using System.Buffers.Binary;

namespace AcDream.Core.Net.Messages;

/// <summary>
/// Inbound <c>ObjDescEvent</c> GameMessage (opcode <c>0xF625</c>). ACE
/// broadcasts this whenever a creature/player's appearance changes after
/// the initial <see cref="CreateObject"/> spawn — equip / unequip
/// (Creature_Equipment.cs:365), tailoring (Tailoring.cs:504), recipe
/// results (RecipeManager.cs:403), character-option toggles. Skunkwors
/// protocol docs: "F625: Change Model — Sent whenever a character changes
/// their clothes. It contains the entire description of what they're
/// wearing (and possibly their facial features as well). This message is
/// only sent for changes; when the character is first created, the body
/// of this message is included inside the creation message."
///
/// <para>Retail handles it via <c>SmartBox::HandleObjDescEvent</c>
/// (named-retail symbol 0x453340). acdream silently dropped it through
/// 2026-05-06 — the bug was that retail-driven characters observed from
/// acdream rendered with the wrong skin/hair palettes because the
/// follow-up appearance updates were never applied.</para>
///
/// <para>Wire layout (ACE WorldObject_Networking.cs:48-54
/// <c>SerializeUpdateModelData</c>):</para>
/// <list type="bullet">
///   <item>u32 opcode (0xF625)</item>
///   <item>u32 guid — target object</item>
///   <item>ModelData block — see <see cref="CreateObject.ReadModelData"/></item>
///   <item>u16 instanceSequence</item>
///   <item>u16 visualDescSequence (OBJDESC_TS)</item>
/// </list>
/// </summary>
public static class ObjDescEvent
{
    public const uint Opcode = 0xF625u;

    /// <summary>
    /// One ObjDescEvent: target guid, new ModelData, and retail's packed
    /// INSTANCE_TS/OBJDESC_TS pair.
    /// </summary>
    public readonly record struct Parsed(
        uint Guid,
        CreateObject.ModelData ModelData,
        ushort InstanceSequence,
        ushort ObjDescSequence);

    /// <summary>
    /// Parse an ObjDescEvent body (must start with the 4-byte opcode).
    /// Returns null on truncation or wrong opcode.
    /// </summary>
    public static Parsed? TryParse(ReadOnlySpan<byte> body)
    {
        try
        {
            int pos = 0;
            if (body.Length - pos < 4) return null;
            uint opcode = BinaryPrimitives.ReadUInt32LittleEndian(body.Slice(pos));
            pos += 4;
            if (opcode != Opcode) return null;

            if (body.Length - pos < 4) return null;
            uint guid = BinaryPrimitives.ReadUInt32LittleEndian(body.Slice(pos));
            pos += 4;

            var modelData = CreateObject.ReadModelData(body, ref pos);

            // PhysicsTimestampPack::UnPack 0x00516F50 is exactly two u16s.
            // Reject a missing or overlong tail so cursor mistakes cannot turn
            // into an apparently valid, ungated appearance update.
            if (body.Length - pos != 4) return null;
            ushort instance = BinaryPrimitives.ReadUInt16LittleEndian(body.Slice(pos));
            ushort objDesc = BinaryPrimitives.ReadUInt16LittleEndian(body.Slice(pos + 2));
            return new Parsed(guid, modelData, instance, objDesc);
        }
        catch
        {
            return null;
        }
    }
}