using System;
using System.Buffers.Binary;
using System.Text;

namespace AcDream.Core.Net.Messages;

/// <summary>
/// Outbound chat GameActions — Talk (local /say), Tell (whisper),
/// ChatChannel (allegiance / general).
///
/// <para>
/// All three ride inside the <c>0xF7B1</c> GameAction envelope with
/// the same leading fields:
/// <code>
///   u32 0xF7B1
///   u32 gameActionSequence
///   u32 subOpcode
///   &lt;payload&gt;
/// </code>
/// </para>
///
/// <para>
/// String16L wire shape mirrors the other outbound AC messages:
/// <code>
///   u16 length      // byte count (not char count)
///   byte[] ascii    // ASCII bytes, no terminator
///   pad to 4-byte boundary
/// </code>
/// </para>
///
/// <para>
/// Source of truth: r08 §3 rows 0x0015 / 0x005D / 0x0147.
/// </para>
/// </summary>
public static class ChatRequests
{
    public const uint GameActionEnvelope = 0xF7B1u;
    public const uint TalkOpcode        = 0x0015u;
    public const uint TellOpcode        = 0x005Du;
    public const uint ChatChannelOpcode = 0x0147u;

    /// <summary>Send a local /say message (heard by anyone within ~20m).</summary>
    public static byte[] BuildTalk(uint gameActionSequence, string message)
    {
        ArgumentNullException.ThrowIfNull(message);
        byte[] msg = PackString16L(message);
        byte[] body = new byte[12 + msg.Length];
        BinaryPrimitives.WriteUInt32LittleEndian(body,           GameActionEnvelope);
        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(4), gameActionSequence);
        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(8), TalkOpcode);
        Array.Copy(msg, 0, body, 12, msg.Length);
        return body;
    }

    /// <summary>Send a /tell (whisper) by target character name.</summary>
    /// <remarks>
    /// Wire field order is <b>message FIRST, then target</b> — that's
    /// what ACE's <c>GameActionTell.Handle</c> reads (see
    /// <c>references/ACE/Source/ACE.Server/Network/GameAction/Actions/GameActionTell.cs</c>
    /// lines 17-18:
    /// <code>
    ///   var message = clientMessage.Payload.ReadString16L();
    ///   var target  = clientMessage.Payload.ReadString16L();
    /// </code>
    /// Filed after a 2026-04-25 live trace where every <c>/tell +Je hello</c>
    /// failed with WeenieError 0x052B (CharacterNotAvailable). With the
    /// previous (target-first) wire shape, ACE was reading our message
    /// payload "hello" as the recipient name, looking it up, finding no
    /// character "hello" online, and returning the not-available error.
    /// Swapping the field order makes <c>/tell &lt;name&gt; &lt;text&gt;</c>
    /// actually deliver.
    /// </remarks>
    public static byte[] BuildTell(uint gameActionSequence, string targetName, string message)
    {
        ArgumentNullException.ThrowIfNull(targetName);
        ArgumentNullException.ThrowIfNull(message);
        byte[] msg  = PackString16L(message);
        byte[] name = PackString16L(targetName);
        byte[] body = new byte[12 + msg.Length + name.Length];
        BinaryPrimitives.WriteUInt32LittleEndian(body,           GameActionEnvelope);
        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(4), gameActionSequence);
        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(8), TellOpcode);
        Array.Copy(msg,  0, body, 12, msg.Length);
        Array.Copy(name, 0, body, 12 + msg.Length, name.Length);
        return body;
    }

    /// <summary>Send to a chat channel (allegiance, general, trade, etc).</summary>
    public static byte[] BuildChatChannel(uint gameActionSequence, uint channelId, string message)
    {
        ArgumentNullException.ThrowIfNull(message);
        byte[] msg = PackString16L(message);
        byte[] body = new byte[16 + msg.Length];
        BinaryPrimitives.WriteUInt32LittleEndian(body,            GameActionEnvelope);
        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(4),  gameActionSequence);
        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(8),  ChatChannelOpcode);
        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(12), channelId);
        Array.Copy(msg, 0, body, 16, msg.Length);
        return body;
    }

    private static byte[] PackString16L(string s)
    {
        // Windows-1252 matches retail (and holtburger's encoding_rs::WINDOWS_1252).
        byte[] data = Encoding.GetEncoding(1252).GetBytes(s);
        if (data.Length > ushort.MaxValue)
            throw new ArgumentException("String too long for 16-bit length prefix.", nameof(s));

        int recordSize = 2 + data.Length;
        int padding = (4 - (recordSize & 3)) & 3;
        byte[] result = new byte[recordSize + padding];
        BinaryPrimitives.WriteUInt16LittleEndian(result, (ushort)data.Length);
        Array.Copy(data, 0, result, 2, data.Length);
        // trailing bytes are already zero from new[]
        return result;
    }
}