jellyfin-plugin-recently-added-guard/Jellyfin.Plugin.RecentlyAddedGuard/Core/CorrectionLog.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;

namespace Jellyfin.Plugin.RecentlyAddedGuard.Core;

/// <summary>
/// One restored (or, in dry-run mode, detected) upgrade.
/// </summary>
/// <param name="ItemId">The corrected item.</param>
/// <param name="Name">The display name.</param>
/// <param name="PreviousDate">The "date added" the upgrade brought along (UTC).</param>
/// <param name="RestoredDate">The first "date added" from the ledger (UTC).</param>
/// <param name="CorrectedAt">When the correction happened (UTC).</param>
/// <param name="DryRun"><c>true</c> if only detected, not applied.</param>
public sealed record CorrectionRecord(
    Guid ItemId,
    string Name,
    DateTime PreviousDate,
    DateTime RestoredDate,
    DateTime CorrectedAt,
    bool DryRun);

/// <summary>
/// In-memory ring buffer of the most recent corrections, for the configuration page.
/// </summary>
public sealed class CorrectionLog
{
    /// <summary>
    /// The default number of kept corrections.
    /// </summary>
    public const int DefaultCapacity = 100;

    private readonly Lock _sync = new();
    private readonly List<CorrectionRecord> _records;
    private readonly int _capacity;
    private long _corrections;
    private long _dryRunDetections;

    /// <summary>
    /// Initializes a new instance of the <see cref="CorrectionLog"/> class.
    /// </summary>
    /// <param name="capacity">The number of kept corrections.</param>
    public CorrectionLog(int capacity = DefaultCapacity)
    {
        ArgumentOutOfRangeException.ThrowIfNegativeOrZero(capacity);
        _capacity = capacity;
        _records = new List<CorrectionRecord>(capacity);
    }

    /// <summary>
    /// Gets the number of applied corrections since the server started.
    /// </summary>
    public long CorrectionsSinceStart => Interlocked.Read(ref _corrections);

    /// <summary>
    /// Gets the number of upgrades detected in dry-run mode since the server started.
    /// </summary>
    public long DryRunDetectionsSinceStart => Interlocked.Read(ref _dryRunDetections);

    /// <summary>
    /// Records a correction, dropping the oldest one when full. Restoring the same date of the same item again
    /// (Jellyfin saved the item with its new date once more, e.g. from a metadata refresh that was already running)
    /// replaces the earlier record and is not counted twice.
    /// </summary>
    /// <param name="record">The correction.</param>
    /// <returns><c>true</c> for a new correction, <c>false</c> for a repeated one.</returns>
    public bool Add(CorrectionRecord record)
    {
        ArgumentNullException.ThrowIfNull(record);

        lock (_sync)
        {
            var existing = _records.FindIndex(r => r.ItemId == record.ItemId
                && r.RestoredDate == record.RestoredDate
                && r.DryRun == record.DryRun);
            if (existing >= 0)
            {
                _records.RemoveAt(existing);
                _records.Add(record);
                return false;
            }

            if (_records.Count == _capacity)
            {
                _records.RemoveAt(0);
            }

            _records.Add(record);
        }

        if (record.DryRun)
        {
            Interlocked.Increment(ref _dryRunDetections);
        }
        else
        {
            Interlocked.Increment(ref _corrections);
        }

        return true;
    }

    /// <summary>
    /// Returns the kept corrections, newest first.
    /// </summary>
    /// <returns>The corrections.</returns>
    public IReadOnlyList<CorrectionRecord> GetRecent()
    {
        lock (_sync)
        {
            return _records.AsEnumerable().Reverse().ToList();
        }
    }
}