namespace SQLInsight.Core.Data;

/// <summary>A session using tempdb (user objects plus internal objects: sorts, hash tables, spools). Login and program are sensitive.</summary>
/// <param name="Since">When the session logged in; with <see cref="SessionId"/> it identifies the session across samples.</param>
public sealed record TempdbSession(int SessionId, DateTimeOffset Since, double UsedMegabytes, string? LoginName, string? ProgramName, string? HostName, string? Status)
{
    public string Key => $"{SessionId}|{Since.UtcTicks}";
}

/// <summary>The oldest open snapshot transaction: it keeps every row version made since it began.</summary>
public sealed record SnapshotTransaction(int SessionId, long ElapsedSeconds, string? LoginName, string? ProgramName);

/// <summary>Version store use per database (from sys.dm_tran_top_version_generators).</summary>
public sealed record VersionGenerator(string DatabaseName, double Megabytes);

/// <summary>tempdb space and the version store, every 5 minutes.</summary>
public sealed record TempdbSnapshot
{
    public required DateTimeOffset CapturedAt { get; init; }

    /// <summary>The size of tempdb's data files now.</summary>
    public double SizeMegabytes { get; init; }
    public double FreeMegabytes { get; init; }
    public double VersionStoreMegabytes { get; init; }
    public double UserObjectsMegabytes { get; init; }
    public double InternalObjectsMegabytes { get; init; }

    /// <summary>Whether a data file can still grow (autogrowth on and not at its maximum size).</summary>
    public bool CanGrow { get; init; } = true;

    public IReadOnlyList<TempdbSession> TopSessions { get; init; } = [];
    public SnapshotTransaction? OldestSnapshotTransaction { get; init; }
    public IReadOnlyList<VersionGenerator> VersionGenerators { get; init; } = [];

    /// <summary>Version store sizes of the latest samples (MB), oldest first: the trend.</summary>
    public IReadOnlyList<double> VersionStoreHistory { get; init; } = [];

    /// <summary>Since when each session has been above <see cref="SessionThresholdMegabytes"/>, by <see cref="TempdbSession.Key"/>.</summary>
    public IReadOnlyDictionary<string, DateTimeOffset> HeavySessionsSince { get; init; } = new Dictionary<string, DateTimeOffset>();

    /// <summary>The version store above this is a finding (SQLInsight:Tempdb:VersionStoreMegabytes).</summary>
    public int VersionStoreThresholdMegabytes { get; init; } = 1024;

    /// <summary>A session using more tempdb than this for <see cref="TempdbAnalysis.HeavySessionMinutes"/> is a finding.</summary>
    public int SessionThresholdMegabytes { get; init; } = 1024;

    public double FreePercent => SizeMegabytes <= 0 ? 100 : 100 * FreeMegabytes / SizeMegabytes;
}

public static class TempdbAnalysis
{
    public const int HistoryLength = 12;
    public const int HeavySessionMinutes = 15;

    /// <summary>The version store has grown in each of the last six samples, by a quarter GB in all.</summary>
    public static bool IsGrowing(IReadOnlyList<double> history)
    {
        ArgumentNullException.ThrowIfNull(history);
        if (history.Count < 6)
        {
            return false;
        }

        var last = history.Skip(history.Count - 6).ToList();
        return last.Zip(last.Skip(1)).All(p => p.Second > p.First) && last[^1] - last[0] >= 256;
    }

    /// <summary>Carries the trend and the heavy sessions over from the previous sample.</summary>
    public static TempdbSnapshot Advance(TempdbSnapshot? previous, TempdbSnapshot current)
    {
        ArgumentNullException.ThrowIfNull(current);
        var history = (previous?.VersionStoreHistory ?? []).Append(current.VersionStoreMegabytes).TakeLast(HistoryLength).ToList();
        var heavy = new Dictionary<string, DateTimeOffset>(StringComparer.Ordinal);
        foreach (var s in current.TopSessions.Where(s => s.UsedMegabytes >= current.SessionThresholdMegabytes))
        {
            heavy[s.Key] = previous?.HeavySessionsSince.TryGetValue(s.Key, out var since) == true ? since : current.CapturedAt;
        }

        return current with { VersionStoreHistory = history, HeavySessionsSince = heavy };
    }
}