using System.Buffers.Binary;

namespace AcDream.Core.Net.Messages;

/// <summary>
/// Inbound <c>ObjectDelete</c> GameMessage (opcode <c>0xF747</c>).
///
/// <para>
/// Retail dispatch path:
/// <c>CM_Physics::DispatchSB_DeleteObject</c> 0x006AC6A0 reads guid from
/// <c>buf+4</c> and instance sequence from <c>buf+8</c>, then calls
/// <c>SmartBox::HandleDeleteObject</c> 0x00451EA0. ACE emits the same
/// layout from <c>GameMessageDeleteObject</c>.
/// </para>
/// </summary>
public static class DeleteObject
{
    public const uint Opcode = 0xF747u;

    /// <param name="FromPickup">
    /// <see langword="true"/> when this delete was sourced from a
    /// <c>PickupEvent (0xF74A)</c> — the object left the 3-D world view but
    /// the weenie record still exists (it is moving into a container).
    /// <see langword="false"/> (default) when sourced from <c>DeleteObject
    /// (0xF747)</c> — the weenie was destroyed and must be evicted from
    /// <see cref="AcDream.Core.Items.ClientObjectTable"/>.
    /// Retail two-table model: PickupEvent removes from <c>object_table</c>;
    /// DeleteObject removes from <c>weenie_object_table</c>.
    /// </param>
    public readonly record struct Parsed(uint Guid, ushort InstanceSequence, bool FromPickup = false);

    /// <summary>
    /// Parse a 0xF747 body. <paramref name="body"/> must start with the
    /// 4-byte opcode, matching every other parser in this namespace.
    /// The returned <see cref="Parsed.FromPickup"/> is always <see langword="false"/>
    /// — this parser handles true destroys only.
    /// </summary>
    public static Parsed? TryParse(ReadOnlySpan<byte> body)
    {
        if (body.Length < 10)
            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));
        return new Parsed(guid, instanceSequence, FromPickup: false);
    }
}