using System.Security.Cryptography;
using System.Text;
using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
using ZB.MOM.WW.OtOpcUa.Core.AlarmHistorian;

namespace ZB.MOM.WW.OtOpcUa.Driver.Historian.Sql.Common;

/// <summary>
///     The deterministic alarm-event id and numeric severity the SQL historian backends store, so the store-and-forward
///     drain's at-least-once redelivery (and both pair nodes writing the same event) converge on one row.
/// </summary>
public static class AlarmEventIds
{
    /// <summary>
    ///     Lowercase-hex SHA-256 over length-prefixed UTF-8 fields (little-endian int32 byte length, -1 for null):
    ///     EquipmentPath, AlarmId, EventKind, TimestampUtc.Ticks (decimal), User, Comment, Message. Length-prefixing keeps
    ///     field boundaries unambiguous (<c>"a|b"+"c"</c> ≠ <c>"a"+"b|c"</c>).
    /// </summary>
    /// <param name="e">The event.</param>
    /// <returns>64 hex characters.</returns>
    public static string ComputeEventId(AlarmHistorianEvent e)
    {
        using var ms = new MemoryStream();
        Field(ms, e.EquipmentPath);
        Field(ms, e.AlarmId);
        Field(ms, e.EventKind);
        Field(ms, e.TimestampUtc.Ticks.ToString(System.Globalization.CultureInfo.InvariantCulture));
        Field(ms, e.User);
        Field(ms, e.Comment);
        Field(ms, e.Message);
        return Convert.ToHexStringLower(SHA256.HashData(ms.ToArray()));
    }

    /// <summary>The numeric Part 9 priority stored for a severity (same values the gateway path sends).</summary>
    /// <param name="s">Alarm severity.</param>
    /// <returns>200 / 500 / 700 / 900; 500 for anything else.</returns>
    public static int ToNumericSeverity(AlarmSeverity s) => s switch
    {
        AlarmSeverity.Low => 200,
        AlarmSeverity.Medium => 500,
        AlarmSeverity.High => 700,
        AlarmSeverity.Critical => 900,
        _ => 500,
    };

    private static void Field(MemoryStream ms, string? value)
    {
        Span<byte> len = stackalloc byte[4];
        if (value is null)
        {
            System.Buffers.Binary.BinaryPrimitives.WriteInt32LittleEndian(len, -1);
            ms.Write(len);
            return;
        }

        var bytes = Encoding.UTF8.GetBytes(value);
        System.Buffers.Binary.BinaryPrimitives.WriteInt32LittleEndian(len, bytes.Length);
        ms.Write(len);
        ms.Write(bytes);
    }
}