using System.Diagnostics.Metrics;
using ZB.MOM.WW.ScadaBridge.Commons.Observability;

namespace ZB.MOM.WW.ScadaBridge.LoadHarness.Metrics;

/// <summary>
/// Captures the production <c>scadabridge.site.stream.batch_size</c> histogram via a
/// <see cref="MeterListener"/>, so a wire-path run can report the actual frame/batch
/// distribution the R2 batcher produced — not a client-side reconstruction. The
/// instrument is recorded once per emitted gRPC frame (negotiated streams only), with
/// the frame's event count as the value, which makes <c>count = frames</c> and
/// <c>sum = events</c> exactly.
///
/// <para>
/// The harness hosts every site's <see cref="ZB.MOM.WW.ScadaBridge.Communication.Grpc.SiteStreamGrpcServer"/>
/// in-process, so this listener sees every wire frame the run emits and nothing else —
/// no other component in the harness records this instrument.
/// </para>
/// </summary>
public sealed class BatchSizeAggregator : IDisposable
{
    /// <summary>
    /// Sizes above this are clamped into the last bucket. The production cap is
    /// <c>GrpcStreamBatchMaxEvents</c> = 100, so 512 leaves generous headroom for a
    /// configured-up run without an unbounded array.
    /// </summary>
    public const int MaxTrackedBatchSize = 512;

    private readonly MeterListener _listener;
    private readonly long[] _framesBySize = new long[MaxTrackedBatchSize + 1];

    /// <summary>Starts listening. Must be created BEFORE the first stream opens.</summary>
    public BatchSizeAggregator()
    {
        _listener = new MeterListener
        {
            InstrumentPublished = (instrument, listener) =>
            {
                if (instrument.Meter.Name == ScadaBridgeTelemetry.MeterName &&
                    instrument.Name == "scadabridge.site.stream.batch_size")
                {
                    listener.EnableMeasurementEvents(instrument);
                }
            },
        };
        _listener.SetMeasurementEventCallback<int>((_, value, _, _) =>
        {
            var size = Math.Clamp(value, 1, MaxTrackedBatchSize);
            Interlocked.Increment(ref _framesBySize[size]);
        });
        _listener.Start();
    }

    /// <summary>Copies the cumulative per-size frame counts.</summary>
    /// <returns>Index = events-per-frame (clamped), value = frames emitted at that size.</returns>
    public long[] Snapshot()
    {
        var copy = new long[_framesBySize.Length];
        for (var i = 0; i < copy.Length; i++)
            copy[i] = Interlocked.Read(ref _framesBySize[i]);
        return copy;
    }

    /// <inheritdoc />
    public void Dispose() => _listener.Dispose();
}

/// <summary>
/// Batch-size distribution over one measurement window, computed from two
/// <see cref="BatchSizeAggregator.Snapshot"/>s. Percentiles are exact (integer sizes,
/// full counts — nothing is bucketed away).
/// </summary>
/// <param name="Frames">gRPC frames written in the window.</param>
/// <param name="Events">Events those frames carried.</param>
/// <param name="BatchedFrames">Frames that carried more than one event.</param>
/// <param name="MeanEventsPerFrame">Events / Frames.</param>
/// <param name="P50">Median events-per-frame.</param>
/// <param name="P95">95th percentile events-per-frame.</param>
/// <param name="P99">99th percentile events-per-frame.</param>
/// <param name="Max">Largest frame observed in the window.</param>
public sealed record BatchSizeWindow(
    long Frames,
    long Events,
    long BatchedFrames,
    double MeanEventsPerFrame,
    int P50,
    int P95,
    int P99,
    int Max)
{
    /// <summary>Computes the window distribution from cumulative start/end snapshots.</summary>
    /// <param name="start">Snapshot at window start.</param>
    /// <param name="end">Snapshot at window end.</param>
    /// <returns>The distribution of frames emitted between the two snapshots.</returns>
    public static BatchSizeWindow From(long[] start, long[] end)
    {
        long frames = 0, events = 0, batched = 0;
        var max = 0;
        var delta = new long[end.Length];
        for (var size = 1; size < end.Length; size++)
        {
            var n = end[size] - start[size];
            if (n <= 0) continue;
            delta[size] = n;
            frames += n;
            events += n * size;
            if (size > 1) batched += n;
            max = size;
        }

        return new BatchSizeWindow(
            frames, events, batched,
            frames == 0 ? 0 : events / (double)frames,
            Percentile(delta, frames, 50),
            Percentile(delta, frames, 95),
            Percentile(delta, frames, 99),
            max);
    }

    private static int Percentile(long[] delta, long frames, double percentile)
    {
        if (frames == 0) return 0;
        var target = (long)Math.Ceiling(frames * percentile / 100.0);
        if (target < 1) target = 1;

        long cumulative = 0;
        for (var size = 1; size < delta.Length; size++)
        {
            cumulative += delta[size];
            if (cumulative >= target)
                return size;
        }

        return delta.Length - 1;
    }
}