using System.Text.Json;
using Fleet.Core;
using Fleet.Hosting;
using Monitor.Protocol;

namespace Monitor.Probe;

/// <summary>
/// Probe 要连接的 Agent 列表：由面板作为期望状态（<c>probe_agents</c>）直接下发；面板连不上时用上一次保存的 <c>agents.json</c>。
/// 列表变化时用 <see cref="ConfigStore"/> 写回 <c>agents.json</c>（这是配置，不是测量数据；只在变化时写，保护路由器的闪存）。
/// </summary>
public sealed class AgentDirectory
{
    public const string FileName = "agents.json";

    private readonly object _gate = new();
    private readonly ConfigStore _store;
    private readonly INodeLog _log;
    private List<KnownAgent> _agents = [];
    private long _version;

    public AgentDirectory(ConfigStore store, string configDirectory, INodeLog log)
    {
        _store = store;
        _log = log;
        string cache = Path.Combine(configDirectory, FileName);
        if (File.Exists(cache))
        {
            try
            {
                AgentsCache saved = NodeConfig.Load(cache, ProbeConfigContext.Default.AgentsCache);
                _agents = saved.Agents;
                _version = saved.Version;
            }
            catch (Exception ex) when (ex is IOException or InvalidDataException or JsonException or InvalidOperationException)
            {
                _log.Warn("agents.json is unreadable, waiting for the panel", ("err", ex.Message));
            }
        }
    }

    /// <summary>列表变化（调和连接）。</summary>
    public event Action? Changed;

    public long Version
    {
        get
        {
            lock (_gate)
            {
                return _version;
            }
        }
    }

    public IReadOnlyList<KnownAgent> Agents
    {
        get
        {
            lock (_gate)
            {
                return [.. _agents];
            }
        }
    }

    /// <summary>应用面板下发的列表。版本不比当前新的忽略（乱序到达的旧推送）。</summary>
    public void Apply(ProbeAgentList result)
    {
        var next = result.Agents.Select(a => new KnownAgent { AgentId = a.AgentId.Value, Host = a.Host, TcpPort = a.TcpPort, Spki = a.Spki }).ToList();
        lock (_gate)
        {
            if (result.Version < _version || (result.Version == _version && Same(next, _agents)))
            {
                return;
            }

            _version = result.Version;
            _agents = next;
        }

        try
        {
            _store.Write(FileName, JsonSerializer.SerializeToUtf8Bytes(new AgentsCache { Version = result.Version, Agents = next }, ProbeConfigContext.Default.AgentsCache));
        }
        catch (Exception ex) when (ex is IOException or UnauthorizedAccessException)
        {
            _log.Warn("could not save agents.json", ("err", ex.Message));
        }

        _log.Info("agent list updated", ("version", result.Version), ("agents", next.Count));
        Changed?.Invoke();
    }

    private static bool Same(List<KnownAgent> a, List<KnownAgent> b) =>
        a.Count == b.Count && a.Zip(b).All(p => Equal(p.First, p.Second));

    public static bool Equal(KnownAgent a, KnownAgent b) =>
        a.AgentId == b.AgentId && a.Host == b.Host && a.TcpPort == b.TcpPort && a.Spki == b.Spki;
}