using System;
using System.Buffers.Binary;

namespace AcDream.Core.Net.Messages;

/// <summary>
/// Inbound <c>SetState</c> GameMessage (opcode <c>0xF74B</c>). The server
/// broadcasts this whenever a previously-spawned entity's
/// <c>PhysicsState</c> bitmask changes after <c>CreateObject</c> — chiefly
/// when a door opens / closes (server flips <c>ETHEREAL_PS = 0x4</c>) or a
/// spell projectile becomes ethereal post-impact.
///
/// <para>
/// Wire layout (per
/// <c>references/holtburger/crates/holtburger-protocol/src/messages/object/messages/properties.rs:117-122</c>,
/// matched by every other acdream parser):
/// </para>
/// <list type="bullet">
///   <item><b>u32 opcode</b> — 0xF74B</item>
///   <item><b>u32 objectGuid</b></item>
///   <item><b>u32 physicsState</b> — bitmask (acclient.h:2815 / 2819)</item>
///   <item><b>u16 instanceSequence</b> — stale-packet rejection</item>
///   <item><b>u16 stateSequence</b> — stale-packet rejection</item>
/// </list>
///
/// <para>
/// Total body size: 16 bytes (4-byte opcode + 12-byte payload).
/// </para>
///
/// <para>
/// Server-side reference:
/// <c>references/ACE/Source/ACE.Server/Network/GameMessages/Messages/GameMessageSetState.cs:8-15</c>
/// — ACE writes the same field order using its <c>UShortSequence.CurrentBytes</c>
/// helper (which calls <c>BitConverter.GetBytes((ushort)value)</c> = 2 bytes per
/// sequence field), so its wire output matches holtburger's 12-byte payload.
/// A one-shot <c>ACDREAM_PROBE_BUILDING</c> hex-dump in
/// <see cref="WorldSession"/>'s dispatcher (added in the same commit) emits
/// the first SetState body bytes for runtime confirmation.
/// </para>
///
/// <para>
/// Named-retail anchor: <c>CPhysicsObj::set_state</c> at
/// <c>docs/research/named-retail/acclient_2013_pseudo_c.txt:283044</c>
/// describes the runtime state-store on the in-memory object
/// (<c>this->state = arg2</c>). The wire format for this opcode is
/// confirmed by holtburger's <c>SetStateData</c> struct; the named-retail
/// decomp does not cover the deserialization path for this opcode.
/// </para>
/// </summary>
public static class SetState
{
    public const uint Opcode = 0xF74Bu;

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

    /// <summary>
    /// Parse a 0xF74B body. <paramref name="body"/> must start with the
    /// 4-byte opcode (matches the convention used by VectorUpdate /
    /// UpdateMotion / UpdatePosition). Returns null on truncation or
    /// opcode mismatch.
    /// </summary>
    public static Parsed? TryParse(ReadOnlySpan<byte> body)
    {
        if (body.Length < 16) return null;
        try
        {
            uint opcode = BinaryPrimitives.ReadUInt32LittleEndian(body.Slice(0, 4));
            if (opcode != Opcode) return null;

            uint guid = BinaryPrimitives.ReadUInt32LittleEndian(body.Slice(4, 4));
            uint state = BinaryPrimitives.ReadUInt32LittleEndian(body.Slice(8, 4));
            ushort instSeq = BinaryPrimitives.ReadUInt16LittleEndian(body.Slice(12, 2));
            ushort stateSeq = BinaryPrimitives.ReadUInt16LittleEndian(body.Slice(14, 2));

            return new Parsed(guid, state, instSeq, stateSeq);
        }
        catch
        {
            return null;
        }
    }
}