Nonemm/src/Nonemm.Spotting/Telnet/LineAssembler.cs

using System.Text;

namespace Nonemm.Spotting.Telnet;

/// Splits the text coming off a telnet stream into lines and keeps the tail
/// that has no line ending yet. Cluster nodes write their login prompt without
/// one, so the tail has to be readable before the line is finished.
public sealed class LineAssembler
{
    private readonly StringBuilder pending = new();
    private bool afterCarriageReturn;

    /// What has arrived since the last line ending.
    public string Pending => pending.ToString();

    /// Breaks on CR, LF or CRLF. Most nodes send CRLF, but N1MM splits on CR
    /// alone, which says some of them leave the LF out.
    public IReadOnlyList<string> Add(string text)
    {
        List<string> lines = [];
        foreach (char character in text)
        {
            bool wasAfterCarriageReturn = afterCarriageReturn;
            afterCarriageReturn = character == '\r';
            switch (character)
            {
                case '\n' when wasAfterCarriageReturn:
                    break;
                case '\n' or '\r':
                    lines.Add(pending.ToString());
                    pending.Clear();
                    break;
                default:
                    pending.Append(character);
                    break;
            }
        }
        return lines;
    }

    public void Clear()
    {
        pending.Clear();
        afterCarriageReturn = false;
    }
}