using System;
using AcDream.Core.Chat;
using AcDream.UI.Abstractions.Panels.Chat;

namespace AcDream.App.UI;

/// <summary>
/// Copies the chat transcript into retail's <c>@log</c> file.
/// </summary>
/// <remarks>
/// <para>
/// Retail's log write sits INSIDE <c>ClientSystem::AddTextToScroll</c>
/// (<c>fprintf(s_pLogFile, "%ls%ls\n", …) @0x00563E5B</c>) — downstream of
/// composition and upstream of glyph layout. So the log records the finished
/// display line, timestamp included, and does not re-derive one.
/// <see cref="ChatLog.Append"/> is acdream's equivalent single fan-in.
/// </para>
/// <para>
/// Attaching happens on OPEN rather than at startup, which is what retail's
/// own help promises: "All the information that appears in your chat window
/// AFTER you type this command will be copied into a text file."
/// </para>
/// </remarks>
public sealed class ChatTranscriptLogWriter
{
    private readonly ChatSessionLog _log;
    private ChatLog? _source;

    public ChatTranscriptLogWriter(ChatSessionLog log)
        => _log = log ?? throw new ArgumentNullException(nameof(log));

    /// <summary>
    /// Starts copying <paramref name="source"/>, detaching from whatever was
    /// attached before. Re-attaching to the same transcript does not double
    /// up.
    /// </summary>
    public void Attach(ChatLog source)
    {
        ArgumentNullException.ThrowIfNull(source);
        Detach();
        _source = source;
        source.EntryAppended += Write;
    }

    /// <summary>
    /// Stops copying. Detaches from the instance actually attached to, not
    /// from whatever is current — a transcript replaced mid-log must not leave
    /// a handler behind on the old one.
    /// </summary>
    public void Detach()
    {
        if (_source is null)
            return;

        _source.EntryAppended -= Write;
        _source = null;
    }

    private void Write(ChatEntry entry)
    {
        ChatLog? source = _source;
        if (source is null)
            return;

        // The SAME gate the window uses, so a log never disagrees with the
        // transcript it is a copy of.
        bool stamped = source.DisplayTimestampsSource?.Invoke() == true;

        _log.Write(
            stamped ? ChatLog.FormatTimestampPrefix(entry.Received) : null,
            ChatVM.FormatEntry(entry));
    }
}