using System.Buffers.Binary;

namespace AcDream.Core.Net.Messages;

/// <summary>
/// Inbound <c>PickupEvent</c> GameMessage (opcode <c>0xF74A</c>).
///
/// <para>
/// ACE emits this from <c>Player_Tracking.RemoveTrackedObject(wo, fromPickup: true)</c>
/// when a player picks up a world item — distinguishes the despawn
/// from a generic <c>0xF747 DeleteObject</c> (timeout / death /
/// out-of-LOS). Downstream effect on the client view is the same
/// (remove the entity from the world), so <see cref="WorldSession"/>
/// routes both opcodes to the same <c>EntityDeleted</c> event.
/// </para>
///
/// <para>
/// Wire layout (ACE <c>GameMessagePickupEvent.cs</c>):
/// <code>
///   u32 0xF74A
///   u32 guid
///   u16 objectInstanceSequence
///   u16 objectPositionSequence
/// </code>
/// </para>
/// </summary>
public static class PickupEvent
{
    public const uint Opcode = 0xF74Au;

    public readonly record struct Parsed(
        uint Guid, ushort InstanceSequence, ushort PositionSequence);

    public static Parsed? TryParse(ReadOnlySpan<byte> body)
    {
        if (body.Length < 12)
            return null;

        uint opcode = BinaryPrimitives.ReadUInt32LittleEndian(body.Slice(0, 4));
        if (opcode != Opcode)
            return null;

        uint guid = BinaryPrimitives.ReadUInt32LittleEndian(body.Slice(4, 4));
        ushort instanceSequence = BinaryPrimitives.ReadUInt16LittleEndian(body.Slice(8, 2));
        ushort positionSequence = BinaryPrimitives.ReadUInt16LittleEndian(body.Slice(10, 2));
        return new Parsed(guid, instanceSequence, positionSequence);
    }
}