sqlinsight-pro/src/SQLInsight.Core/Data/PerformanceCounters.cs

namespace SQLInsight.Core.Data;

/// <summary>
/// Selected SQL Server performance counters (<c>sys.dm_os_performance_counters</c> / <c>sysperfinfo</c>).
/// Point-in-time counters are used as-is; per-second counters are cumulative and need a delta.
/// </summary>
public sealed record PerformanceCounterSnapshot
{
    public required DateTimeOffset CapturedAt { get; init; }
    public required DateTime SqlServerStartTime { get; init; }
    public long? MemoryGrantsPending { get; init; }
    public long? MemoryGrantsOutstanding { get; init; }
    public long? PageLifeExpectancySeconds { get; init; }
    public long? UserConnections { get; init; }
    public long? ProcessesBlocked { get; init; }

    /// <summary>Cumulative since start.</summary>
    public long? BatchRequests { get; init; }

    /// <summary>Cumulative since start.</summary>
    public long? SqlCompilations { get; init; }

    /// <summary>Cumulative since start.</summary>
    public long? SqlRecompilations { get; init; }
}

public sealed record PerformanceCounterRates
{
    public required DateTimeOffset From { get; init; }
    public required DateTimeOffset To { get; init; }
    public double? BatchRequestsPerSecond { get; init; }
    public double? CompilationsPerSecond { get; init; }
    public double? RecompilationsPerSecond { get; init; }

    public static PerformanceCounterRates? Compute(PerformanceCounterSnapshot? previous, PerformanceCounterSnapshot current)
    {
        ArgumentNullException.ThrowIfNull(current);
        if (previous is null || previous.SqlServerStartTime != current.SqlServerStartTime || current.CapturedAt <= previous.CapturedAt)
        {
            return null;
        }

        var seconds = (current.CapturedAt - previous.CapturedAt).TotalSeconds;
        return new PerformanceCounterRates
        {
            From = previous.CapturedAt,
            To = current.CapturedAt,
            BatchRequestsPerSecond = Rate(previous.BatchRequests, current.BatchRequests, seconds),
            CompilationsPerSecond = Rate(previous.SqlCompilations, current.SqlCompilations, seconds),
            RecompilationsPerSecond = Rate(previous.SqlRecompilations, current.SqlRecompilations, seconds),
        };
    }

    private static double? Rate(long? before, long? after, double seconds) =>
        before is { } b && after is { } a && a >= b ? (a - b) / seconds : null;
}