using System.Buffers.Binary;
using System.Text;

namespace ZB.MOM.WW.OtOpcUa.Driver.AbCip.Tests;

/// <summary>
///     Builds an <c>@udt/&lt;id&gt;</c> template buffer in the layout libplctag really returns —
///     the layout pinned by the real-controller captures in <c>CipTemplateObjectWireCaptureTests</c>.
///     Keep it in step with those captures, never with the decoder: a builder that restates the
///     decoder's assumption proves nothing (#468, and the template decoder after it).
/// </summary>
internal static class TemplateBufferBuilder
{
    /// <summary>Build a template buffer.</summary>
    /// <param name="udtName">UDT type name (written as <c>name;n</c> + an encoded tail, as a controller does).</param>
    /// <param name="instanceSize">Bytes per instance.</param>
    /// <param name="members">Per member: name, CIP type word (atomic code, or <c>0x8000 | nestedTemplateId</c>),
    ///     array length (0 = scalar; &gt; 0 sets the array flag and the info word), byte offset.</param>
    /// <returns>The buffer.</returns>
    public static byte[] Build(
        string udtName, uint instanceSize, params (string name, ushort type, ushort arraySize, uint offset)[] members)
    {
        const int header = 14;
        var buf = new MemoryStream();
        var head = new byte[header + (8 * members.Length)];
        BinaryPrimitives.WriteUInt16LittleEndian(head.AsSpan(0), 0x0123);          // template id
        BinaryPrimitives.WriteUInt32LittleEndian(head.AsSpan(2), 0);               // def size (words)
        BinaryPrimitives.WriteUInt32LittleEndian(head.AsSpan(6), instanceSize);
        BinaryPrimitives.WriteUInt16LittleEndian(head.AsSpan(10), (ushort)members.Length);
        BinaryPrimitives.WriteUInt16LittleEndian(head.AsSpan(12), 0xABCD);         // struct handle
        for (var i = 0; i < members.Length; i++)
        {
            var o = header + (i * 8);
            var (_, type, arraySize, offset) = members[i];
            var typeWord = arraySize > 0 ? (ushort)(type | 0x2000) : type;
            BinaryPrimitives.WriteUInt16LittleEndian(head.AsSpan(o), arraySize);
            BinaryPrimitives.WriteUInt16LittleEndian(head.AsSpan(o + 2), typeWord);
            BinaryPrimitives.WriteUInt32LittleEndian(head.AsSpan(o + 4), offset);
        }
        buf.Write(head);
        buf.Write(Encoding.ASCII.GetBytes(udtName + ";nEAIA\0"));
        foreach (var m in members) buf.Write(Encoding.ASCII.GetBytes(m.name + "\0"));
        return buf.ToArray();
    }
}