using ZB.MOM.WW.MxGateway.Server.Dashboard.Hubs;

namespace ZB.MOM.WW.MxGateway.Tests.Gateway.Dashboard;

/// <summary>
/// Verifies <see cref="DashboardSnapshotHubConnectionCounter"/>, the seam
/// <see cref="DashboardSnapshotPublisher"/> reads before building a snapshot.
/// An over-count leaves the publisher ticking for nobody; an under-count is
/// worse — it idle-gates a dashboard that is actually open, so the page silently
/// stops updating. The counter is exercised directly rather than through the hub,
/// mirroring the <see cref="EventsHubViewerRegistry"/> precedent: a SignalR
/// <c>Hub</c> instance needs a caller-clients and connection context fake to
/// invoke <c>OnConnectedAsync</c>, and the hub methods themselves are two lines
/// of delegation to this type.
/// </summary>
public sealed class DashboardSnapshotHubConnectionCounterTests
{
    /// <summary>A fresh counter reports no viewers, so the publisher starts idle.</summary>
    [Fact]
    public void Count_WhenNothingConnected_IsZero()
    {
        DashboardSnapshotHubConnectionCounter counter = new();

        Assert.Equal(0, counter.Count);
    }

    /// <summary>Connect/disconnect pairs move the count and return the post-operation value.</summary>
    [Fact]
    public void IncrementThenDecrement_TracksLiveConnections()
    {
        DashboardSnapshotHubConnectionCounter counter = new();

        Assert.Equal(1, counter.Increment());
        Assert.Equal(2, counter.Increment());
        Assert.Equal(2, counter.Count);

        Assert.Equal(1, counter.Decrement());
        Assert.Equal(0, counter.Decrement());
        Assert.Equal(0, counter.Count);
    }

    /// <summary>
    /// SignalR calls <c>OnDisconnectedAsync</c> for a connection whose
    /// <c>OnConnectedAsync</c> faulted, so unmatched decrements happen. They must
    /// hold the floor at zero rather than driving the count negative.
    /// </summary>
    [Fact]
    public void Decrement_WithoutMatchingIncrement_HoldsAtZero()
    {
        DashboardSnapshotHubConnectionCounter counter = new();

        Assert.Equal(0, counter.Decrement());
        Assert.Equal(0, counter.Decrement());
        Assert.Equal(0, counter.Count);

        // A genuine connection after unmatched disconnects still registers as one.
        Assert.Equal(1, counter.Increment());
    }

    /// <summary>
    /// Many concurrent unmatched decrements must not leave the count below zero:
    /// a negative floor would swallow the next real connection's increment and
    /// keep the publisher idle-gated while a viewer waits. The clamp lives inside
    /// the compare-and-swap, so the floor holds however the calls interleave.
    /// </summary>
    [Fact]
    public void Decrement_UnderConcurrencyFromZero_NeverGoesNegative()
    {
        DashboardSnapshotHubConnectionCounter counter = new();

        Parallel.For(0, 256, _ => counter.Decrement());

        Assert.Equal(0, counter.Count);

        Assert.Equal(1, counter.Increment());
        Assert.Equal(1, counter.Count);
    }

    /// <summary>
    /// Stress check on the invariant the idle gate depends on: real connects
    /// interleaved with unmatched disconnects leave the count in [0, connects], and a
    /// connect after the storm is always visible to the publisher. The failure this
    /// guards is the decrement-then-repair race the CAS retry loop replaced — an early
    /// decrementer's stale repair either erases a live connection's increment or leaves
    /// a negative value behind, and either way an open dashboard freezes behind the
    /// gate.
    /// </summary>
    /// <remarks>
    /// This does not deterministically reproduce that race, and it is not claimed to:
    /// the bad interleaving needs a specific few-instruction overlap that cannot be
    /// forced through the public API, and a merely low count is a legitimate outcome
    /// here (a decrement that runs while the count is positive consumes a real
    /// connection). Verified by experiment: the previous implementation passes this
    /// test. What is asserted are the observable consequences — never negative, the
    /// floor holds, a later connect still registers — with the correctness argument
    /// resting on the clamped CAS retry loop itself.
    /// </remarks>
    [Fact]
    public void IncrementAndDecrement_InterleavedUnderConcurrency_StayWithinTheRealConnectionCount()
    {
        const int LiveConnections = 8;
        const int UnmatchedDisconnects = 128;

        for (int round = 0; round < 50; round++)
        {
            DashboardSnapshotHubConnectionCounter counter = new();

            // A few workers are real connects that must survive; the rest are
            // unmatched disconnects hammering the zero floor around them.
            Parallel.For(0, LiveConnections + UnmatchedDisconnects, index =>
            {
                if (index % 16 == 0 && index / 16 < LiveConnections)
                {
                    counter.Increment();
                    return;
                }

                counter.Decrement();
            });

            Assert.InRange(counter.Count, 0, LiveConnections);

            // Every unmatched decrement has completed, so the surviving connections
            // disconnect cleanly and the counter must land exactly on zero — never
            // below it, and a subsequent connect must be visible to the publisher.
            int remaining = counter.Count;
            for (int i = 0; i < remaining; i++)
            {
                counter.Decrement();
            }

            Assert.Equal(0, counter.Count);
            Assert.Equal(1, counter.Increment());
        }
    }

    /// <summary>Matched connect/disconnect pairs under concurrency settle back at zero.</summary>
    [Fact]
    public void IncrementAndDecrement_MatchedPairsUnderConcurrency_SettleAtZero()
    {
        DashboardSnapshotHubConnectionCounter counter = new();

        Parallel.For(0, 64, _ =>
        {
            for (int pass = 0; pass < 50; pass++)
            {
                counter.Increment();
                counter.Decrement();
            }
        });

        Assert.Equal(0, counter.Count);
    }
}