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

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

/// <summary>
///     Decoder for the CIP Template Object (class 0x6C) blob returned by a <c>Read Template</c>
///     service. Produces an <see cref="AbCipUdtShape"/> describing the UDT's name, total size,
///     + ordered member list with per-member offset + type + array length.
/// </summary>
/// <remarks>
///     <para>Wire format per Rockwell CIP Vol 1 §5A + Logix 5000 CIP Programming Manual
///     1756-PM019 §"Template Object", cross-checked against libplctag's <c>ab/cip.c</c>
///     <c>handle_read_template_reply</c>:</para>
///
///     <para>Header (fixed-size, little-endian):</para>
///     <list type="table">
///         <item><term>u16</term><description>Member count.</description></item>
///         <item><term>u16</term><description>Struct handle (opaque id).</description></item>
///         <item><term>u32</term><description>Instance size — bytes per structure instance.</description></item>
///         <item><term>u32</term><description>Member-definition total size — not used here.</description></item>
///     </list>
///
///     <para>Then <c>member_count</c> member blocks (8 bytes each):</para>
///     <list type="table">
///         <item><term>u16</term><description>Member info — type code + flags (same encoding
///             as Symbol Object: bit 15 = struct, lower 12 = CIP type code).</description></item>
///         <item><term>u16</term><description>Array size — 0 for scalar members.</description></item>
///         <item><term>u32</term><description>Struct offset — byte offset from struct start.</description></item>
///     </list>
///
///     <para>Then strings: UDT name followed by each member name, each terminated by a
///     semicolon <c>;</c> followed by a null <c>\0</c>. The UDT name may itself contain the
///     sequence <c>UDTName;0\0</c> where <c>0</c> after the semicolon is an ASCII flag byte.
///     Decoder trims to the first semicolon.</para>
/// </remarks>
public static class CipTemplateObjectDecoder
{
    private const int HeaderSize = 12; // u16 + u16 + u32 + u32
    private const int MemberBlockSize = 8; // u16 + u16 + u32

    private const ushort MemberInfoStructFlag = 0x8000;
    private const ushort MemberInfoTypeCodeMask = 0x0FFF;

    /// <summary>
    ///     Decode the raw Template Object blob. Returns <c>null</c> when the header indicates
    ///     zero members or the buffer is too short to hold the fixed header.
    /// </summary>
    public static AbCipUdtShape? Decode(byte[] buffer)
    {
        ArgumentNullException.ThrowIfNull(buffer);
        if (buffer.Length < HeaderSize) return null;

        var memberCount = BinaryPrimitives.ReadUInt16LittleEndian(buffer.AsSpan(0));
        // bytes 2-3: struct handle — opaque, not needed for the shape record
        var instanceSize = BinaryPrimitives.ReadUInt32LittleEndian(buffer.AsSpan(4));
        // bytes 8-11: member-definition total size — inferred from names list instead

        if (memberCount == 0) return null;

        var memberBlocksOffset = HeaderSize;
        var namesOffset = memberBlocksOffset + MemberBlockSize * memberCount;
        if (namesOffset > buffer.Length) return null;

        var stringsSpan = buffer.AsSpan(namesOffset);
        var names = ParseSemicolonTerminatedStrings(stringsSpan);
        if (names.Count == 0) return null;

        // Strings layout: UDT name first, then one per member (in the same order as the
        // member-info blocks). Always consume the first entry as the UDT name; missing
        // trailing member names get <member_N> placeholders below.
        var udtName = names[0];
        var memberNames = names.Skip(1).ToArray();

        var members = new List<AbCipUdtMember>(memberCount);
        for (var i = 0; i < memberCount; i++)
        {
            var blockOffset = memberBlocksOffset + (i * MemberBlockSize);
            var info = BinaryPrimitives.ReadUInt16LittleEndian(buffer.AsSpan(blockOffset));
            var arraySize = BinaryPrimitives.ReadUInt16LittleEndian(buffer.AsSpan(blockOffset + 2));
            var offset = (int)BinaryPrimitives.ReadUInt32LittleEndian(buffer.AsSpan(blockOffset + 4));

            var isStruct = (info & MemberInfoStructFlag) != 0;
            var typeCode = (byte)(info & MemberInfoTypeCodeMask);
            var dataType = isStruct
                ? AbCipDataType.Structure
                : (CipSymbolObjectDecoder.MapTypeCode(typeCode) ?? AbCipDataType.Structure);

            var memberName = i < memberNames.Length ? memberNames[i] : $"<member_{i}>";
            members.Add(new AbCipUdtMember(
                Name: memberName,
                Offset: offset,
                DataType: dataType,
                ArrayLength: arraySize == 0 ? 1 : arraySize));
        }

        return new AbCipUdtShape(
            TypeName: udtName,
            TotalSize: (int)instanceSize,
            Members: members);
    }

    /// <summary>
    ///     Walk a span of <c>NAME;\0NAME;\0…</c> byte sequences. Splits at each semicolon —
    ///     the null byte after each semicolon is optional padding per Rockwell's string
    ///     encoding convention. Stops at a trailing null / end of buffer.
    /// </summary>
    internal static List<string> ParseSemicolonTerminatedStrings(ReadOnlySpan<byte> span)
    {
        var result = new List<string>();
        var start = 0;
        for (var i = 0; i < span.Length; i++)
        {
            var b = span[i];
            if (b == ';')
            {
                if (i > start)
                    result.Add(Encoding.ASCII.GetString(span[start..i]));
                // Skip the optional null/space padding following the semicolon.
                while (i + 1 < span.Length && (span[i + 1] == '\0' || span[i + 1] == ' '))
                    i++;
                start = i + 1;
            }
            else if (b == 0 && start == i)
            {
                // Trailing null at a string boundary — done.
                break;
            }
        }
        // Trailing name without a semicolon (unlikely but observed on some firmwares).
        if (start < span.Length)
        {
            var zeroAt = span[start..].IndexOf((byte)0);
            var end = zeroAt < 0 ? span.Length : start + zeroAt;
            if (end > start)
                result.Add(Encoding.ASCII.GetString(span[start..end]));
        }
        return result;
    }
}