LumaTunnel

using System.Text.Json;
using LumaTunnel.Shared.Protocol;

namespace LumaTunnel.Server.Core.Persistence;

public sealed class AtomicJsonStore<T>(string path) : IDisposable where T : class
{
    private readonly SemaphoreSlim _gate = new(1, 1);
    private readonly string _path = path;

    public async Task<T> LoadOrCreateAsync(Func<T> factory, CancellationToken cancellationToken = default)
    {
        await _gate.WaitAsync(cancellationToken).ConfigureAwait(false);
        try
        {
            if (!File.Exists(_path))
            {
                var value = factory();
                await SaveCoreAsync(value, cancellationToken).ConfigureAwait(false);
                return value;
            }

            await using var stream = File.OpenRead(_path);
            return await JsonSerializer.DeserializeAsync<T>(stream, TunnelMessageSerializer.Options, cancellationToken).ConfigureAwait(false)
                   ?? throw new InvalidDataException($"The JSON document at '{_path}' is empty.");
        }
        finally
        {
            _gate.Release();
        }
    }

    public async Task SaveAsync(T value, CancellationToken cancellationToken = default)
    {
        await _gate.WaitAsync(cancellationToken).ConfigureAwait(false);
        try
        {
            await SaveCoreAsync(value, cancellationToken).ConfigureAwait(false);
        }
        finally
        {
            _gate.Release();
        }
    }

    private async Task SaveCoreAsync(T value, CancellationToken cancellationToken)
    {
        var directory = Path.GetDirectoryName(_path) ?? throw new InvalidOperationException("Store path has no directory.");
        Directory.CreateDirectory(directory);
        var tempPath = Path.Combine(directory, $".{Path.GetFileName(_path)}.{Guid.NewGuid():N}.tmp");
        try
        {
            await using (var stream = new FileStream(tempPath, FileMode.CreateNew, FileAccess.Write, FileShare.None, 16 * 1024, FileOptions.WriteThrough | FileOptions.Asynchronous))
            {
                await JsonSerializer.SerializeAsync(stream, value, TunnelMessageSerializer.Options, cancellationToken).ConfigureAwait(false);
                await stream.FlushAsync(cancellationToken).ConfigureAwait(false);
            }

            File.Move(tempPath, _path, true);
        }
        finally
        {
            if (File.Exists(tempPath))
                File.Delete(tempPath);
        }
    }

    public void Dispose()
    {
        _gate.Dispose();
        GC.SuppressFinalize(this);
    }
}