Case-insensitive ANTLRStringStream for ANTLR 3

CaseInsensitiveStringStream.cs

using Antlr.Runtime;

public class CaseInsensitiveStringStream : ANTLRStringStream
{
    // the string used for lookahead (performance improvement by not having to call Char.ToLowerInvariant())
    private readonly string _lastring;

    public CaseInsensitiveStringStream(string input, string sourceName)
        : base(input, sourceName)
    {
        _lastring = input.ToLowerInvariant();
    }

    public override int LA(int i)
    {
        if (i == 0)
            return 0;

        if (i < 0)
        {
            i++; // e.g., translate LA(-1) to use offset i=0; then data[p+0-1]
            if ((p + i - 1) < 0)
            {
                return (int)CharStreamConstants.EndOfFile; // invalid; no char before first char
            }
        }

        if ((p + i - 1) >= n)
        {
            return (int)CharStreamConstants.EndOfFile;
        }

        return _lastring[p + i - 1];
    }
}