using System.Buffers.Binary;

namespace AcDream.Core.Net.Messages;

/// <summary>
/// Inbound <c>ServerName</c> GameMessage (opcode <c>0xF7E1</c>). ACE sends
/// this in the SAME batch as <see cref="CharacterList"/>, right after
/// <c>AuthConnectResponse</c> completes — it is the world (server) name the
/// retail character-select screen's "World" box shows.
///
/// <para>
/// Retail wire path: <c>CM_Login::DispatchUI_WorldInfo@0x006ad860</c> checks
/// the leading opcode against <c>0xf7e1</c>, unpacks the trailing
/// <c>PStringBase&lt;char&gt;</c>, and calls
/// <c>ClientUISystem::Handle_Login__WorldInfo@0x005641a0(currentConnections,
/// maxConnections, worldName)</c>, which forwards only the name to
/// <c>ECM_Login::SendNotice_WorldName@0x00692b10</c> (notice id
/// <c>0x186a2</c>). <c>gmCharacterManagementUI</c> registers for that notice
/// in its ctor (<c>0x004ec8f0</c>) and both
/// <c>RecvNotice_WorldName@0x004ec360</c> and its own
/// <c>UpdateWorldName@0x004ec120</c> resolve child element <c>0x1000039B</c>
/// (<c>UIElement::GetChildRecursive(m_rootField, 0x1000039b)</c>, dynamic-cast
/// to <c>UIElement_Text</c>) and call
/// <c>UIElement_Text::SetText(Client::GetInstance()->GetWorldName())</c> —
/// <c>Client::GetWorldName@0x00401ca0</c>/<c>SetWorldName@0x00402090</c> just
/// hold the string the notice delivered. The two leading dwords
/// (currentConnections/maxConnections) are read off the wire by the
/// dispatcher but never consumed by the character-management screen itself.
/// </para>
///
/// <para>
/// ACE: <c>GameMessageOpcode.ServerName = 0xF7E1</c>
/// (<c>ACE.Server/Network/GameMessages/GameMessageOpcode.cs</c>);
/// <c>GameMessageServerName</c>
/// (<c>ACE.Server/Network/GameMessages/Messages/GameMessageServerName.cs</c>)
/// writes <c>i32 currentConnections, i32 maxConnections, String16L
/// serverName</c>; sent from
/// <c>AuthenticationHandler.SendConnectResponse</c>
/// (<c>ACE.Server/Network/Handlers/AuthenticationHandler.cs:258</c>)
/// alongside <c>GameMessageCharacterList</c> and
/// <c>GameMessageDDDInterrogation</c>. holtburger's
/// <c>ServerNameData</c>
/// (<c>holtburger-protocol/src/messages/character/types.rs</c>) parses the
/// same three fields and cross-checks the field order/types.
/// </para>
///
/// <code>
///   u32 opcode              (0xF7E1)
///   i32 currentConnections
///   i32 maxConnections
///   String16L worldName
/// </code>
/// </summary>
public static class ServerName
{
    public const uint Opcode = 0xF7E1u;

    public readonly record struct Parsed(
        int CurrentConnections,
        int MaxConnections,
        string WorldName);

    /// <summary>
    /// Parse a ServerName body. <paramref name="body"/> must start with the
    /// 4-byte opcode (0xF7E1) — i.e. pass the full reassembled GameMessage
    /// output from <see cref="AcDream.Core.Net.Packets.FragmentAssembler"/>.
    /// </summary>
    public static Parsed Parse(ReadOnlySpan<byte> body)
    {
        int pos = 0;

        uint opcode = ReadU32(body, ref pos);
        if (opcode != Opcode)
            throw new FormatException($"expected ServerName opcode 0x{Opcode:X4}, got 0x{opcode:X8}");

        int currentConnections = unchecked((int)ReadU32(body, ref pos));
        int maxConnections = unchecked((int)ReadU32(body, ref pos));
        string worldName = StringReader.ReadString16L(body, ref pos);

        return new Parsed(currentConnections, maxConnections, worldName);
    }

    private static uint ReadU32(ReadOnlySpan<byte> source, ref int pos)
    {
        if (source.Length - pos < 4) throw new FormatException("truncated u32");
        uint value = BinaryPrimitives.ReadUInt32LittleEndian(source.Slice(pos));
        pos += 4;
        return value;
    }
}