using System;
using System.Collections.Generic;
using System.Text;

namespace AcDream.Core.Net.Tests.Messages;

/// <summary>
/// Test-only faithful mirror of ACE's server-side wire writer, so golden-byte
/// conformance tests generate their fixtures the same way the authoritative
/// server generates them. Hand-rolled hex would only prove that our parser
/// agrees with whoever typed the hex; generating from the oracle's own
/// algorithm proves it agrees with the server.
///
/// <para>
/// Every primitive below is a line-for-line port of
/// <c>ACE/Source/ACE.Server/Network/Extensions.cs</c> (read 2026-07-29 from
/// the ACE checkout). Cited line numbers are that file's:
/// </para>
/// <list type="bullet">
///   <item><c>CalculatePadMultiple</c> — Extensions.cs:10</item>
///   <item><c>WriteString16L</c> — Extensions.cs:12-21</item>
///   <item><c>WritePackedDword</c> — Extensions.cs:23-34</item>
///   <item><c>Pad</c> — Extensions.cs:51</item>
///   <item><c>Align</c> — Extensions.cs:55-58</item>
///   <item><c>WriteGuid</c> — Extensions.cs:121 (writes <c>guid.Full</c>, a u32)</item>
/// </list>
///
/// <para>
/// ACE's underlying <see cref="System.IO.BinaryWriter"/> is little-endian for
/// the fixed-width overloads, which is what <c>Write(uint)</c>,
/// <c>Write(ushort)</c> and <c>Write(float)</c> reproduce here.
/// </para>
/// </summary>
internal sealed class AceWireWriter
{
    private readonly List<byte> _buffer = new();

    public int Length => _buffer.Count;

    /// <summary>ACE <c>Extensions.cs:10</c>.</summary>
    private static uint CalculatePadMultiple(uint length, uint multiple)
        => multiple * ((length + multiple - 1u) / multiple) - length;

    /// <summary>BinaryWriter.Write(uint) — little-endian.</summary>
    public AceWireWriter Write(uint value)
    {
        _buffer.Add((byte)(value & 0xFF));
        _buffer.Add((byte)((value >> 8) & 0xFF));
        _buffer.Add((byte)((value >> 16) & 0xFF));
        _buffer.Add((byte)((value >> 24) & 0xFF));
        return this;
    }

    /// <summary>BinaryWriter.Write(ushort) — little-endian.</summary>
    public AceWireWriter Write(ushort value)
    {
        _buffer.Add((byte)(value & 0xFF));
        _buffer.Add((byte)((value >> 8) & 0xFF));
        return this;
    }

    /// <summary>BinaryWriter.Write(float) — little-endian IEEE-754.</summary>
    public AceWireWriter Write(float value)
        => Write((uint)BitConverter.SingleToInt32Bits(value));

    /// <summary>
    /// ACE <c>Extensions.cs:121</c> — <c>WriteGuid</c> writes the full 32-bit
    /// guid with no packing.
    /// </summary>
    public AceWireWriter WriteGuid(uint guid) => Write(guid);

    /// <summary>
    /// ACE <c>Extensions.cs:12-21</c>. Writes a u16 length, the CP1252 bytes,
    /// then pads so that (2 + length) is a multiple of 4 — the comment in ACE
    /// reads "client expects string length to be a multiple of 4 including the
    /// 2 bytes for length".
    /// </summary>
    public AceWireWriter WriteString16L(string? data)
    {
        data ??= "";
        byte[] bytes = Encoding.GetEncoding(1252).GetBytes(data);
        Write((ushort)data.Length);
        _buffer.AddRange(bytes);
        return Pad(CalculatePadMultiple(sizeof(ushort) + (uint)data.Length, 4u));
    }

    /// <summary>ACE <c>Extensions.cs:23-34</c>.</summary>
    public AceWireWriter WritePackedDword(uint value)
    {
        if (value <= 32767)
            return Write((ushort)value);

        uint packed = (value << 16) | ((value >> 16) | 0x8000);
        return Write(packed);
    }

    /// <summary>ACE <c>Extensions.cs:51</c>.</summary>
    public AceWireWriter Pad(uint pad)
    {
        for (uint i = 0; i < pad; i++)
            _buffer.Add(0);
        return this;
    }

    /// <summary>ACE <c>Extensions.cs:55-58</c> — pad the stream to a 4-byte boundary.</summary>
    public AceWireWriter Align() => Pad(CalculatePadMultiple((uint)_buffer.Count, 4u));

    public byte[] ToArray() => _buffer.ToArray();

    /// <summary>
    /// Start a top-level GameMessage body: the 4-byte opcode, exactly as
    /// <c>ACE.Server.Network.GameMessages.GameMessage</c>'s constructor writes
    /// it before the per-message payload.
    /// </summary>
    public static AceWireWriter GameMessage(uint opcode)
        => new AceWireWriter().Write(opcode);

    /// <summary>
    /// Start a GameEvent body (top-level opcode <c>0xF7B0</c>), mirroring
    /// <c>ACE/Source/ACE.Server/Network/GameEvent/GameEventMessage.cs:21-25</c>:
    /// <c>WriteGuid(guid)</c>, <c>Write(session.GameEventSequence++)</c>,
    /// <c>Write((uint)EventType)</c>.
    /// </summary>
    public static AceWireWriter GameEvent(uint guid, uint eventSequence, uint eventType)
        => new AceWireWriter()
            .Write(0xF7B0u)
            .WriteGuid(guid)
            .Write(eventSequence)
            .Write(eventType);
}