using System.Buffers.Binary;
namespace ZB.MOM.WW.OtOpcUa.Driver.Ab.Transport.Tests.Captures;

/// <summary>One EtherNet/IP encapsulation frame from a golden capture.</summary>
/// <param name="FromClient"><c>true</c> for a client→server (<c>&gt;</c>) frame.</param>
/// <param name="Bytes">The full frame: 24-byte encapsulation header plus data.</param>
internal sealed record CapturedFrame(bool FromClient, byte[] Bytes);

/// <summary>
///     Loads the golden libplctag ⇄ ab_server captures in <c>Captures/*.hex</c>. See
///     <c>Captures/README.md</c> for how each was taken.
/// </summary>
internal static class CaptureFile
{
    /// <summary>The directory the <c>.hex</c> fixtures are copied to.</summary>
    public static string Dir => Path.Combine(AppContext.BaseDirectory, "Captures");

    /// <summary>Loads every frame of a capture, in wire order; <c>#</c> lines are skipped.</summary>
    /// <param name="name">The capture's file name, e.g. <c>logix-read-dint.hex</c>.</param>
    /// <returns>The frames.</returns>
    public static IReadOnlyList<CapturedFrame> Load(string name) =>
        File.ReadAllLines(Path.Combine(Dir, name))
            .Where(l => l.Length > 2 && l[0] is '>' or '<')
            .Select(l => new CapturedFrame(l[0] == '>', Convert.FromHexString(l[2..].Trim())))
            .ToList();

    /// <summary>The CIP payload of a SendRRData / SendUnitData frame (the last CPF item's data,
    /// minus the 2-byte sequence count for connected data).</summary>
    /// <param name="f">The frame.</param>
    /// <returns>The CIP payload, or empty for any other encapsulation command.</returns>
    public static byte[] CipPayload(CapturedFrame f)
    {
        var b = f.Bytes;
        var cmd = BinaryPrimitives.ReadUInt16LittleEndian(b.AsSpan(0));         
        if (cmd is not (0x6F or 0x70)) return [];       // only SendRRData / SendUnitData carry CPF items
        var pos = 24 + 4 + 2;                           // header + interface handle + timeout
        var itemCount = BinaryPrimitives.ReadUInt16LittleEndian(b.AsSpan(pos)); pos += 2;
        byte[] last = [];
        for (var i = 0; i < itemCount; i++)
        {
            var type = BinaryPrimitives.ReadUInt16LittleEndian(b.AsSpan(pos));
            var len = BinaryPrimitives.ReadUInt16LittleEndian(b.AsSpan(pos + 2));
            last = b.AsSpan(pos + 4, len).ToArray();
            if (type == 0x00B1) last = last[2..];       // strip connected sequence count
            pos += 4 + len;
        }
        return last;
    }
}