IsDigit(c))
                        {
                            var start = i;
                            for (int j = 0; j < 3; j++)
                            {
                                i++;
                                if (i >= literal.Length) break;
                                c = literal[i];
                                if (!char.IsDigit(c)) break;
                            }

                            Console.WriteLine((char)int.Parse(literal[start..i]));

                            builder.Append((char)int.Parse(literal[start..i]));
                            i--;
                        }
                        else
                        {
                            result = null;
                            return false;
                        }
                        break;
                }
            }
            else
            {
                builder.Append(c);
            }
        }

        result = builder.ToString();
        return true;
    }
}