using System.Collections.Concurrent;
using System.Net;
using System.Net.Sockets;
using System.Text.Json;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;
using ZB.MOM.WW.CBDDC.Core;
using ZB.MOM.WW.CBDDC.Core.Network;
using ZB.MOM.WW.CBDDC.Core.Storage;
using ZB.MOM.WW.CBDDC.Core.Sync;
using ZB.MOM.WW.CBDDC.Network;
using ZB.MOM.WW.CBDDC.Network.Security;
using ZB.MOM.WW.CBDDC.Persistence.Surreal;

namespace ZB.MOM.WW.CBDDC.E2E.Tests;

public class ClusterCrudSyncE2ETests
{
    /// <summary>
    ///     Verifies two real peers replicate create, update, and delete operations in both directions.
    /// </summary>
    [Fact]
    public async Task TwoPeers_ShouldReplicateCrudBidirectionally()
    {
        var clusterToken = Guid.NewGuid().ToString("N");
        int nodeAPort = GetAvailableTcpPort();
        int nodeBPort = GetAvailableTcpPort();
        while (nodeBPort == nodeAPort) nodeBPort = GetAvailableTcpPort();

        var nodeA = TestPeerNode.Create(
            "node-a",
            nodeAPort,
            clusterToken,
            [
                new KnownPeerConfiguration
                {
                    NodeId = "node-b",
                    Host = "127.0.0.1",
                    Port = nodeBPort
                }
            ]);

        await using var nodeB = TestPeerNode.Create(
            "node-b",
            nodeBPort,
            clusterToken,
            [
                new KnownPeerConfiguration
                {
                    NodeId = "node-a",
                    Host = "127.0.0.1",
                    Port = nodeAPort
                }
            ]);

        await nodeA.StartAsync();
        await nodeB.StartAsync();

        const int timeoutSeconds = 45;

        var nodeAUserId = "user-from-a";
        await nodeA.UpsertUserAsync(new User
        {
            Id = nodeAUserId,
            Name = "Alice",
            Age = 30,
            Address = new Address { City = "Austin" }
        });

        await AssertEventuallyAsync(() =>
        {
            var replicated = nodeB.ReadUser(nodeAUserId);
            return replicated is not null
                   && replicated.Name == "Alice"
                   && replicated.Age == 30
                   && replicated.Address?.City == "Austin";
        }, timeoutSeconds, "Node B did not receive create from node A.", () => BuildDiagnostics(nodeA, nodeB));

        await AssertEventuallyAsync(
            () => nodeA.ReadUser(nodeAUserId) is not null,
            timeoutSeconds,
            "Node A could not read back its own created user.",
            () => BuildDiagnostics(nodeA, nodeB));

        await nodeA.DeleteUserAsync(nodeAUserId);

        await nodeA.UpsertUserAsync(new User
        {
            Id = nodeAUserId,
            Name = "Alice Updated",
            Age = 31,
            Address = new Address { City = "Dallas" }
        });

        await AssertEventuallyAsync(() =>
        {
            var replicated = nodeB.ReadUser(nodeAUserId);
            return replicated is not null
                   && replicated.Name == "Alice Updated"
                   && replicated.Age == 31
                   && replicated.Address?.City == "Dallas";
        }, timeoutSeconds, "Node B did not receive update from node A.", () => BuildDiagnostics(nodeA, nodeB));

        await nodeA.DeleteUserAsync(nodeAUserId);

        await AssertEventuallyAsync(
            () => nodeB.ReadUser(nodeAUserId) is null,
            timeoutSeconds,
            "Node B did not receive delete from node A.",
            () => BuildDiagnostics(nodeA, nodeB));

        var nodeBUserId = "user-from-b";
        await nodeB.UpsertUserAsync(new User
        {
            Id = nodeBUserId,
            Name = "Bob",
            Age = 40,
            Address = new Address { City = "Boston" }
        });

        await AssertEventuallyAsync(() =>
        {
            var replicated = nodeA.ReadUser(nodeBUserId);
            return replicated is not null
                   && replicated.Name == "Bob"
                   && replicated.Age == 40
                   && replicated.Address?.City == "Boston";
        }, timeoutSeconds, "Node A did not receive create from node B.", () => BuildDiagnostics(nodeA, nodeB));

        await AssertEventuallyAsync(
            () => nodeB.ReadUser(nodeBUserId) is not null,
            timeoutSeconds,
            "Node B could not read back its own created user.",
            () => BuildDiagnostics(nodeA, nodeB));

        await nodeB.DeleteUserAsync(nodeBUserId);

        await nodeB.UpsertUserAsync(new User
        {
            Id = nodeBUserId,
            Name = "Bob Updated",
            Age = 41,
            Address = new Address { City = "Denver" }
        });

        await AssertEventuallyAsync(() =>
        {
            var replicated = nodeA.ReadUser(nodeBUserId);
            return replicated is not null
                   && replicated.Name == "Bob Updated"
                   && replicated.Age == 41
                   && replicated.Address?.City == "Denver";
        }, timeoutSeconds, "Node A did not receive update from node B.", () => BuildDiagnostics(nodeA, nodeB));

        await nodeB.DeleteUserAsync(nodeBUserId);

        await AssertEventuallyAsync(
            () => nodeA.ReadUser(nodeBUserId) is null,
            timeoutSeconds,
            "Node A did not receive delete from node B.",
            () => BuildDiagnostics(nodeA, nodeB));
    }

    /// <summary>
    ///     Verifies a reconnecting peer catches up mutations that happened while it was offline.
    /// </summary>
    [Fact]
    public async Task PeerReconnect_ShouldCatchUpMissedChanges()
    {
        var clusterToken = Guid.NewGuid().ToString("N");
        int nodeAPort = GetAvailableTcpPort();
        int nodeBPort = GetAvailableTcpPort();
        while (nodeBPort == nodeAPort) nodeBPort = GetAvailableTcpPort();

        var nodeA = TestPeerNode.Create(
            "node-a",
            nodeAPort,
            clusterToken,
            [
                new KnownPeerConfiguration
                {
                    NodeId = "node-b",
                    Host = "127.0.0.1",
                    Port = nodeBPort
                }
            ]);

        await using var nodeB = TestPeerNode.Create(
            "node-b",
            nodeBPort,
            clusterToken,
            [
                new KnownPeerConfiguration
                {
                    NodeId = "node-a",
                    Host = "127.0.0.1",
                    Port = nodeAPort
                }
            ]);

        await nodeA.StartAsync();
        await nodeB.StartAsync();

        await nodeB.StopAsync();

        const string userId = "reconnect-user";
        await nodeA.UpsertUserAsync(new User
        {
            Id = userId,
            Name = "Offline Create",
            Age = 20,
            Address = new Address { City = "Rome" }
        });

        await nodeA.UpsertUserAsync(new User
        {
            Id = userId,
            Name = "Offline Update",
            Age = 21,
            Address = new Address { City = "Milan" }
        });

        await nodeA.UpsertUserAsync(new User
        {
            Id = userId,
            Name = "Offline Final",
            Age = 22,
            Address = new Address { City = "Turin" }
        });

        await nodeB.StartAsync();

        await AssertEventuallyAsync(() =>
        {
            var replicated = nodeB.ReadUser(userId);
            return replicated is not null &&
                   replicated.Name == "Offline Final" &&
                   replicated.Age == 22 &&
                   replicated.Address?.City == "Turin";
        }, 60, "Node B did not catch up missed reconnect mutations.", () => BuildDiagnostics(nodeA, nodeB));
    }

    /// <summary>
    ///     Verifies a burst of rapid multi-node mutations converges to a deterministic final state.
    /// </summary>
    [Fact]
    public async Task MultiChangeBurst_ShouldConvergeDeterministically()
    {
        var clusterToken = Guid.NewGuid().ToString("N");
        int nodeAPort = GetAvailableTcpPort();
        int nodeBPort = GetAvailableTcpPort();
        while (nodeBPort == nodeAPort) nodeBPort = GetAvailableTcpPort();

        await using var nodeA = TestPeerNode.Create(
            "node-a",
            nodeAPort,
            clusterToken,
            [
                new KnownPeerConfiguration
                {
                    NodeId = "node-b",
                    Host = "127.0.0.1",
                    Port = nodeBPort
                }
            ]);

        await using var nodeB = TestPeerNode.Create(
            "node-b",
            nodeBPort,
            clusterToken,
            [
                new KnownPeerConfiguration
                {
                    NodeId = "node-a",
                    Host = "127.0.0.1",
                    Port = nodeAPort
                }
            ]);

        await nodeA.StartAsync();
        await nodeB.StartAsync();

        const int burstCount = 8;
        for (var i = 0; i < burstCount; i++)
        {
            string aId = $"burst-a-{i:D2}";
            string bId = $"burst-b-{i:D2}";

            await nodeA.UpsertUserAsync(new User
            {
                Id = aId,
                Name = $"A-{i}",
                Age = 30 + i,
                Address = new Address { City = $"CityA-{i}" }
            });

            await nodeB.UpsertUserAsync(new User
            {
                Id = bId,
                Name = $"B-{i}",
                Age = 40 + i,
                Address = new Address { City = $"CityB-{i}" }
            });
        }

        await AssertEventuallyAsync(
            () => nodeA.Context.Users.FindAll().Count() == burstCount * 2 &&
                  nodeB.Context.Users.FindAll().Count() == burstCount * 2,
            60,
            "Burst convergence did not reach expected document counts.",
            () => BuildDiagnostics(nodeA, nodeB));

        await AssertEventuallyAsync(() =>
        {
            for (var i = 0; i < burstCount; i++)
            {
                var aOnB = nodeB.ReadUser($"burst-a-{i:D2}");
                var bOnA = nodeA.ReadUser($"burst-b-{i:D2}");
                if (aOnB is null || bOnA is null) return false;
                if (aOnB.Name != $"A-{i}" || bOnA.Name != $"B-{i}") return false;
            }

            return true;
        }, 60, "Burst convergence content mismatch.", () => BuildDiagnostics(nodeA, nodeB));
    }

    /// <summary>
    ///     Verifies recovery safety when a process crashes after oplog commit but before checkpoint advance.
    /// </summary>
    [Fact]
    public async Task CrashBetweenOplogAndCheckpoint_ShouldReplaySafelyOnRestart()
    {
        var clusterToken = Guid.NewGuid().ToString("N");
        int nodeAPort = GetAvailableTcpPort();
        int nodeBPort = GetAvailableTcpPort();
        while (nodeBPort == nodeAPort) nodeBPort = GetAvailableTcpPort();

        string sharedWorkDir = Path.Combine(Path.GetTempPath(), $"cbddc-e2e-crash-{Guid.NewGuid():N}");
        Directory.CreateDirectory(sharedWorkDir);

        await using var nodeA = TestPeerNode.Create(
            "node-a",
            nodeAPort,
            clusterToken,
            [
                new KnownPeerConfiguration
                {
                    NodeId = "node-b",
                    Host = "127.0.0.1",
                    Port = nodeBPort
                }
            ],
            workDirOverride: sharedWorkDir,
            preserveWorkDirOnDispose: true,
            useFaultInjectedCheckpointStore: true);
        bool nodeADisposed = false;

        try
        {
            await using var nodeB = TestPeerNode.Create(
                "node-b",
                nodeBPort,
                clusterToken,
                [
                    new KnownPeerConfiguration
                    {
                        NodeId = "node-a",
                        Host = "127.0.0.1",
                        Port = nodeAPort
                    }
                ]);

            await nodeA.StartAsync();
            await nodeB.StartAsync();

            const string userId = "crash-window-user";
            var payload = new User
            {
                Id = userId,
                Name = "Crash Recovered",
                Age = 45,
                Address = new Address { City = "Naples" }
            };

            await Should.ThrowAsync<InvalidOperationException>(() => nodeA.UpsertUserAsync(payload));
            nodeA.ReadUser(userId).ShouldNotBeNull();
            nodeA.GetLocalOplogCountForKey("Users", userId).ShouldBe(1);

            await nodeA.StopAsync();
            await nodeA.DisposeAsync();
            nodeADisposed = true;

            TestPeerNode? recoveredNodeA = null;
            for (var attempt = 0; attempt < 10; attempt++)
                try
                {
                    recoveredNodeA = TestPeerNode.Create(
                        "node-a",
                        nodeAPort,
                        clusterToken,
                        [
                            new KnownPeerConfiguration
                            {
                                NodeId = "node-b",
                                Host = "127.0.0.1",
                                Port = nodeBPort
                            }
                        ],
                        workDirOverride: sharedWorkDir);
                    break;
                }
                catch (Exception ex) when (IsRocksDbLockContention(ex) && attempt < 9)
                {
                    await Task.Delay(100);
                }

            recoveredNodeA.ShouldNotBeNull();
            await using (recoveredNodeA)
            {
                await recoveredNodeA.StartAsync();

                await AssertEventuallyAsync(() =>
                {
                    var replicated = nodeB.ReadUser(userId);
                    return replicated is not null
                           && replicated.Name == payload.Name
                           && replicated.Age == payload.Age
                           && replicated.Address?.City == payload.Address?.City;
                }, 60, "Node B did not converge after crash-window recovery.", () => BuildDiagnostics(recoveredNodeA, nodeB));

            await AssertEventuallyAsync(
                () => recoveredNodeA.GetOplogCountForKey("Users", userId) == 1 &&
                      nodeB.GetOplogCountForKey("Users", userId) == 1,
                60,
                "Crash-window recovery created duplicate oplog entries.",
                () => BuildDiagnostics(recoveredNodeA, nodeB));
            }
        }
        finally
        {
            if (!nodeADisposed) await nodeA.DisposeAsync();
        }
    }

    private static async Task AssertEventuallyAsync(
        Func<bool> predicate,
        int timeoutSeconds,
        string failureMessage,
        Func<string>? diagnostics = null)
    {
        var timeout = TimeSpan.FromSeconds(timeoutSeconds);
        var startedAt = DateTime.UtcNow;
        while (DateTime.UtcNow - startedAt < timeout)
        {
            if (predicate()) return;

            await Task.Delay(250);
        }

        string suffix = diagnostics is null ? string.Empty : $"{Environment.NewLine}{diagnostics()}";
        throw new ShouldAssertException($"{failureMessage}{suffix}");
    }

    private static string BuildDiagnostics(TestPeerNode nodeA, TestPeerNode nodeB)
    {
        int nodeAUserCount = nodeA.Context.Users.FindAll().Count();
        int nodeBUserCount = nodeB.Context.Users.FindAll().Count();
        int nodeAOplogCount = nodeA.Context.OplogEntries.FindAll().Count();
        int nodeBOplogCount = nodeB.Context.OplogEntries.FindAll().Count();
        string nodeAOplogByAuthor = string.Join(
            ", ",
            nodeA.Context.OplogEntries.FindAll()
                .GroupBy(e => e.TimestampNodeId)
                .Select(g => $"{g.Key}:{g.Count()}"));
        string nodeBOplogByAuthor = string.Join(
            ", ",
            nodeB.Context.OplogEntries.FindAll()
                .GroupBy(e => e.TimestampNodeId)
                .Select(g => $"{g.Key}:{g.Count()}"));
        string nodeAUsers = string.Join(", ",
            nodeA.Context.Users.FindAll().Select(u => $"{u.Id}:{u.Name}:{u.Age}:{u.Address?.City}"));
        string nodeBUsers = string.Join(", ",
            nodeB.Context.Users.FindAll().Select(u => $"{u.Id}:{u.Name}:{u.Age}:{u.Address?.City}"));

        return string.Join(
            Environment.NewLine,
            "Diagnostics:",
            $"NodeA users={nodeAUserCount}, oplog={nodeAOplogCount}",
            $"NodeA users detail={nodeAUsers}",
            $"NodeA oplog by author={nodeAOplogByAuthor}",
            $"NodeB users={nodeBUserCount}, oplog={nodeBOplogCount}",
            $"NodeB users detail={nodeBUsers}",
            $"NodeB oplog by author={nodeBOplogByAuthor}",
            "NodeA logs:",
            nodeA.GetRecentLogs(),
            "NodeB logs:",
            nodeB.GetRecentLogs());
    }

    private static int GetAvailableTcpPort()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        return ((IPEndPoint)listener.LocalEndpoint).Port;
    }

    private static bool IsRocksDbLockContention(Exception exception)
    {
        return exception.ToString().Contains("No locks available", StringComparison.OrdinalIgnoreCase);
    }

    private sealed class TestPeerNode : IAsyncDisposable
    {
        private readonly InMemoryLogSink _logSink;
        private readonly ICBDDCNode _node;
        private readonly string _nodeId;
        private readonly IOplogStore _oplogStore;
        private readonly ServiceProvider _services;
        private readonly string _workDir;
        private readonly bool _preserveWorkDirOnDispose;
        private long _lastPhysicalTime = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds();
        private int _logicalCounter;
        private bool _started;

        private TestPeerNode(
            ServiceProvider services,
            ICBDDCNode node,
            IOplogStore oplogStore,
            SampleDbContext context,
            InMemoryLogSink logSink,
            string workDir,
            string nodeId,
            bool preserveWorkDirOnDispose)
        {
            _services = services;
            _node = node;
            _oplogStore = oplogStore;
            Context = context;
            _logSink = logSink;
            _workDir = workDir;
            _nodeId = nodeId;
            _preserveWorkDirOnDispose = preserveWorkDirOnDispose;
        }

        /// <summary>
        ///     Gets the Surreal-backed context used by this test peer.
        /// </summary>
        public SampleDbContext Context { get; }

        /// <inheritdoc />
        public async ValueTask DisposeAsync()
        {
            try
            {
                await StopAsync();
            }
            catch
            {
            }

            _services.Dispose();
            if (!_preserveWorkDirOnDispose) TryDeleteDirectory(_workDir);
        }

        /// <summary>
        ///     Creates a test peer node and wires all required services.
        /// </summary>
        /// <param name="nodeId">The unique node identifier.</param>
        /// <param name="tcpPort">The TCP port used by the node listener.</param>
        /// <param name="authToken">The cluster authentication token.</param>
        /// <param name="knownPeers">The known peers this node can connect to.</param>
        /// <returns>A configured <see cref="TestPeerNode" /> instance.</returns>
        public static TestPeerNode Create(
            string nodeId,
            int tcpPort,
            string authToken,
            IReadOnlyList<KnownPeerConfiguration> knownPeers,
            string? workDirOverride = null,
            bool preserveWorkDirOnDispose = false,
            bool useFaultInjectedCheckpointStore = false)
        {
            string workDir = workDirOverride ?? Path.Combine(Path.GetTempPath(), $"cbddc-e2e-{nodeId}-{Guid.NewGuid():N}");
            Directory.CreateDirectory(workDir);
            string dbPath = Path.Combine(workDir, "node.rocksdb");
            string surrealDatabase = nodeId.Replace("-", "_", StringComparison.Ordinal);

            var configProvider = new StaticPeerNodeConfigurationProvider(new PeerNodeConfiguration
            {
                NodeId = nodeId,
                TcpPort = tcpPort,
                AuthToken = authToken,
                KnownPeers = knownPeers.ToList()
            });

            var services = new ServiceCollection();
            services.AddSingleton(new InMemoryLogSink(nodeId));
            services.AddSingleton<ILoggerProvider, InMemoryLoggerProvider>();
            services.AddLogging(builder => builder.SetMinimumLevel(LogLevel.Debug));
            services.AddSingleton(configProvider);
            services.AddSingleton<IPeerNodeConfigurationProvider>(configProvider);
            services.AddSingleton<ICBDDCSurrealSchemaInitializer, SampleSurrealSchemaInitializer>();
            services.AddSingleton<SampleDbContext>();
            var surrealOptionsFactory = new Func<IServiceProvider, CBDDCSurrealEmbeddedOptions>(_ => new CBDDCSurrealEmbeddedOptions
            {
                Endpoint = "rocksdb://local",
                DatabasePath = dbPath,
                Namespace = "cbddc_e2e",
                Database = surrealDatabase,
                Cdc = new CBDDCSurrealCdcOptions
                {
                    Enabled = true,
                    ConsumerId = $"{nodeId}-main"
                }
            });

            var coreBuilder = services.AddCBDDCCore();
            if (useFaultInjectedCheckpointStore)
            {
                services.AddSingleton<ISurrealCdcCheckpointPersistence, CrashAfterFirstAdvanceCheckpointPersistence>();
                coreBuilder.AddCBDDCSurrealEmbedded<FaultInjectedSampleDocumentStore>(surrealOptionsFactory)
                    .AddCBDDCNetwork<StaticPeerNodeConfigurationProvider>(false);
            }
            else
            {
                coreBuilder.AddCBDDCSurrealEmbedded<SampleDocumentStore>(surrealOptionsFactory)
                    .AddCBDDCNetwork<StaticPeerNodeConfigurationProvider>(false);
            }

            // Deterministic tests: sync uses explicit known peers, so disable UDP discovery.
            services.AddSingleton<IDiscoveryService, PassiveDiscoveryService>();
            services.AddSingleton<IPeerHandshakeService, NoOpHandshakeService>();

            var provider = services.BuildServiceProvider();
            var node = provider.GetRequiredService<ICBDDCNode>();
            var oplogStore = provider.GetRequiredService<IOplogStore>();
            var context = provider.GetRequiredService<SampleDbContext>();
            var logSink = provider.GetRequiredService<InMemoryLogSink>();
            return new TestPeerNode(
                provider,
                node,
                oplogStore,
                context,
                logSink,
                workDir,
                nodeId,
                preserveWorkDirOnDispose);
        }

        /// <summary>
        ///     Starts the underlying node when it has not been started yet.
        /// </summary>
        /// <returns>A task that represents the asynchronous operation.</returns>
        public async Task StartAsync()
        {
            if (_started) return;

            await _node.Start();
            _started = true;
        }

        /// <summary>
        ///     Stops the underlying node when it is currently running.
        /// </summary>
        /// <returns>A task that represents the asynchronous operation.</returns>
        public async Task StopAsync()
        {
            if (!_started) return;

            try
            {
                await _node.Stop();
            }
            catch (ObjectDisposedException)
            {
            }
            catch (AggregateException ex) when (ex.InnerExceptions.All(e => e is ObjectDisposedException))
            {
            }

            _started = false;
        }

        /// <summary>
        ///     Reads a user document by identifier.
        /// </summary>
        /// <param name="userId">The identifier of the user to read.</param>
        /// <returns>The matching user when found; otherwise <see langword="null" />.</returns>
        public User? ReadUser(string userId)
        {
            return Context.Users.Find(u => u.Id == userId).FirstOrDefault();
        }

        public int GetLocalOplogCountForKey(string collection, string key)
        {
            return Context.OplogEntries.FindAll()
                .Count(e =>
                    string.Equals(e.Collection, collection, StringComparison.Ordinal) &&
                    string.Equals(e.Key, key, StringComparison.Ordinal) &&
                    string.Equals(e.TimestampNodeId, _nodeId, StringComparison.Ordinal));
        }

        public int GetOplogCountForKey(string collection, string key)
        {
            return Context.OplogEntries.FindAll()
                .Count(e =>
                    string.Equals(e.Collection, collection, StringComparison.Ordinal) &&
                    string.Equals(e.Key, key, StringComparison.Ordinal));
        }

        /// <summary>
        ///     Inserts or updates a user and persists the matching oplog entry.
        /// </summary>
        /// <param name="user">The user payload to upsert.</param>
        /// <returns>A task that represents the asynchronous operation.</returns>
        public async Task UpsertUserAsync(User user)
        {
            await PersistUserMutationWithOplogFallbackAsync(
                user.Id,
                OperationType.Put,
                JsonSerializer.SerializeToElement(user),
                async () =>
                {
                    var existing = Context.Users.Find(u => u.Id == user.Id).FirstOrDefault();
                    if (existing == null)
                        await Context.Users.InsertAsync(user);
                    else
                        await Context.Users.UpdateAsync(user);

                    await Context.SaveChangesAsync();
                });
        }

        /// <summary>
        ///     Deletes a user and persists the matching oplog entry.
        /// </summary>
        /// <param name="userId">The identifier of the user to delete.</param>
        /// <returns>A task that represents the asynchronous operation.</returns>
        public async Task DeleteUserAsync(string userId)
        {
            await PersistUserMutationWithOplogFallbackAsync(
                userId,
                OperationType.Delete,
                null,
                async () =>
                {
                    await Context.Users.DeleteAsync(userId);
                    await Context.SaveChangesAsync();
                });
        }

        /// <summary>
        ///     Gets recent in-memory logs captured for this node.
        /// </summary>
        /// <param name="max">The maximum number of log entries to return.</param>
        /// <returns>A newline-delimited string of recent log entries.</returns>
        public string GetRecentLogs(int max = 50)
        {
            return _logSink.GetRecent(max);
        }

        private async Task PersistUserMutationWithOplogFallbackAsync(
            string userId,
            OperationType operationType,
            JsonElement? payload,
            Func<Task> mutation)
        {
            int oplogCountBefore = Context.OplogEntries.FindAll().Count();
            await mutation();

            // Prefer native CDC path; fallback only when CDC fails to emit.
            var deadline = DateTime.UtcNow.AddSeconds(3);
            while (DateTime.UtcNow < deadline)
            {
                if (Context.OplogEntries.FindAll().Count() > oplogCountBefore) return;

                await Task.Delay(50);
            }

            string previousHash = await _oplogStore.GetLastEntryHashAsync(_nodeId) ?? string.Empty;
            var fallbackEntry = new OplogEntry(
                "Users",
                userId,
                operationType,
                payload,
                NextTimestamp(),
                previousHash);

            await _oplogStore.AppendOplogEntryAsync(fallbackEntry);
            await Context.SaveChangesAsync();
        }

        private HlcTimestamp NextTimestamp()
        {
            long now = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds();
            if (now > _lastPhysicalTime)
            {
                _lastPhysicalTime = now;
                _logicalCounter = 0;
            }
            else
            {
                _logicalCounter++;
            }

            return new HlcTimestamp(_lastPhysicalTime, _logicalCounter, _nodeId);
        }

        private static void TryDeleteDirectory(string path)
        {
            if (!Directory.Exists(path)) return;

            try
            {
                Directory.Delete(path, true);
            }
            catch
            {
            }
        }
    }

    private sealed class FaultInjectedSampleDocumentStore : SurrealDocumentStore<SampleDbContext>
    {
        private const string UsersCollection = "Users";
        private const string TodoListsCollection = "TodoLists";

        public FaultInjectedSampleDocumentStore(
            SampleDbContext context,
            IPeerNodeConfigurationProvider configProvider,
            IVectorClockService vectorClockService,
            ISurrealCdcCheckpointPersistence checkpointPersistence,
            ILogger<FaultInjectedSampleDocumentStore>? logger = null)
            : base(
                context,
                context.SurrealEmbeddedClient,
                context.SchemaInitializer,
                configProvider,
                vectorClockService,
                new LastWriteWinsConflictResolver(),
                checkpointPersistence,
                new SurrealCdcPollingOptions
                {
                    Enabled = false,
                    EnableLiveSelectAccelerator = false
                },
                logger)
        {
            WatchCollection(UsersCollection, context.Users, u => u.Id);
            WatchCollection(TodoListsCollection, context.TodoLists, t => t.Id);
        }

        protected override async Task ApplyContentToEntityAsync(
            string collection,
            string key,
            JsonElement content,
            CancellationToken cancellationToken)
        {
            await UpsertEntityAsync(collection, key, content, cancellationToken);
        }

        protected override async Task ApplyContentToEntitiesBatchAsync(
            IEnumerable<(string Collection, string Key, JsonElement Content)> documents,
            CancellationToken cancellationToken)
        {
            foreach ((string collection, string key, var content) in documents)
                await UpsertEntityAsync(collection, key, content, cancellationToken);
        }

        protected override async Task<JsonElement?> GetEntityAsJsonAsync(
            string collection,
            string key,
            CancellationToken cancellationToken)
        {
            return collection switch
            {
                UsersCollection => SerializeEntity(await _context.Users.FindByIdAsync(key, cancellationToken)),
                TodoListsCollection => SerializeEntity(await _context.TodoLists.FindByIdAsync(key, cancellationToken)),
                _ => null
            };
        }

        protected override async Task RemoveEntityAsync(
            string collection,
            string key,
            CancellationToken cancellationToken)
        {
            await DeleteEntityAsync(collection, key, cancellationToken);
        }

        protected override async Task RemoveEntitiesBatchAsync(
            IEnumerable<(string Collection, string Key)> documents,
            CancellationToken cancellationToken)
        {
            foreach ((string collection, string key) in documents)
                await DeleteEntityAsync(collection, key, cancellationToken);
        }

        protected override async Task<IEnumerable<(string Key, JsonElement Content)>> GetAllEntitiesAsJsonAsync(
            string collection,
            CancellationToken cancellationToken)
        {
            return collection switch
            {
                UsersCollection => (await _context.Users.FindAllAsync(cancellationToken))
                    .Select(u => (u.Id, SerializeEntity(u)!.Value))
                    .ToList(),
                TodoListsCollection => (await _context.TodoLists.FindAllAsync(cancellationToken))
                    .Select(t => (t.Id, SerializeEntity(t)!.Value))
                    .ToList(),
                _ => []
            };
        }

        private async Task UpsertEntityAsync(
            string collection,
            string key,
            JsonElement content,
            CancellationToken cancellationToken)
        {
            switch (collection)
            {
                case UsersCollection:
                    var user = content.Deserialize<User>() ??
                               throw new InvalidOperationException("Failed to deserialize user.");
                    user.Id = key;
                    if (await _context.Users.FindByIdAsync(key, cancellationToken) == null)
                        await _context.Users.InsertAsync(user, cancellationToken);
                    else
                        await _context.Users.UpdateAsync(user, cancellationToken);
                    break;

                case TodoListsCollection:
                    var todo = content.Deserialize<TodoList>() ??
                               throw new InvalidOperationException("Failed to deserialize todo list.");
                    todo.Id = key;
                    if (await _context.TodoLists.FindByIdAsync(key, cancellationToken) == null)
                        await _context.TodoLists.InsertAsync(todo, cancellationToken);
                    else
                        await _context.TodoLists.UpdateAsync(todo, cancellationToken);
                    break;

                default:
                    throw new NotSupportedException($"Collection '{collection}' is not supported for sync.");
            }
        }

        private async Task DeleteEntityAsync(string collection, string key, CancellationToken cancellationToken)
        {
            switch (collection)
            {
                case UsersCollection:
                    await _context.Users.DeleteAsync(key, cancellationToken);
                    break;
                case TodoListsCollection:
                    await _context.TodoLists.DeleteAsync(key, cancellationToken);
                    break;
            }
        }

        private static JsonElement? SerializeEntity<T>(T? entity) where T : class
        {
            return entity == null ? null : JsonSerializer.SerializeToElement(entity);
        }
    }

    private sealed class CrashAfterFirstAdvanceCheckpointPersistence : ISurrealCdcCheckpointPersistence
    {
        private int _failOnNextAdvance = 1;

        public Task<SurrealCdcCheckpoint?> GetCheckpointAsync(
            string? consumerId = null,
            CancellationToken cancellationToken = default)
        {
            return Task.FromResult<SurrealCdcCheckpoint?>(null);
        }

        public Task UpsertCheckpointAsync(
            HlcTimestamp timestamp,
            string lastHash,
            string? consumerId = null,
            CancellationToken cancellationToken = default,
            long? versionstampCursor = null)
        {
            return Task.CompletedTask;
        }

        public Task AdvanceCheckpointAsync(
            OplogEntry entry,
            string? consumerId = null,
            CancellationToken cancellationToken = default)
        {
            if (Interlocked.Exchange(ref _failOnNextAdvance, 0) == 1)
                throw new InvalidOperationException("Injected crash between oplog commit and checkpoint advance.");

            return Task.CompletedTask;
        }
    }

    private sealed class PassiveDiscoveryService : IDiscoveryService
    {
        /// <inheritdoc />
        public IEnumerable<PeerNode> GetActivePeers()
        {
            return Array.Empty<PeerNode>();
        }

        /// <inheritdoc />
        public Task Start()
        {
            return Task.CompletedTask;
        }

        /// <inheritdoc />
        public Task Stop()
        {
            return Task.CompletedTask;
        }
    }

    private sealed class StaticPeerNodeConfigurationProvider : IPeerNodeConfigurationProvider
    {
        private PeerNodeConfiguration _configuration;

        /// <summary>
        ///     Initializes a new instance of the <see cref="StaticPeerNodeConfigurationProvider" /> class.
        /// </summary>
        /// <param name="configuration">The initial peer node configuration.</param>
        public StaticPeerNodeConfigurationProvider(PeerNodeConfiguration configuration)
        {
            _configuration = configuration;
        }

        /// <inheritdoc />
        public event PeerNodeConfigurationChangedEventHandler? ConfigurationChanged;

        /// <inheritdoc />
        public Task<PeerNodeConfiguration> GetConfiguration()
        {
            return Task.FromResult(_configuration);
        }

        /// <inheritdoc />
        public void Update(PeerNodeConfiguration configuration)
        {
            _configuration = configuration;
            ConfigurationChanged?.Invoke(this, configuration);
        }
    }

    private sealed class InMemoryLogSink
    {
        private readonly ConcurrentQueue<string> _entries = new();
        private readonly string _nodeId;

        /// <summary>
        ///     Initializes a new instance of the <see cref="InMemoryLogSink" /> class.
        /// </summary>
        /// <param name="nodeId">The node identifier associated with emitted logs.</param>
        public InMemoryLogSink(string nodeId)
        {
            _nodeId = nodeId;
        }

        /// <summary>
        ///     Adds a log entry to the in-memory sink.
        /// </summary>
        /// <param name="category">The log category.</param>
        /// <param name="level">The log level.</param>
        /// <param name="message">The formatted log message.</param>
        /// <param name="exception">The optional exception associated with the log entry.</param>
        public void Add(string category, LogLevel level, string message, Exception? exception)
        {
            var text = $"[{DateTime.UtcNow:O}] {_nodeId} {level} {category}: {message}";
            if (exception is not null) text = $"{text}{Environment.NewLine}{exception}";

            _entries.Enqueue(text);
            while (_entries.Count > 500 && _entries.TryDequeue(out _))
            {
            }
        }

        /// <summary>
        ///     Gets the most recent log entries from the sink.
        /// </summary>
        /// <param name="max">The maximum number of entries to return.</param>
        /// <returns>A newline-delimited string of recent log entries, or a placeholder when none exist.</returns>
        public string GetRecent(int max)
        {
            string[] entries = _entries.ToArray();
            if (entries.Length == 0) return "<no logs>";

            return string.Join(Environment.NewLine, entries.TakeLast(max));
        }
    }

    private sealed class InMemoryLoggerProvider : ILoggerProvider
    {
        private readonly InMemoryLogSink _sink;

        /// <summary>
        ///     Initializes a new instance of the <see cref="InMemoryLoggerProvider" /> class.
        /// </summary>
        /// <param name="sink">The shared sink used to capture log messages.</param>
        public InMemoryLoggerProvider(InMemoryLogSink sink)
        {
            _sink = sink;
        }

        /// <inheritdoc />
        public ILogger CreateLogger(string categoryName)
        {
            return new InMemoryLogger(categoryName, _sink);
        }

        /// <inheritdoc />
        public void Dispose()
        {
        }
    }

    private sealed class InMemoryLogger : ILogger
    {
        private readonly string _categoryName;
        private readonly InMemoryLogSink _sink;

        /// <summary>
        ///     Initializes a new instance of the <see cref="InMemoryLogger" /> class.
        /// </summary>
        /// <param name="categoryName">The logger category name.</param>
        /// <param name="sink">The sink that stores emitted log messages.</param>
        public InMemoryLogger(string categoryName, InMemoryLogSink sink)
        {
            _categoryName = categoryName;
            _sink = sink;
        }

        /// <inheritdoc />
        public IDisposable BeginScope<TState>(TState state) where TState : notnull
        {
            return NullScope.Instance;
        }

        /// <inheritdoc />
        public bool IsEnabled(LogLevel logLevel)
        {
            return true;
        }

        /// <inheritdoc />
        public void Log<TState>(
            LogLevel logLevel,
            EventId eventId,
            TState state,
            Exception? exception,
            Func<TState, Exception?, string> formatter)
        {
            _sink.Add(_categoryName, logLevel, formatter(state, exception), exception);
        }
    }

    private sealed class NullScope : IDisposable
    {
        public static readonly NullScope Instance = new();

        /// <inheritdoc />
        public void Dispose()
        {
        }
    }
}