using System.Threading.Channels;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Options;

namespace StellaOps.Provcache;

/// <summary>
/// Background service that manages write-behind persistence for Provcache entries.
/// Batches writes to Postgres for efficiency and provides retry logic for transient failures.
/// </summary>
public sealed class WriteBehindQueue : BackgroundService, IWriteBehindQueue
{
    private readonly Channel<WriteBehindItem> _channel;
    private readonly IProvcacheRepository _repository;
    private readonly ProvcacheOptions _options;
    private readonly ILogger<WriteBehindQueue> _logger;
    private readonly TimeProvider _timeProvider;

    // Metrics
    private long _totalEnqueued;
    private long _totalPersisted;
    private long _totalFailed;
    private long _totalRetries;
    private long _totalBatches;
    private long _currentQueueDepth;

    public WriteBehindQueue(
        IProvcacheRepository repository,
        IOptions<ProvcacheOptions> options,
        ILogger<WriteBehindQueue> logger,
        TimeProvider? timeProvider = null)
    {
        _repository = repository ?? throw new ArgumentNullException(nameof(repository));
        _options = options?.Value ?? throw new ArgumentNullException(nameof(options));
        _logger = logger ?? throw new ArgumentNullException(nameof(logger));
        _timeProvider = timeProvider ?? TimeProvider.System;

        // Bounded channel to provide backpressure
        _channel = Channel.CreateBounded<WriteBehindItem>(new BoundedChannelOptions(_options.WriteBehindQueueCapacity)
        {
            FullMode = BoundedChannelFullMode.Wait,
            SingleWriter = false,
            SingleReader = true
        });
    }

    /// <inheritdoc />
    public ValueTask EnqueueAsync(ProvcacheEntry entry, CancellationToken cancellationToken = default)
    {
        ArgumentNullException.ThrowIfNull(entry);

        var item = new WriteBehindItem
        {
            Entry = entry,
            EnqueuedAt = _timeProvider.GetUtcNow(),
            RetryCount = 0
        };

        Interlocked.Increment(ref _totalEnqueued);
        Interlocked.Increment(ref _currentQueueDepth);
        ProvcacheTelemetry.SetWriteBehindQueueSize((int)Interlocked.Read(ref _currentQueueDepth));

        return _channel.Writer.WriteAsync(item, cancellationToken);
    }

    /// <inheritdoc />
    public WriteBehindMetrics GetMetrics()
    {
        return new WriteBehindMetrics
        {
            TotalEnqueued = Interlocked.Read(ref _totalEnqueued),
            TotalPersisted = Interlocked.Read(ref _totalPersisted),
            TotalFailed = Interlocked.Read(ref _totalFailed),
            TotalRetries = Interlocked.Read(ref _totalRetries),
            TotalBatches = Interlocked.Read(ref _totalBatches),
            CurrentQueueDepth = Interlocked.Read(ref _currentQueueDepth),
            Timestamp = _timeProvider.GetUtcNow()
        };
    }

    /// <summary>
    /// Main processing loop that reads from the channel and batches writes.
    /// </summary>
    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        _logger.LogInformation(
            "Write-behind queue started with batch size {BatchSize}, interval {IntervalMs}ms",
            _options.WriteBehindBatchSize,
            _options.WriteBehindFlushIntervalMs);

        var batch = new List<WriteBehindItem>(_options.WriteBehindBatchSize);
        var flushInterval = TimeSpan.FromMilliseconds(_options.WriteBehindFlushIntervalMs);

        while (!stoppingToken.IsCancellationRequested)
        {
            try
            {
                batch.Clear();

                // Read items until batch is full or timeout
                using var cts = CancellationTokenSource.CreateLinkedTokenSource(stoppingToken);
                cts.CancelAfter(flushInterval);

                try
                {
                    while (batch.Count < _options.WriteBehindBatchSize)
                    {
                        var item = await _channel.Reader.ReadAsync(cts.Token).ConfigureAwait(false);
                        batch.Add(item);
                        Interlocked.Decrement(ref _currentQueueDepth);
                    }
                }
                catch (OperationCanceledException) when (!stoppingToken.IsCancellationRequested)
                {
                    // Timeout reached, process current batch
                }

                if (batch.Count > 0)
                {
                    await ProcessBatchAsync(batch, stoppingToken).ConfigureAwait(false);
                }
            }
            catch (OperationCanceledException) when (stoppingToken.IsCancellationRequested)
            {
                break;
            }
            catch (Exception ex)
            {
                _logger.LogError(ex, "Error in write-behind queue processing loop");
                await Task.Delay(1000, stoppingToken).ConfigureAwait(false); // Backoff on error
            }
        }

        // Drain remaining items on shutdown
        await DrainAsync(CancellationToken.None).ConfigureAwait(false);

        _logger.LogInformation("Write-behind queue stopped");
    }

    /// <summary>
    /// Processes a batch of items with retry logic.
    /// </summary>
    private async Task ProcessBatchAsync(List<WriteBehindItem> batch, CancellationToken cancellationToken)
    {
        var entries = batch.Select(b => b.Entry).ToList();
        using var activity = ProvcacheTelemetry.StartWriteBehindFlushActivity(batch.Count);

        try
        {
            await _repository.UpsertManyAsync(entries, cancellationToken).ConfigureAwait(false);

            Interlocked.Add(ref _totalPersisted, batch.Count);
            Interlocked.Increment(ref _totalBatches);
            ProvcacheTelemetry.RecordWriteBehind("ok", batch.Count);
            ProvcacheTelemetry.SetWriteBehindQueueSize((int)Interlocked.Read(ref _currentQueueDepth));

            _logger.LogDebug(
                "Write-behind batch persisted {Count} entries",
                batch.Count);
        }
        catch (Exception ex)
        {
            ProvcacheTelemetry.MarkError(activity, ex.Message);
            _logger.LogWarning(
                ex,
                "Write-behind batch failed for {Count} entries, scheduling retries",
                batch.Count);

            // Re-enqueue failed items for retry
            foreach (var item in batch)
            {
                if (item.RetryCount < _options.WriteBehindMaxRetries)
                {
                    var retryItem = item with { RetryCount = item.RetryCount + 1 };
                    Interlocked.Increment(ref _totalRetries);
                    ProvcacheTelemetry.RecordWriteBehind("retry", 1);

                    if (_channel.Writer.TryWrite(retryItem))
                    {
                        Interlocked.Increment(ref _currentQueueDepth);
                        ProvcacheTelemetry.SetWriteBehindQueueSize((int)Interlocked.Read(ref _currentQueueDepth));
                    }
                    else
                    {
                        Interlocked.Increment(ref _totalFailed);
                        ProvcacheTelemetry.RecordWriteBehind("failed", 1);
                        _logger.LogError(
                            "Write-behind queue full, dropping entry for VeriKey {VeriKey}",
                            item.Entry.VeriKey);
                    }
                }
                else
                {
                    Interlocked.Increment(ref _totalFailed);
                    ProvcacheTelemetry.RecordWriteBehind("failed", 1);
                    _logger.LogError(
                        "Write-behind max retries exceeded for VeriKey {VeriKey}",
                        item.Entry.VeriKey);
                }
            }
        }
    }

    /// <summary>
    /// Drains remaining items from the queue during shutdown.
    /// </summary>
    private async Task DrainAsync(CancellationToken cancellationToken)
    {
        var batch = new List<WriteBehindItem>();

        while (_channel.Reader.TryRead(out var item))
        {
            batch.Add(item);
            Interlocked.Decrement(ref _currentQueueDepth);

            if (batch.Count >= _options.WriteBehindBatchSize)
            {
                await ProcessBatchAsync(batch, cancellationToken).ConfigureAwait(false);
                batch.Clear();
            }
        }

        if (batch.Count > 0)
        {
            await ProcessBatchAsync(batch, cancellationToken).ConfigureAwait(false);
        }

        _logger.LogInformation("Write-behind queue drained");
    }
}

/// <summary>
/// Interface for write-behind queue operations.
/// </summary>
public interface IWriteBehindQueue
{
    /// <summary>
    /// Enqueues an entry for asynchronous persistence.
    /// </summary>
    ValueTask EnqueueAsync(ProvcacheEntry entry, CancellationToken cancellationToken = default);

    /// <summary>
    /// Gets current queue metrics.
    /// </summary>
    WriteBehindMetrics GetMetrics();
}

/// <summary>
/// Item in the write-behind queue with retry metadata.
/// </summary>
internal sealed record WriteBehindItem
{
    public required ProvcacheEntry Entry { get; init; }
    public required DateTimeOffset EnqueuedAt { get; init; }
    public required int RetryCount { get; init; }
}

/// <summary>
/// Metrics for the write-behind queue.
/// </summary>
public sealed record WriteBehindMetrics
{
    public required long TotalEnqueued { get; init; }
    public required long TotalPersisted { get; init; }
    public required long TotalFailed { get; init; }
    public required long TotalRetries { get; init; }
    public required long TotalBatches { get; init; }
    public required long CurrentQueueDepth { get; init; }
    public required DateTimeOffset Timestamp { get; init; }
}