using System.Diagnostics;
using Elsa.Diagnostics.StructuredLogs.Contracts;
using Elsa.Diagnostics.StructuredLogs.Models;
using Elsa.Diagnostics.StructuredLogs.Persistence.Relational.Contracts;
using Elsa.Diagnostics.StructuredLogs.Persistence.Relational.Options;
using Elsa.Diagnostics.StructuredLogs.Persistence.Relational.Stores;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Options;

namespace Elsa.Diagnostics.StructuredLogs.Persistence.Relational.Services;

public class StructuredLogWriteBuffer(
    RelationalStructuredLogStore store,
    IOptions<RelationalStructuredLogOptions> options) : IStructuredLogStore, IStructuredLogWriteBuffer, IHostedService, IAsyncDisposable
{
    private readonly object _lifecycleLock = new();
    private readonly Queue<StructuredLogEvent> _queue = new();
    private readonly SemaphoreSlim _signal = new(0);
    private CancellationTokenSource _stopTokenSource = new();
    private Task? _backgroundTask;
    private long _droppedWriteCount;
    private int _activeStartCount;
    private int _disposed;

    public long DroppedWriteCount => Interlocked.Read(ref _droppedWriteCount);

    public ValueTask WriteAsync(StructuredLogEvent logEvent, CancellationToken cancellationToken = default)
    {
        var shouldSignal = false;

        lock (_queue)
        {
            if (_queue.Count >= Math.Max(1, options.Value.WriteQueue.Capacity))
            {
                Interlocked.Increment(ref _droppedWriteCount);
                return ValueTask.CompletedTask;
            }

            shouldSignal = _queue.Count == 0;
            _queue.Enqueue(logEvent);
        }

        if (shouldSignal)
            _signal.Release();

        return ValueTask.CompletedTask;
    }

    public async ValueTask WriteManyAsync(IReadOnlyCollection<StructuredLogEvent> logEvents, CancellationToken cancellationToken = default)
    {
        foreach (var logEvent in logEvents)
            await WriteAsync(logEvent, cancellationToken);
    }

    public ValueTask<RecentStructuredLogsResult> QueryAsync(StructuredLogFilter filter, CancellationToken cancellationToken = default)
    {
        return store.QueryAsync(filter, cancellationToken);
    }

    public ValueTask<IReadOnlyCollection<StructuredLogSource>> ListSourcesAsync(CancellationToken cancellationToken = default)
    {
        return store.ListSourcesAsync(cancellationToken);
    }

    public Task StartAsync(CancellationToken cancellationToken)
    {
        lock (_lifecycleLock)
        {
            _activeStartCount++;

            if (_backgroundTask is { IsCompleted: false })
                return Task.CompletedTask;

            if (_stopTokenSource.IsCancellationRequested)
            {
                _stopTokenSource.Dispose();
                _stopTokenSource = new();
            }

            var stopToken = _stopTokenSource.Token;
            _backgroundTask = Task.Run(() => ProcessQueueAsync(stopToken), CancellationToken.None);
        }

        return Task.CompletedTask;
    }

    public async Task StopAsync(CancellationToken cancellationToken)
    {
        Task? backgroundTask;
        CancellationTokenSource stopTokenSource;

        lock (_lifecycleLock)
        {
            if (_activeStartCount == 0)
                return;

            _activeStartCount--;

            if (_activeStartCount > 0)
                return;

            backgroundTask = _backgroundTask;
            stopTokenSource = _stopTokenSource;
        }

        await stopTokenSource.CancelAsync();

        if (backgroundTask != null)
        {
            try
            {
                await backgroundTask.WaitAsync(cancellationToken);
            }
            catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested || stopTokenSource.IsCancellationRequested)
            {
                // Expected during shutdown; remaining queued writes are flushed below.
            }
        }

        using var timeoutTokenSource = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
        timeoutTokenSource.CancelAfter(options.Value.WriteQueue.ShutdownFlushTimeout);
        try
        {
            await FlushAsync(timeoutTokenSource.Token);
        }
        catch (OperationCanceledException) when (timeoutTokenSource.IsCancellationRequested)
        {
            CountPendingWritesAsDropped();
        }
    }

    public async ValueTask FlushAsync(CancellationToken cancellationToken = default)
    {
        while (true)
        {
            var batch = DequeueBatch();
            if (batch.Count == 0)
                return;

            try
            {
                await store.WriteManyAsync(batch, cancellationToken);
            }
            catch
            {
                Interlocked.Add(ref _droppedWriteCount, batch.Count);
                throw;
            }
        }
    }

    public async ValueTask DisposeAsync()
    {
        if (Interlocked.Exchange(ref _disposed, 1) == 1)
            return;

        Task? backgroundTask;
        CancellationTokenSource stopTokenSource;

        lock (_lifecycleLock)
        {
            backgroundTask = _backgroundTask;
            stopTokenSource = _stopTokenSource;
        }

        await stopTokenSource.CancelAsync();
        using var timeoutTokenSource = new CancellationTokenSource(options.Value.WriteQueue.ShutdownFlushTimeout);

        if (backgroundTask != null)
        {
            try
            {
                await backgroundTask.WaitAsync(timeoutTokenSource.Token);
            }
            catch (OperationCanceledException) when (timeoutTokenSource.IsCancellationRequested || stopTokenSource.IsCancellationRequested)
            {
                CountPendingWritesAsDropped();
            }
        }

        try
        {
            await FlushAsync(timeoutTokenSource.Token);
        }
        catch (OperationCanceledException) when (timeoutTokenSource.IsCancellationRequested)
        {
            CountPendingWritesAsDropped();
        }

        _signal.Dispose();
        stopTokenSource.Dispose();
    }

    private async Task ProcessQueueAsync(CancellationToken cancellationToken)
    {
        while (!cancellationToken.IsCancellationRequested)
        {
            try
            {
                await _signal.WaitAsync(options.Value.WriteQueue.FlushInterval, cancellationToken);

                if (cancellationToken.IsCancellationRequested)
                    return;

                await FlushAsync();
            }
            catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
            {
                return;
            }
            catch (ObjectDisposedException) when (cancellationToken.IsCancellationRequested)
            {
                return;
            }
            catch (Exception e)
            {
                Trace.TraceError("Failed to flush structured log writes: {0}", e);

                if (cancellationToken.IsCancellationRequested)
                    return;
            }
        }
    }

    private IReadOnlyCollection<StructuredLogEvent> DequeueBatch()
    {
        var batchSize = Math.Max(1, options.Value.WriteQueue.BatchSize);
        var batch = new List<StructuredLogEvent>(batchSize);

        lock (_queue)
        {
            while (_queue.Count > 0 && batch.Count < batchSize)
                batch.Add(_queue.Dequeue());
        }

        return batch;
    }

    private void CountPendingWritesAsDropped()
    {
        lock (_queue)
        {
            if (_queue.Count == 0)
                return;

            Interlocked.Add(ref _droppedWriteCount, _queue.Count);
            _queue.Clear();
        }
    }
}