using System;
using System.Security.Cryptography;
using System.Text;

namespace ZB.MOM.WW.MxGateway.Worker.MxAccess;

/// <summary>
///     Derives a deterministic synthetic <see cref="Guid"/> from an alarm
///     reference for the subtag-provider fallback path, which has no native
///     MxAccess alarm GUID. Hashing the reference yields a stable identity so
///     repeated transitions for the same alarm reference correlate downstream
///     (acknowledge, snapshot, OPC UA mapping) without an alarmmgr-supplied GUID.
/// </summary>
public static class SyntheticAlarmGuid
{
    /// <summary>
    ///     Produces a stable <see cref="Guid"/> for the given alarm reference.
    ///     The same reference always maps to the same GUID; distinct references
    ///     map to distinct GUIDs with overwhelming probability.
    /// </summary>
    /// <param name="reference">
    ///     The fully-qualified alarm reference (for example
    ///     <c>"Galaxy!Area.Tag.HiHi"</c>). Treated as UTF-8 bytes.
    /// </param>
    /// <returns>A deterministic, non-empty GUID derived from the reference.</returns>
    /// <exception cref="ArgumentNullException">
    ///     Thrown when <paramref name="reference"/> is <see langword="null"/>.
    /// </exception>
    public static Guid ForReference(string reference)
    {
        if (reference is null) throw new ArgumentNullException(nameof(reference));

        byte[] bytes = Encoding.UTF8.GetBytes(reference);

        // MD5 is used purely for a stable, non-cryptographic identity mapping
        // (reference -> 16-byte GUID), never for security. Its 128-bit output
        // fits a GUID exactly, which is why it is preferred here.
        using MD5 md5 = MD5.Create();
        byte[] hash = md5.ComputeHash(bytes);
        return new Guid(hash);
    }
}