using System;
using System.IO;
using System.Threading;

namespace Aivfo.OperationLog
{
    /// <summary>
    /// 配置热加载（G3-3 / 14§10）：后台定时轮询一个 JSON 配置文件，文件变化即热应用到
    /// <see cref="OperationLogOptions"/>（全局开关/级别 + 模块级开关），无需改代码重编。
    /// <para>
    /// "集中下发"落地方式：日志微服务 / 运维把统一配置文件下发（覆盖）到各端
    /// <see cref="OperationLogOptions.ConfigFilePath"/> 指向的路径即可；本组件负责读取并热生效。
    /// </para>
    /// 全 try 兜底：读/解析失败沿用旧配置，绝不抛、绝不影响业务。
    /// </summary>
    internal sealed class OperationLogConfigWatcher : IDisposable
    {
        private readonly OperationLogOptions _options;
        private readonly Action<string> _onSelfError;
        private readonly Timer _timer;
        private DateTime _lastWriteUtc = DateTime.MinValue;
        private long _lastLength = -1;
        private volatile bool _disposed;

        public OperationLogConfigWatcher(OperationLogOptions options, Action<string> onSelfError)
        {
            _options = options;
            _onSelfError = onSelfError;

            var periodMs = Math.Max(3, _options.ConfigReloadSeconds) * 1000;
            // 启动即先读一次（dueTime=0），随后周期轮询。
            _timer = new Timer(Poll, null, 0, periodMs);
        }

        private void Poll(object state)
        {
            if (_disposed) return;
            try
            {
                var path = _options.ConfigFilePath;
                if (string.IsNullOrWhiteSpace(path) || !File.Exists(path)) return;

                // 仅在 文件大小或写时间 变化时才重读重应用（省 IO、避免无谓刷新）。
                var info = new FileInfo(path);
                var writeUtc = info.LastWriteTimeUtc;
                var len = info.Length;
                if (writeUtc == _lastWriteUtc && len == _lastLength) return;

                string json = SafeReadAllText(path);
                if (json == null) return;

                _options.ApplyConfigJson(json);
                _lastWriteUtc = writeUtc;
                _lastLength = len;
                _onSelfError?.Invoke($"操作日志配置已热加载: {path}");
            }
            catch (Exception ex)
            {
                try { _onSelfError?.Invoke("配置热加载失败(沿用旧配置): " + ex.Message); } catch { }
            }
        }

        /// <summary>容错读取：文件可能正被写，读失败返回 null（下轮再试）。</summary>
        private static string SafeReadAllText(string path)
        {
            try
            {
                using (var fs = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
                using (var sr = new StreamReader(fs, System.Text.Encoding.UTF8))
                {
                    return sr.ReadToEnd();
                }
            }
            catch { return null; }
        }

        public void Dispose()
        {
            if (_disposed) return;
            _disposed = true;
            try { _timer?.Dispose(); } catch { /* ignore */ }
        }
    }
}