using AVEVA.Historian.Client.Models;
using AVEVA.Historian.Client.Redundancy;

namespace AVEVA.Historian.Client.Tests;

/// <summary>
/// Unit tests for the R4.4 multi-historian redundancy client. No server required — members are
/// driven through a controllable <see cref="FakeMember"/>.
/// </summary>
public sealed class RedundancyTests
{
    // ---- read failover -------------------------------------------------------------------

    [Fact]
    public async Task ReadRaw_PrefersPrimary_WhenHealthy()
    {
        var primary = new FakeMember("primary") { Rows = [Sample("primary", 1)] };
        var secondary = new FakeMember("secondary") { Rows = [Sample("secondary", 2)] };
        await using var cluster = new HistorianRedundantClient([primary, secondary], NoWatchdog());

        List<HistorianSample> rows = await CollectAsync(cluster.ReadRawAsync("T", Day(0), Day(1), 10));

        Assert.Equal(["primary"], rows.Select(r => r.TagName));
        Assert.Equal(1, primary.RawCalls);
        Assert.Equal(0, secondary.RawCalls);
    }

    [Fact]
    public async Task ReadRaw_FailsOverToSecondary_WhenPrimaryDownOnConnect()
    {
        var primary = new FakeMember("primary") { Online = false };
        var secondary = new FakeMember("secondary") { Rows = [Sample("secondary", 1), Sample("secondary", 2)] };
        await using var cluster = new HistorianRedundantClient([primary, secondary], NoWatchdog());

        List<HistorianSample> rows = await CollectAsync(cluster.ReadRawAsync("T", Day(0), Day(1), 10));

        Assert.Equal(["secondary", "secondary"], rows.Select(r => r.TagName));
        Assert.Equal(1, primary.RawCalls);
        Assert.Equal(1, secondary.RawCalls);

        // The failed primary is demoted; status reflects it.
        HistorianClusterStatus status = cluster.GetStatus();
        Assert.False(status.Members.Single(m => m.Name == "primary").IsHealthy);
        Assert.Equal("secondary", status.ActiveMember);
    }

    [Fact]
    public async Task ReadRaw_MidStreamFailure_DoesNotFailOver_AndPropagates()
    {
        // Primary yields one row then throws mid-stream. Failing over would duplicate/skip rows, so the
        // error must propagate rather than silently switch to the secondary.
        var primary = new FakeMember("primary") { Rows = [Sample("primary", 1), Sample("primary", 2)], ThrowAfter = 1 };
        var secondary = new FakeMember("secondary") { Rows = [Sample("secondary", 9)] };
        await using var cluster = new HistorianRedundantClient([primary, secondary], NoWatchdog());

        var seen = new List<HistorianSample>();
        await Assert.ThrowsAnyAsync<Exception>(async () =>
        {
            await foreach (HistorianSample s in cluster.ReadRawAsync("T", Day(0), Day(1), 10))
            {
                seen.Add(s);
            }
        });

        Assert.Equal(["primary"], seen.Select(r => r.TagName)); // the one row before the failure
        Assert.Equal(0, secondary.RawCalls); // never failed over mid-stream
    }

    [Fact]
    public async Task ReadRaw_AllMembersFail_ThrowsAggregated()
    {
        var primary = new FakeMember("primary") { Online = false };
        var secondary = new FakeMember("secondary") { Online = false };
        await using var cluster = new HistorianRedundantClient([primary, secondary], NoWatchdog());

        HistorianAllMembersFailedException ex = await Assert.ThrowsAsync<HistorianAllMembersFailedException>(
            async () => await CollectAsync(cluster.ReadRawAsync("T", Day(0), Day(1), 10)));

        Assert.Equal(nameof(HistorianRedundantClient.ReadRawAsync), ex.Operation);
        Assert.IsType<AggregateException>(ex.InnerException);
        Assert.Equal(2, ((AggregateException)ex.InnerException!).InnerExceptions.Count);
    }

    [Fact]
    public async Task ReadAtTime_ScalarFailover_UsesSecondary()
    {
        var primary = new FakeMember("primary") { Online = false };
        var secondary = new FakeMember("secondary") { Rows = [Sample("secondary", 5)] };
        await using var cluster = new HistorianRedundantClient([primary, secondary], NoWatchdog());

        IReadOnlyList<HistorianSample> rows = await cluster.ReadAtTimeAsync("T", [Day(0)]);
        Assert.Equal(["secondary"], rows.Select(r => r.TagName));
    }

    // ---- probe ---------------------------------------------------------------------------

    [Fact]
    public async Task Probe_ReturnsTrue_WhenAnyMemberUp_FalseWhenAllDown()
    {
        var up = new FakeMember("up");
        var down = new FakeMember("down") { Online = false };

        await using (var cluster = new HistorianRedundantClient([down, up], NoWatchdog()))
        {
            Assert.True(await cluster.ProbeAsync());
        }

        await using (var cluster = new HistorianRedundantClient([down, new FakeMember("down2") { Online = false }], NoWatchdog()))
        {
            Assert.False(await cluster.ProbeAsync());
        }
    }

    // ---- fan-out writes ------------------------------------------------------------------

    [Fact]
    public async Task Write_FansOutToAllMembers_AckAll_RequiresEveryMember()
    {
        var a = new FakeMember("a");
        var b = new FakeMember("b");
        await using var cluster = new HistorianRedundantClient([a, b], NoWatchdog());

        HistorianRedundantWriteResult result = await cluster.AddHistoricalValuesAsync("T", [new HistorianHistoricalValue(Day(0), 1)]);

        Assert.True(result.Succeeded);
        Assert.Equal(2, result.Outcomes.Count);
        Assert.All(result.Outcomes, o => Assert.True(o.Accepted));
        Assert.Equal(1, a.WriteCalls);
        Assert.Equal(1, b.WriteCalls);
    }

    [Fact]
    public async Task Write_AckAll_FailsWhenOneMemberDown_ButOtherStillReceivesIt()
    {
        var a = new FakeMember("a");
        var b = new FakeMember("b") { Online = false };
        await using var cluster = new HistorianRedundantClient([a, b], NoWatchdog());

        HistorianRedundantWriteResult result = await cluster.AddHistoricalValuesAsync("T", [new HistorianHistoricalValue(Day(0), 1)]);

        Assert.False(result.Succeeded);             // AckAll not met
        Assert.Single(result.Accepted, o => o.Member == "a");
        Assert.Single(result.Failed, o => o.Member == "b");
    }

    [Fact]
    public async Task Write_AckAny_SucceedsWhenOneMemberAccepts()
    {
        var a = new FakeMember("a") { Online = false };
        var b = new FakeMember("b");
        var options = NoWatchdog() with { WriteAcknowledgement = HistorianWriteAcknowledgement.Any };
        await using var cluster = new HistorianRedundantClient([a, b], options);

        HistorianRedundantWriteResult result = await cluster.AddHistoricalValuesAsync("T", [new HistorianHistoricalValue(Day(0), 1)]);
        Assert.True(result.Succeeded);
    }

    [Fact]
    public async Task Write_PreferredOnly_WritesSingleMember()
    {
        var a = new FakeMember("a");
        var b = new FakeMember("b");
        var options = NoWatchdog() with { WriteFanout = HistorianWriteFanout.PreferredOnly };
        await using var cluster = new HistorianRedundantClient([a, b], options);

        HistorianRedundantWriteResult result = await cluster.AddHistoricalValuesAsync("T", [new HistorianHistoricalValue(Day(0), 1)]);

        Assert.True(result.Succeeded);
        Assert.Single(result.Outcomes);
        Assert.Equal(1, a.WriteCalls);
        Assert.Equal(0, b.WriteCalls);
    }

    [Fact]
    public async Task Write_RejectingMember_ReportsNotAccepted_WithoutThrowing()
    {
        var a = new FakeMember("a") { RejectWrite = true };
        await using var cluster = new HistorianRedundantClient([a], NoWatchdog());

        HistorianRedundantWriteResult result = await cluster.AddHistoricalValuesAsync("T", [new HistorianHistoricalValue(Day(0), 1)]);

        Assert.False(result.Succeeded);
        Assert.False(result.Outcomes.Single().Accepted);
        Assert.NotNull(result.Outcomes.Single().Error);
    }

    // ---- health recovery -----------------------------------------------------------------

    [Fact]
    public async Task DemotedMember_IsRestored_ByHealthCheck_AndPreferredAgain()
    {
        var primary = new FakeMember("primary") { Online = false, Rows = [Sample("primary", 1)] };
        var secondary = new FakeMember("secondary") { Rows = [Sample("secondary", 2)] };
        await using var cluster = new HistorianRedundantClient([primary, secondary], NoWatchdog());

        // First read fails over and demotes the primary.
        await CollectAsync(cluster.ReadRawAsync("T", Day(0), Day(1), 10));
        Assert.False(cluster.GetStatus().Members.Single(m => m.Name == "primary").IsHealthy);

        // Primary recovers; an explicit health check restores it to the healthy pool.
        primary.Online = true;
        HistorianClusterStatus status = await cluster.CheckHealthAsync();
        Assert.True(status.Members.Single(m => m.Name == "primary").IsHealthy);
        Assert.Equal("primary", status.ActiveMember);

        // Subsequent reads prefer the restored primary again.
        List<HistorianSample> rows = await CollectAsync(cluster.ReadRawAsync("T", Day(0), Day(1), 10));
        Assert.Equal(["primary"], rows.Select(r => r.TagName));
    }

    [Fact]
    public async Task Watchdog_RestoresMember_AfterRecovery()
    {
        var primary = new FakeMember("primary") { Online = false };
        var secondary = new FakeMember("secondary") { Rows = [Sample("secondary", 1)] };
        var options = new HistorianRedundancyOptions { RunWatchdog = true, WatchdogInterval = TimeSpan.FromMilliseconds(100) };
        await using var cluster = new HistorianRedundantClient([primary, secondary], options);

        await CollectAsync(cluster.ReadRawAsync("T", Day(0), Day(1), 10)); // demotes primary
        await cluster.StartAsync();

        primary.Online = true;
        bool restored = await WaitUntilAsync(() => cluster.GetStatus().Members.Single(m => m.Name == "primary").IsHealthy, TimeSpan.FromSeconds(3));
        Assert.True(restored);
    }

    [Fact]
    public void Constructor_RejectsEmptyMemberList()
    {
        Assert.Throws<ArgumentException>(() => new HistorianRedundantClient([], NoWatchdog()));
    }

    // ---- helpers -------------------------------------------------------------------------

    private static HistorianRedundancyOptions NoWatchdog() => new() { RunWatchdog = false };

    private static DateTime Day(int d) => new(2026, 1, 1 + d, 0, 0, 0, DateTimeKind.Utc);

    private static HistorianSample Sample(string tag, double value) =>
        new(tag, Day(0), value, null, 0, 0, 192, 100);

    private static async Task<List<HistorianSample>> CollectAsync(IAsyncEnumerable<HistorianSample> source)
    {
        var list = new List<HistorianSample>();
        await foreach (HistorianSample s in source)
        {
            list.Add(s);
        }

        return list;
    }

    private static async Task<bool> WaitUntilAsync(Func<bool> condition, TimeSpan timeout)
    {
        DateTime deadline = DateTime.UtcNow + timeout;
        while (DateTime.UtcNow < deadline)
        {
            if (condition())
            {
                return true;
            }

            await Task.Delay(25);
        }

        return condition();
    }

    private sealed class FakeMember : IHistorianMember
    {
        public FakeMember(string name) => Name = name;

        public string Name { get; }
        public bool Online { get; set; } = true;
        public bool RejectWrite { get; set; }
        public IReadOnlyList<HistorianSample> Rows { get; set; } = [];

        /// <summary>If &gt;= 0, yield that many rows then throw mid-stream.</summary>
        public int ThrowAfter { get; set; } = -1;

        public int ProbeCalls;
        public int RawCalls;
        public int WriteCalls;

        public Task<bool> ProbeAsync(CancellationToken cancellationToken)
        {
            ProbeCalls++;
            if (!Online)
            {
                throw new IOException("member down");
            }

            return Task.FromResult(true);
        }

        public async IAsyncEnumerable<HistorianSample> ReadRawAsync(string tag, DateTime startUtc, DateTime endUtc, int maxValues, [System.Runtime.CompilerServices.EnumeratorCancellation] CancellationToken cancellationToken)
        {
            RawCalls++;
            if (!Online)
            {
                throw new IOException("member down"); // throws on first MoveNext (before any yield)
            }

            int n = 0;
            foreach (HistorianSample s in Rows)
            {
                if (ThrowAfter >= 0 && n == ThrowAfter)
                {
                    throw new IOException("mid-stream failure");
                }

                yield return s;
                n++;
                await Task.Yield();
            }
        }

        public IAsyncEnumerable<HistorianAggregateSample> ReadAggregateAsync(string tag, DateTime startUtc, DateTime endUtc, RetrievalMode mode, TimeSpan interval, CancellationToken cancellationToken) =>
            throw new NotSupportedException();

        public Task<IReadOnlyList<HistorianSample>> ReadAtTimeAsync(string tag, IReadOnlyList<DateTime> timestampsUtc, CancellationToken cancellationToken)
        {
            RawCalls++;
            if (!Online)
            {
                throw new IOException("member down");
            }

            return Task.FromResult(Rows);
        }

        public IAsyncEnumerable<HistorianEvent> ReadEventsAsync(DateTime startUtc, DateTime endUtc, CancellationToken cancellationToken) =>
            throw new NotSupportedException();

        public IAsyncEnumerable<string> BrowseTagNamesAsync(string filter, CancellationToken cancellationToken) =>
            throw new NotSupportedException();

        public Task<HistorianTagMetadata?> GetTagMetadataAsync(string tag, CancellationToken cancellationToken) =>
            throw new NotSupportedException();

        public Task<bool> AddHistoricalValuesAsync(string tag, IReadOnlyList<HistorianHistoricalValue> values, CancellationToken cancellationToken)
        {
            WriteCalls++;
            if (!Online)
            {
                throw new IOException("member down");
            }

            return Task.FromResult(!RejectWrite);
        }

        public Task<bool> SendEventAsync(HistorianEvent historianEvent, CancellationToken cancellationToken)
        {
            WriteCalls++;
            if (!Online)
            {
                throw new IOException("member down");
            }

            return Task.FromResult(!RejectWrite);
        }
    }
}