using System;
using System.Buffers.Binary;

namespace AcDream.Core.Net.Messages;

/// <summary>
/// Parsed header of a retail AC <c>0xF7B0</c> GameEvent envelope (S→C).
///
/// <para>
/// Wire layout
/// (references/ACE/Source/ACE.Server/Network/GameEvent/GameEventMessage.cs):
/// <code>
///   u32 0xF7B0                // GameMessage opcode (GameEvent envelope)
///   u32 guid                  // session's player guid (0 if none yet)
///   u32 gameEventSequence     // per-session, incremented by server
///   u32 eventType             // GameEventType sub-opcode (94 values)
///   &lt;payload&gt;                // variable; type-dispatched
/// </code>
/// </para>
///
/// <para>
/// Payload is returned as a <see cref="ReadOnlyMemory{T}"/> view into the
/// original message body — no copy. Per-event parsers slice from this.
/// </para>
/// </summary>
public readonly record struct GameEventEnvelope(
    uint PlayerGuid,
    uint Sequence,
    GameEventType EventType,
    ReadOnlyMemory<byte> Payload)
{
    /// <summary>GameMessage opcode of the outer envelope.</summary>
    public const uint Opcode = 0xF7B0u;

    /// <summary>Header size (4×u32 = 16 bytes before payload).</summary>
    public const int HeaderSize = 16;

    /// <summary>
    /// Parse a raw 0xF7B0 GameMessage body into the header + payload view.
    /// Returns null if the body is shorter than the header or the outer
    /// opcode doesn't match.
    /// </summary>
    public static GameEventEnvelope? TryParse(byte[] body)
    {
        ArgumentNullException.ThrowIfNull(body);
        return TryParseBorrowed(body);
    }

    /// <summary>
    /// Internal receive-path parser. The returned payload borrows
    /// <paramref name="body"/> and must not escape synchronous dispatch.
    /// </summary>
    internal static GameEventEnvelope? TryParseBorrowed(
        ReadOnlyMemory<byte> body)
    {
        if (body.Length < HeaderSize) return null;

        ReadOnlySpan<byte> span = body.Span;
        uint outer = BinaryPrimitives.ReadUInt32LittleEndian(span);
        if (outer != Opcode) return null;

        uint guid      = BinaryPrimitives.ReadUInt32LittleEndian(span.Slice(4));
        uint sequence  = BinaryPrimitives.ReadUInt32LittleEndian(span.Slice(8));
        uint eventType = BinaryPrimitives.ReadUInt32LittleEndian(span.Slice(12));

        ReadOnlyMemory<byte> payload = body.Slice(HeaderSize);
        return new GameEventEnvelope(guid, sequence, (GameEventType)eventType, payload);
    }
}