using System.Globalization;

namespace ZB.MOM.WW.MxGateway.Server.Security.Authorization;

/// <summary>
///     Accumulates the constraint denials of one bulk command so they are audited as a single
///     aggregated event rather than one event per denied item.
/// </summary>
/// <remarks>
///     A bulk command can carry thousands of items, and the audit writer is a bounded, drop-on-full
///     channel. One audit event per denied item let a single low-privilege key flood that channel and
///     push every other audit record out of it. The summary keeps the forensic signal — how many items
///     were denied, which constraints denied them, and a bounded sample of the targets — at a fixed
///     cost of one audit event per command.
/// </remarks>
public sealed class ConstraintDenialSummary
{
    /// <summary>The maximum number of denied targets retained as a sample.</summary>
    public const int MaxSampleTargets = 20;

    private readonly List<string> _constraintNames = [];
    private readonly List<string> _sampleTargets = [];

    /// <summary>Initializes a new instance of the <see cref="ConstraintDenialSummary"/> class.</summary>
    /// <param name="totalCount">The number of items the bulk command carried.</param>
    public ConstraintDenialSummary(int totalCount)
    {
        TotalCount = totalCount;
    }

    /// <summary>Gets the number of items the bulk command carried.</summary>
    public int TotalCount { get; }

    /// <summary>Gets the number of items denied.</summary>
    public int DeniedCount { get; private set; }

    /// <summary>Gets the distinct constraint names that denied at least one item, in first-seen order.</summary>
    public IReadOnlyList<string> ConstraintNames => _constraintNames;

    /// <summary>Gets up to <see cref="MaxSampleTargets"/> denied targets, in item order.</summary>
    public IReadOnlyList<string> SampleTargets => _sampleTargets;

    /// <summary>Gets a value indicating whether more items were denied than the sample retains.</summary>
    public bool SampleTruncated => DeniedCount > _sampleTargets.Count;

    /// <summary>Records one denied tag-address item.</summary>
    /// <param name="target">The denied tag address.</param>
    /// <param name="failure">The constraint failure that denied it.</param>
    public void Add(string target, ConstraintFailure failure)
    {
        ArgumentNullException.ThrowIfNull(failure);
        AddConstraint(failure.ConstraintName);
        if (_sampleTargets.Count < MaxSampleTargets)
        {
            _sampleTargets.Add(target);
        }

        DeniedCount++;
    }

    /// <summary>Records one denied item-handle item.</summary>
    /// <param name="itemHandle">The denied MXAccess item handle.</param>
    /// <param name="failure">The constraint failure that denied it.</param>
    /// <remarks>The handle is formatted only while the sample still has room.</remarks>
    public void Add(int itemHandle, ConstraintFailure failure)
    {
        ArgumentNullException.ThrowIfNull(failure);
        AddConstraint(failure.ConstraintName);
        if (_sampleTargets.Count < MaxSampleTargets)
        {
            _sampleTargets.Add(itemHandle.ToString(CultureInfo.InvariantCulture));
        }

        DeniedCount++;
    }

    private void AddConstraint(string constraintName)
    {
        // The constraint vocabulary is a handful of fixed names, so a linear scan beats a set.
        for (int index = 0; index < _constraintNames.Count; index++)
        {
            if (string.Equals(_constraintNames[index], constraintName, StringComparison.Ordinal))
            {
                return;
            }
        }

        _constraintNames.Add(constraintName);
    }
}