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

namespace AcDream.Core.Net.Messages;

/// <summary>
/// Inbound <c>CharacterList</c> GameMessage (opcode <c>0xF658</c>).
/// The server sends one of these right after ConnectResponse completes
/// the handshake — it's the client's "pick which character to log in"
/// data. Contains one entry per character on the account plus account-
/// level settings (slot count, Turbine chat toggle, ToD flag).
///
/// <para>
/// Wire layout (ported from ACE's <c>GameMessageCharacterList.cs</c>
/// writer — see NOTICE.md for attribution):
/// </para>
///
/// <code>
///   u32       0                (leading padding, always 0)
///   u32       characterCount
///   for each character:
///     u32     characterId      (GUID)
///     String16L name           (prefixed with '+' for admin chars)
///     u32     deleteTimeDelta  (0 if not scheduled for deletion)
///   u32       0                (trailing padding)
///   u32       slotCount        (max characters per account)
///   String16L accountName
///   u32       useTurbineChat   (bool)
///   u32       hasThroneOfDestiny (bool — always 1 on retail-era ACE)
/// </code>
/// </summary>
public static class CharacterList
{
    public const uint Opcode = 0xF658u;

    public readonly record struct Character(uint Id, string Name, uint DeleteTimeDelta);

    public sealed record Parsed(
        IReadOnlyList<Character> Characters,
        uint SlotCount,
        string AccountName,
        bool UseTurbineChat,
        bool HasThroneOfDestiny);

    /// <summary>
    /// Parse a CharacterList body. <paramref name="body"/> must start with
    /// the 4-byte opcode (0xF658) — i.e. pass the full reassembled
    /// GameMessage output from <see cref="Packets.FragmentAssembler"/>.
    /// </summary>
    public static Parsed Parse(ReadOnlySpan<byte> body)
    {
        int pos = 0;

        uint opcode = ReadU32(body, ref pos);
        if (opcode != Opcode)
            throw new FormatException($"expected CharacterList opcode 0x{Opcode:X4}, got 0x{opcode:X8}");

        _ = ReadU32(body, ref pos);  // leading 0
        uint count = ReadU32(body, ref pos);
        if (count > 255)
            throw new FormatException($"character count {count} exceeds sanity limit");

        var characters = new Character[count];
        for (int i = 0; i < count; i++)
        {
            uint id = ReadU32(body, ref pos);
            string name = ReadString16L(body, ref pos);
            uint deleteDelta = ReadU32(body, ref pos);
            characters[i] = new Character(id, name, deleteDelta);
        }

        _ = ReadU32(body, ref pos);  // trailing 0
        uint slotCount = ReadU32(body, ref pos);
        string accountName = ReadString16L(body, ref pos);
        bool useTurbineChat = ReadU32(body, ref pos) != 0;
        bool hasThroneOfDestiny = ReadU32(body, ref pos) != 0;

        return new Parsed(characters, slotCount, accountName, useTurbineChat, hasThroneOfDestiny);
    }

    private static uint ReadU32(ReadOnlySpan<byte> source, ref int pos)
    {
        if (source.Length - pos < 4) throw new FormatException("truncated u32");
        uint value = BinaryPrimitives.ReadUInt32LittleEndian(source.Slice(pos));
        pos += 4;
        return value;
    }

    private static string ReadString16L(ReadOnlySpan<byte> source, ref int pos)
    {
        if (source.Length - pos < 2) throw new FormatException("truncated String16L length");
        ushort length = BinaryPrimitives.ReadUInt16LittleEndian(source.Slice(pos));
        pos += 2;
        if (source.Length - pos < length) throw new FormatException("truncated String16L body");
        // Windows-1252 matches retail (and holtburger's encoding_rs::WINDOWS_1252).
        string result = Encoding.GetEncoding(1252).GetString(source.Slice(pos, length));
        pos += length;
        int recordSize = 2 + length;
        int padding = (4 - (recordSize & 3)) & 3;
        pos += padding;
        return result;
    }
}