using System;
using System.Buffers.Binary;

namespace AcDream.Core.Net.Messages;

/// <summary>
/// Inbound <c>0xF7E0 ServerMessage</c> top-level GameMessage.
/// General-purpose server-broadcast text — admin announcements,
/// combat logs, and routine error messages routed by the server
/// instead of via WeenieError.
///
/// <para>
/// This is a standalone GameMessage — NOT wrapped in 0xF7B0
/// GameEvent envelope. Dispatched directly from
/// <see cref="WorldSession.ProcessDatagram"/>.
/// </para>
///
/// <para>
/// Wire layout (port from holtburger
/// <c>references/holtburger/.../messages/chat/types.rs::ServerMessageData</c>,
/// see also opcodes.rs:167):
/// <code>
///   u32 opcode               // 0xF7E0
///   string16L message
///   u32 chatType             // ChatMessageType (Broadcast / System / etc)
/// </code>
/// </para>
/// </summary>
public static class ServerMessage
{
    public const uint Opcode = 0xF7E0u;

    public readonly record struct Parsed(string Message, uint ChatType);

    public static Parsed? TryParse(ReadOnlySpan<byte> body)
    {
        if (body.Length < 8) return null;
        uint opcode = BinaryPrimitives.ReadUInt32LittleEndian(body);
        if (opcode != Opcode) return null;

        try
        {
            int pos = 4;
            string message = StringReader.ReadString16L(body, ref pos);
            if (body.Length - pos < 4) return null;
            uint chatType = BinaryPrimitives.ReadUInt32LittleEndian(body.Slice(pos));
            return new Parsed(message, chatType);
        }
        catch { return null; }
    }
}