using Microsoft.Extensions.Logging;
using ScadaLink.Commons.Types.Enums;

namespace ScadaLink.StoreAndForward;

/// <summary>
/// WP-9/10: Core store-and-forward service.
///
/// Lifecycle:
///   1. Caller attempts immediate delivery via IDeliveryHandler
///   2. On transient failure → buffer in SQLite → retry loop
///   3. On success → remove from buffer
///   4. On reaching MaxRetries → park (a MaxRetries of 0 means "no limit" — the
///      message is retried until delivered and is never parked for retry exhaustion)
///   5. Permanent failures are returned to caller immediately (never buffered)
///
/// WP-10: Fixed retry interval (not exponential). Per-source-entity retry settings.
///        Background timer-based retry sweep.
///
/// WP-12: Parked messages queryable, retryable, and discardable.
///
/// WP-14: Buffer depth reported as health metric. Activity logged to site event log.
///
/// WP-15: CachedCall idempotency is the caller's responsibility.
///        This service does not deduplicate — if the same message is enqueued twice,
///        it will be delivered twice. Callers using ExternalSystem.CachedCall() must
///        design their payloads to be idempotent (e.g., include unique request IDs
///        and handle duplicate detection on the remote end).
/// </summary>
public class StoreAndForwardService
{
    private readonly StoreAndForwardStorage _storage;
    private readonly StoreAndForwardOptions _options;
    private readonly ReplicationService? _replication;
    private readonly ILogger<StoreAndForwardService> _logger;
    private Timer? _retryTimer;
    private int _retryInProgress;

    /// <summary>
    /// WP-10: Delivery handler delegate. The return value / exception is interpreted
    /// the same way on both the immediate-delivery path (<see cref="EnqueueAsync"/>)
    /// and the background retry path (<c>RetryMessageAsync</c>):
    /// <list type="bullet">
    ///   <item><description><c>true</c> — delivered successfully. The message is
    ///   removed from the buffer (or, on the immediate path, never buffered).</description></item>
    ///   <item><description><c>false</c> — permanent failure. On the immediate path
    ///   the message is NOT buffered; on a retry the message is already buffered and
    ///   is parked immediately (no further retries).</description></item>
    ///   <item><description>throws — transient failure. On the immediate path the
    ///   message is buffered for retry; on a retry the retry count is incremented and
    ///   the message is parked once <see cref="StoreAndForwardMessage.MaxRetries"/> is
    ///   reached.</description></item>
    /// </list>
    /// </summary>
    private readonly Dictionary<StoreAndForwardCategory, Func<StoreAndForwardMessage, Task<bool>>> _deliveryHandlers = new();

    /// <summary>
    /// WP-14: Event callback for logging S&amp;F activity to site event log.
    /// </summary>
    public event Action<string, StoreAndForwardCategory, string>? OnActivity;

    public StoreAndForwardService(
        StoreAndForwardStorage storage,
        StoreAndForwardOptions options,
        ILogger<StoreAndForwardService> logger,
        ReplicationService? replication = null)
    {
        _storage = storage;
        _options = options;
        _logger = logger;
        _replication = replication;
    }

    /// <summary>
    /// Registers a delivery handler for a given message category. See the
    /// <c>_deliveryHandlers</c> field documentation for the true/false/throws contract,
    /// which applies identically on the immediate and retry paths.
    /// </summary>
    public void RegisterDeliveryHandler(
        StoreAndForwardCategory category,
        Func<StoreAndForwardMessage, Task<bool>> handler)
    {
        _deliveryHandlers[category] = handler;
    }

    /// <summary>
    /// Initializes storage and starts the background retry timer.
    /// </summary>
    public async Task StartAsync()
    {
        await _storage.InitializeAsync();
        _retryTimer = new Timer(
            _ => _ = RetryPendingMessagesAsync(),
            null,
            _options.RetryTimerInterval,
            _options.RetryTimerInterval);

        _logger.LogInformation(
            "Store-and-forward service started. Retry interval: {Interval}s",
            _options.DefaultRetryInterval.TotalSeconds);
    }

    /// <summary>
    /// Stops the background retry timer.
    /// </summary>
    public async Task StopAsync()
    {
        if (_retryTimer != null)
        {
            await _retryTimer.DisposeAsync();
            _retryTimer = null;
        }
    }

    /// <summary>
    /// WP-10: Enqueues a message for store-and-forward delivery.
    /// Attempts immediate delivery first. On transient failure, buffers for retry.
    /// On permanent failure (handler returns false), returns false immediately.
    ///
    /// WP-10: Retry-count lifecycle — the immediate (or caller-made) delivery attempt
    /// is attempt 0 and is not counted; the background retry sweep increments
    /// <see cref="StoreAndForwardMessage.RetryCount"/> on each retry. A buffered
    /// message is parked once <c>RetryCount</c> reaches <paramref name="maxRetries"/>
    /// — <b>but only when <paramref name="maxRetries"/> is greater than 0</b>. A
    /// <paramref name="maxRetries"/> of <c>0</c> means <b>no limit</b>: the message is
    /// retried on every sweep until it is delivered and is <b>never parked</b> on a
    /// retry-count basis. It is therefore <i>not</i> a "do not retry" value — callers
    /// that want delivery abandoned after a bounded number of attempts must pass a
    /// positive <paramref name="maxRetries"/>.
    ///
    /// WP-15: CachedCall idempotency note — this method does not deduplicate.
    /// The caller (e.g., ExternalSystem.CachedCall()) is responsible for ensuring
    /// that the remote system can handle duplicate deliveries safely.
    /// </summary>
    /// <param name="category">Message category (selects the delivery handler).</param>
    /// <param name="target">Target system name (external system / notification list / DB connection).</param>
    /// <param name="payloadJson">JSON-serialized call payload, treated opaquely.</param>
    /// <param name="originInstanceName">Instance that originated the message (WP-13: survives instance deletion).</param>
    /// <param name="maxRetries">
    /// Maximum background retry-sweep attempts before the message is parked.
    /// <b><c>0</c> = no limit</b> — the message is retried on every sweep until
    /// delivered and is never parked for exhausting retries; it is <b>not</b> a
    /// "never retry" value. <c>null</c> uses <see cref="StoreAndForwardOptions.DefaultMaxRetries"/>.
    /// Must be positive to bound delivery attempts. Mirrors the
    /// <see cref="StoreAndForwardMessage.MaxRetries"/> contract.
    /// </param>
    /// <param name="retryInterval">Fixed interval between retry sweeps for this message; <c>null</c> uses the configured default.</param>
    /// <param name="attemptImmediateDelivery">
    /// When <c>false</c>, the caller has already made its own delivery attempt and the
    /// message is buffered directly for the retry sweep (the handler is not invoked here).
    /// </param>
    public async Task<StoreAndForwardResult> EnqueueAsync(
        StoreAndForwardCategory category,
        string target,
        string payloadJson,
        string? originInstanceName = null,
        int? maxRetries = null,
        TimeSpan? retryInterval = null,
        bool attemptImmediateDelivery = true)
    {
        var message = new StoreAndForwardMessage
        {
            Id = Guid.NewGuid().ToString("N"),
            Category = category,
            Target = target,
            PayloadJson = payloadJson,
            RetryCount = 0,
            MaxRetries = maxRetries ?? _options.DefaultMaxRetries,
            RetryIntervalMs = (long)(retryInterval ?? _options.DefaultRetryInterval).TotalMilliseconds,
            CreatedAt = DateTimeOffset.UtcNow,
            Status = StoreAndForwardMessageStatus.Pending,
            OriginInstanceName = originInstanceName
        };

        // Attempt immediate delivery — unless the caller has already made a
        // delivery attempt of its own (attemptImmediateDelivery: false). In that
        // case re-invoking the handler here would dispatch the request twice.
        if (attemptImmediateDelivery && _deliveryHandlers.TryGetValue(category, out var handler))
        {
            try
            {
                var success = await handler(message);
                if (success)
                {
                    RaiseActivity("Delivered", category, $"Immediate delivery to {target}");
                    return new StoreAndForwardResult(true, message.Id, false);
                }

                // Permanent failure — do not buffer
                return new StoreAndForwardResult(false, message.Id, false);
            }
            catch (Exception ex)
            {
                // Transient failure — buffer for retry. The immediate attempt is
                // attempt 0; RetryCount tracks only sweep retries, so it stays 0
                // here (StoreAndForward-003).
                _logger.LogWarning(ex,
                    "Immediate delivery to {Target} failed (transient), buffering for retry",
                    target);

                message.LastAttemptAt = DateTimeOffset.UtcNow;
                message.LastError = ex.Message;
                await BufferAsync(message);

                RaiseActivity("Queued", category, $"Buffered for retry: {target} ({ex.Message})");
                return new StoreAndForwardResult(true, message.Id, true);
            }
        }

        // Either no handler is registered yet, or the caller already attempted
        // delivery itself — buffer for the background retry sweep to deliver.
        // The initial attempt (caller-made, or skipped because no handler is
        // registered) is attempt 0; RetryCount tracks only sweep retries and
        // therefore stays 0 here (StoreAndForward-003).
        if (!attemptImmediateDelivery)
        {
            message.LastAttemptAt = DateTimeOffset.UtcNow;
        }
        await BufferAsync(message);
        RaiseActivity("Queued", category, attemptImmediateDelivery
            ? $"No handler registered, buffered: {target}"
            : $"Buffered for retry: {target}");
        return new StoreAndForwardResult(true, message.Id, true);
    }

    /// <summary>
    /// Persists a message to the local SQLite buffer and (WP-11) replicates the
    /// add to the standby node so a failover does not lose the buffered message.
    /// </summary>
    private async Task BufferAsync(StoreAndForwardMessage message)
    {
        await _storage.EnqueueAsync(message);
        _replication?.ReplicateEnqueue(message);
    }

    /// <summary>
    /// WP-10: Background retry sweep. Processes all pending messages that are due for retry.
    /// </summary>
    internal async Task RetryPendingMessagesAsync()
    {
        // Prevent overlapping retry sweeps
        if (Interlocked.CompareExchange(ref _retryInProgress, 1, 0) != 0)
            return;

        try
        {
            var messages = await _storage.GetMessagesForRetryAsync();
            if (messages.Count == 0) return;

            _logger.LogDebug("Retry sweep: {Count} messages due for retry", messages.Count);

            foreach (var message in messages)
            {
                await RetryMessageAsync(message);
            }
        }
        catch (Exception ex)
        {
            _logger.LogError(ex, "Error during retry sweep");
        }
        finally
        {
            Interlocked.Exchange(ref _retryInProgress, 0);
        }
    }

    private async Task RetryMessageAsync(StoreAndForwardMessage message)
    {
        if (!_deliveryHandlers.TryGetValue(message.Category, out var handler))
        {
            _logger.LogWarning("No delivery handler for category {Category}", message.Category);
            return;
        }

        try
        {
            var success = await handler(message);
            if (success)
            {
                await _storage.RemoveMessageAsync(message.Id);
                _replication?.ReplicateRemove(message.Id);
                RaiseActivity("Delivered", message.Category,
                    $"Delivered to {message.Target} after {message.RetryCount} retries");
                return;
            }

            // Permanent failure on retry — park immediately.
            // StoreAndForward-005: the sweep observed this row as Pending; only commit
            // the park if it is still Pending so a concurrent operator action that
            // moved it (retry/discard) is not silently overwritten.
            message.Status = StoreAndForwardMessageStatus.Parked;
            message.LastAttemptAt = DateTimeOffset.UtcNow;
            message.LastError = "Permanent failure (handler returned false)";
            var parked = await _storage.UpdateMessageIfStatusAsync(
                message, StoreAndForwardMessageStatus.Pending);
            if (!parked)
            {
                _logger.LogDebug(
                    "Message {MessageId} changed status during delivery; sweep park skipped",
                    message.Id);
                return;
            }
            _replication?.ReplicatePark(message);
            RaiseActivity("Parked", message.Category,
                $"Permanent failure for {message.Target}: handler returned false");
        }
        catch (Exception ex)
        {
            // Transient failure — increment retry, check max
            message.RetryCount++;
            message.LastAttemptAt = DateTimeOffset.UtcNow;
            message.LastError = ex.Message;

            if (message.MaxRetries > 0 && message.RetryCount >= message.MaxRetries)
            {
                // StoreAndForward-005: conditional park — see the permanent-failure
                // branch above for rationale.
                message.Status = StoreAndForwardMessageStatus.Parked;
                var parked = await _storage.UpdateMessageIfStatusAsync(
                    message, StoreAndForwardMessageStatus.Pending);
                if (!parked)
                {
                    _logger.LogDebug(
                        "Message {MessageId} changed status during delivery; sweep park skipped",
                        message.Id);
                    return;
                }
                _replication?.ReplicatePark(message);
                RaiseActivity("Parked", message.Category,
                    $"Max retries ({message.MaxRetries}) reached for {message.Target}");
                _logger.LogWarning(
                    "Message {MessageId} parked after {MaxRetries} retries to {Target}",
                    message.Id, message.MaxRetries, message.Target);
            }
            else
            {
                // StoreAndForward-005: the retry-count increment is also conditional
                // on the row still being Pending so it cannot clobber an operator
                // action that ran during the failed delivery.
                if (!await _storage.UpdateMessageIfStatusAsync(
                        message, StoreAndForwardMessageStatus.Pending))
                {
                    _logger.LogDebug(
                        "Message {MessageId} changed status during delivery; sweep retry-count update skipped",
                        message.Id);
                    return;
                }
                RaiseActivity("Retried", message.Category,
                    $"Retry {message.RetryCount}/{message.MaxRetries} for {message.Target}: {ex.Message}");
            }
        }
    }

    /// <summary>
    /// WP-12: Gets parked messages for central query (Pattern 8).
    /// </summary>
    public async Task<(List<StoreAndForwardMessage> Messages, int TotalCount)> GetParkedMessagesAsync(
        StoreAndForwardCategory? category = null,
        int pageNumber = 1,
        int pageSize = 50)
    {
        return await _storage.GetParkedMessagesAsync(category, pageNumber, pageSize);
    }

    /// <summary>
    /// WP-12: Retries a parked message (moves back to pending queue).
    ///
    /// StoreAndForward-016: an operator requeue is a buffer state change and is
    /// replicated to the standby (as a <see cref="ReplicationOperationType.Requeue"/>)
    /// so a failover preserves the operator's retry intent.
    /// StoreAndForward-017: the activity-log entry carries the message's true
    /// category rather than a hard-coded one.
    /// </summary>
    public async Task<bool> RetryParkedMessageAsync(string messageId)
    {
        var success = await _storage.RetryParkedMessageAsync(messageId);
        if (success)
        {
            // Re-load the requeued row so the activity log gets the real category
            // and the standby gets the post-requeue state (Pending, retry_count = 0).
            var message = await _storage.GetMessageByIdAsync(messageId);
            var category = message?.Category ?? StoreAndForwardCategory.ExternalSystem;
            if (message != null)
            {
                _replication?.ReplicateRequeue(message);
            }
            RaiseActivity("Retry", category,
                $"Parked message {messageId} moved back to queue");
        }
        return success;
    }

    /// <summary>
    /// WP-12: Permanently discards a parked message.
    ///
    /// StoreAndForward-016: an operator discard is a buffer removal and is replicated
    /// to the standby (as a <see cref="ReplicationOperationType.Remove"/>) so the
    /// discarded message does not reappear after a failover.
    /// StoreAndForward-017: the activity-log entry carries the message's true
    /// category rather than a hard-coded one.
    /// </summary>
    public async Task<bool> DiscardParkedMessageAsync(string messageId)
    {
        // Capture the category before the row is deleted so the activity log is
        // labelled correctly.
        var message = await _storage.GetMessageByIdAsync(messageId);
        var success = await _storage.DiscardParkedMessageAsync(messageId);
        if (success)
        {
            _replication?.ReplicateRemove(messageId);
            RaiseActivity("Discard", message?.Category ?? StoreAndForwardCategory.ExternalSystem,
                $"Parked message {messageId} discarded");
        }
        return success;
    }

    /// <summary>
    /// WP-14: Gets buffer depth by category for health reporting.
    /// </summary>
    public async Task<Dictionary<StoreAndForwardCategory, int>> GetBufferDepthAsync()
    {
        return await _storage.GetBufferDepthByCategoryAsync();
    }

    /// <summary>
    /// WP-13: Gets count of S&amp;F messages for a given instance (for verifying survival on deletion).
    /// </summary>
    public async Task<int> GetMessageCountForInstanceAsync(string instanceName)
    {
        return await _storage.GetMessageCountByOriginInstanceAsync(instanceName);
    }

    /// <summary>
    /// WP-14: Raises the S&amp;F activity notification. StoreAndForward-009: the
    /// delegate is snapshotted (so a concurrent unsubscribe cannot NRE) and every
    /// subscriber invocation is wrapped so a slow/throwing subscriber (e.g. the site
    /// event log) cannot abort the caller. Crucially, a subscriber exception raised
    /// from <see cref="EnqueueAsync"/> or <c>RetryMessageAsync</c> must NOT be
    /// misclassified as a transient delivery failure — pre-fix it escaped into the
    /// delivery try/catch and caused a successfully delivered message to be buffered
    /// (or its retry count to be bumped). Activity logging is best-effort.
    /// </summary>
    private void RaiseActivity(string action, StoreAndForwardCategory category, string detail)
    {
        var handlers = OnActivity;
        if (handlers == null) return;

        foreach (var handler in handlers.GetInvocationList().Cast<Action<string, StoreAndForwardCategory, string>>())
        {
            try
            {
                handler(action, category, detail);
            }
            catch (Exception ex)
            {
                _logger.LogWarning(ex,
                    "Store-and-forward activity subscriber threw for action {Action}; ignored",
                    action);
            }
        }
    }
}

/// <summary>
/// Result of an enqueue operation.
/// </summary>
public record StoreAndForwardResult(
    /// <summary>True if the message was accepted (either delivered immediately or buffered).</summary>
    bool Accepted,
    /// <summary>Unique message ID for tracking.</summary>
    string MessageId,
    /// <summary>True if the message was buffered (not delivered immediately).</summary>
    bool WasBuffered);