using System.Collections.Concurrent;
using System.Threading.Channels;

namespace w4c_workflows.Services;

/// <summary>
/// Server-push hub for tenant-scoped realtime events (SSE), mirroring w4c-webapi's hub. A connected
/// client gets its own bounded channel; <see cref="Publish"/> writes a tiny notification into every
/// channel for the tenant. The client decides what to re-fetch, keeping broadcasts cheap. The channel
/// is bounded and drops the oldest notification when a client cannot keep up, so one stalled SSE
/// consumer cannot grow server memory without limit (the next notification makes it re-fetch anyway).
///
/// Used to make the frontend an optional view: when a workflow file is written server-side (e.g. by
/// the AI transform), every subscribed client in the tenant is notified and reloads the file.
/// </summary>
public sealed class RealtimeEventHub
{
    /// <summary>Per-client notification backlog ceiling.</summary>
    private const int ClientChannelCapacity = 256;

    private readonly ConcurrentDictionary<string, ConcurrentDictionary<Guid, Channel<RealtimeEvent>>> _clients = new();

    public (Guid Id, ChannelReader<RealtimeEvent> Reader) AddClient(string tenantId)
    {
        var channel = Channel.CreateBounded<RealtimeEvent>(new BoundedChannelOptions(ClientChannelCapacity)
        {
            SingleReader = true,
            SingleWriter = false,
            FullMode = BoundedChannelFullMode.DropOldest,
        });
        var id = Guid.NewGuid();
        var clients = _clients.GetOrAdd(tenantId, _ => new ConcurrentDictionary<Guid, Channel<RealtimeEvent>>());
        clients[id] = channel;
        return (id, channel.Reader);
    }

    public void RemoveClient(string tenantId, Guid id)
    {
        if (!_clients.TryGetValue(tenantId, out var clients)) return;
        clients.TryRemove(id, out _);
        if (clients.IsEmpty) _clients.TryRemove(tenantId, out _);
    }

    public void Publish(string tenantId, string kind, string id, DateTime at)
    {
        if (!_clients.TryGetValue(tenantId, out var clients)) return;
        var evt = new RealtimeEvent(kind, id, at);
        foreach (var channel in clients.Values)
        {
            channel.Writer.TryWrite(evt);
        }
    }
}

/// <summary>A tenant-scoped change notification: <c>Kind</c> is a component name (e.g. "workflow"),
/// <c>Id</c> is that component's server id/path, <c>At</c> is the server-side write time.</summary>
public record RealtimeEvent(string Kind, string Id, DateTime At);