using System.Security.Cryptography;

namespace Fleet.Core;

public enum NodeLogLevel
{
    Info,
    Warn,
    Error,
}

/// <summary>节点日志：字段名统一为 logfmt（<c>ts level msg agent dialer session task result err</c>）。逐包事件不记录，只累加计数。</summary>
public interface INodeLog
{
    void Write(NodeLogLevel level, string message, params (string Key, object? Value)[] fields);
}

/// <summary>One structured node log line queued for delivery to the control plane.</summary>
public sealed record NodeLogRecord(long AtUtcMs, string Level, string Message, Dictionary<string, string> Fields);

public sealed record NodeLogBatch(IReadOnlyList<NodeLogRecord> Records, long Dropped);

/// <summary>Optional bounded log source used by node heartbeats to upload recent local logs.</summary>
public interface INodeLogBatchSource
{
    NodeLogBatch TakeBatch(int maxRecords);
}

public sealed class NullNodeLog : INodeLog
{
    public static NullNodeLog Instance { get; } = new();

    public void Write(NodeLogLevel level, string message, params (string Key, object? Value)[] fields)
    {
    }
}

public static class NodeLogExtensions
{
    public static void Info(this INodeLog log, string message, params (string Key, object? Value)[] fields) => log.Write(NodeLogLevel.Info, message, fields);

    public static void Warn(this INodeLog log, string message, params (string Key, object? Value)[] fields) => log.Write(NodeLogLevel.Warn, message, fields);

    public static void Error(this INodeLog log, string message, params (string Key, object? Value)[] fields) => log.Write(NodeLogLevel.Error, message, fields);
}

/// <summary>指数退避（带随机抖动）：1 s → 20 s（对端恢复之后最多等 20 秒；节点数量不大，重连风暴不是问题）。连接成功之后调用 <see cref="Reset"/>。</summary>
public sealed class Backoff
{
    private readonly TimeSpan _initial;
    private readonly TimeSpan _max;
    private int _attempt;

    public Backoff(TimeSpan? initial = null, TimeSpan? max = null)
    {
        _initial = initial ?? TimeSpan.FromSeconds(1);
        _max = max ?? TimeSpan.FromSeconds(20);
    }

    public TimeSpan Next()
    {
        double baseMs = Math.Min(_max.TotalMilliseconds, _initial.TotalMilliseconds * Math.Pow(2, Math.Min(_attempt, 20)));
        _attempt++;
        // 抖动：0.5 ~ 1.0 倍，避免所有节点同时重连
        double jitter = 0.5 + (RandomNumberGenerator.GetInt32(0, 1000) / 2000.0);
        return TimeSpan.FromMilliseconds(baseMs * jitter);
    }

    public void Reset() => _attempt = 0;
}

/// <summary>
/// 序号重放窗口（滑动 1024 位）：同一个序号只接受一次；比窗口还旧的序号视为重放。
/// 用于数据面的 UDP 包，每个 (session, stream) 一个。
/// </summary>
public sealed class ReplayWindow
{
    private const int Size = 1024;
    private readonly ulong[] _bits = new ulong[Size / 64];
    private long _highest = -1;

    public bool TryAccept(uint seq)
    {
        if (_highest < 0)
        {
            _highest = seq;
            Set(seq);
            return true;
        }

        if (seq > _highest)
        {
            long shift = seq - _highest;
            if (shift >= Size)
            {
                Array.Clear(_bits);
            }
            else
            {
                for (long s = _highest + 1; s <= seq; s++)
                {
                    Clear((uint)s);
                }
            }

            _highest = seq;
            Set(seq);
            return true;
        }

        if (_highest - seq >= Size)
        {
            return false; // 太旧
        }

        if (IsSet(seq))
        {
            return false; // 重复
        }

        Set(seq);
        return true;
    }

    private void Set(uint seq) => _bits[(int)(seq % Size) / 64] |= 1UL << (int)(seq % 64);

    private void Clear(uint seq) => _bits[(int)(seq % Size) / 64] &= ~(1UL << (int)(seq % 64));

    private bool IsSet(uint seq) => (_bits[(int)(seq % Size) / 64] & (1UL << (int)(seq % 64))) != 0;
}