using System.Collections.Concurrent;
using System.Threading.Channels;
using Fleet.Core;
using MoShiro.Protocol;

namespace MoShiro.Backend.Application;

/// <summary>发布到 SSE 订阅者的信号。SSE 只负责“更快地获得变化”，推送的内容都能由 REST 查询重建。</summary>
public abstract record LiveSignal;

/// <summary>节点上下线等。</summary>
public sealed record NodeSignal(string Kind, string Id, bool Online, long AtMs) : LiveSignal;

/// <summary>某条序列有新的原始块入库。</summary>
public sealed record ChunkSignal(NodeId AgentId, string Series) : LiveSignal;

public sealed record TestSignal(TestId TestId) : LiveSignal;

/// <summary>某个 Agent 的主机资源占用有了新的快照（实时推给展示页）。</summary>
public sealed record HostSignal(NodeId AgentId, HostSnapshot Host) : LiveSignal;

public sealed class LiveSubscription : IDisposable
{
    private readonly LiveHub _hub;
    private readonly Channel<LiveSignal> _channel = Channel.CreateBounded<LiveSignal>(new BoundedChannelOptions(256) { FullMode = BoundedChannelFullMode.DropOldest });

    internal LiveSubscription(LiveHub hub)
    {
        _hub = hub;
    }

    public ChannelReader<LiveSignal> Reader => _channel.Reader;

    internal void Post(LiveSignal signal) => _channel.Writer.TryWrite(signal);

    public void Dispose()
    {
        _hub.Remove(this);
        _channel.Writer.TryComplete();
    }
}

/// <summary>内存中的发布订阅：入库、节点上下线、测试状态变化都经过这里。</summary>
public sealed class LiveHub
{
    private readonly ConcurrentDictionary<LiveSubscription, byte> _subscriptions = new();

    public int SubscriberCount => _subscriptions.Count;

    public LiveSubscription Subscribe()
    {
        var subscription = new LiveSubscription(this);
        _subscriptions[subscription] = 0;
        return subscription;
    }

    public void Publish(LiveSignal signal)
    {
        foreach (LiveSubscription subscription in _subscriptions.Keys)
        {
            subscription.Post(signal);
        }
    }

    internal void Remove(LiveSubscription subscription) => _subscriptions.TryRemove(subscription, out _);
}