namespace MoShiro.Measurement;

/// <summary>探测节奏的状态。<see cref="Dead"/> 是线路几乎全丢：不再按正常频率白白发包，指数退避着探测，等到有回包再恢复。</summary>
public enum ProbeState : byte
{
    Quiet = 0,
    Watch = 1,
    Active = 2,
    Dead = 3,
}

/// <summary>自适应探测频率的参数。频率范围、精度目标和各状态的保持时间都在这里。</summary>
public sealed record AdaptiveRateOptions
{
    /// <summary>平稳时的最低频率（Hz）。</summary>
    public double MinHz { get; init; } = 2;

    /// <summary>
    /// 最高频率（Hz），也是权重的参考网格：每个探测包的权重是它代表的时间除以 <c>1/MaxHz</c>，
    /// 分析时丢包率和 RTT 分位数按权重累计，所以频率怎么变，得到的仍是按时间平均的值，不会偏向探测更密的时段。
    /// </summary>
    public double MaxHz { get; init; } = 10;

    /// <summary>平稳时 RTT 分位数（p99）的 95% 置信区间半宽容差：取 max(下限毫秒, 百分比 × p99)。样本多到超出这个精度才会降频。</summary>
    public double RttTolPct { get; init; } = 2;

    public double RttTolFloorMs { get; init; } = 0.3;

    /// <summary>平稳线路要能证明的丢包上界（百分比，95% 置信）。三法则：零丢包需要 3 / τ 个样本。</summary>
    public double TauPct { get; init; } = 0.2;

    /// <summary>证明窗口（秒）：平稳频率 = 3 / (τ × 窗口)。</summary>
    public double CertifySeconds { get; init; } = 300;

    /// <summary>历史上一直平稳的时段可以拉长证明窗口，同样的精度用更少的包。</summary>
    public double QuietCertifySeconds { get; init; } = 900;

    /// <summary>回包等多久算丢（秒）。</summary>
    public double GraceSeconds { get; init; } = 3;

    /// <summary>刚启动没有历史：先按 Watch 频率跑这么久（秒）。</summary>
    public double WarmupSeconds { get; init; } = 300;

    /// <summary>一次丢包事件之后，Watch 至少保持多久（秒）。</summary>
    public double WatchHoldSeconds { get; init; } = 1800;

    /// <summary>进入 Active 之后，没有新的丢包事件要保持多久才降到 Watch（秒）。</summary>
    public double ActiveHoldSeconds { get; init; } = 900;

    /// <summary>证明窗口内有这么多个不同的秒出现丢包，就算持续丢包，升到 Active。</summary>
    public int ActiveLossCount { get; init; } = 3;

    /// <summary>判断线路全丢的窗口（秒）和最少样本数、丢包比例。</summary>
    public double DeadWindowSeconds { get; init; } = 10;

    public int DeadMinSent { get; init; } = 10;

    public double DeadLossRatio { get; init; } = 0.95;

    /// <summary>Dead 状态的探测间隔：从 1 秒起翻倍，最长这么久（秒），带 ±20% 抖动。</summary>
    public double DeadMaxIntervalSeconds { get; init; } = 30;

    /// <summary>RTT 比长期水平高出这个倍数（再加下面的绝对值）持续几秒，就当作链路在变差，至少升到 Watch。</summary>
    public double RttFactor { get; init; } = 1.5;

    public double RttFloorMs { get; init; } = 5;

    /// <summary>用 RTT p99 的置信区间限制降频。样本稀少的任务（每分钟几次）估不出区间，关掉后平稳频率只由丢包上界决定。</summary>
    public bool RttGuard { get; init; } = true;

    /// <summary>先验：时间 → 这个时刻丢包的历史概率（0 到 1）；null 表示没有历史。</summary>
    public Func<long, double?>? Prior { get; init; }

    /// <summary>先验里的"丢包高发"阈值：达到它在到来之前 <see cref="PriorLeadSeconds"/> 提前预热。</summary>
    public double PriorWatchAt { get; init; } = 0.15;

    public double PriorActiveAt { get; init; } = 0.5;

    public double PriorLeadSeconds { get; init; } = 1800;
}

/// <summary>状态或频率变了：<see cref="Reason"/> 是给日志和审计看的固定短语。</summary>
public sealed record ProbeTransition(ProbeState From, ProbeState To, double Hz, string Reason, double LossPct, int Sent, int Lost);

/// <summary>
/// 自适应探测频率：按"要证明的精度"决定频率，丢包多了自动升频，线路全丢就指数退避。
/// 平稳线路只需要能证明丢包低于 τ 的样本数（三法则 N = 3/τ），有丢包的线路用满频率保精度和丢包的细节（连续丢包、乱序）。
/// 状态靠丢包事件的频率升降，带滞回，不会在阈值附近来回跳；时间画像可以让高发时段提前预热。
/// 纯逻辑：时间由调用方传入，不依赖时钟，方便测试。
/// </summary>
public sealed class AdaptiveRate
{
    private const long UsPerSecond = 1_000_000;
    private const int Buckets = 1200; // 20 分钟，够覆盖最长的统计窗口

    private readonly AdaptiveRateOptions _options;
    private readonly Random _random;
    private readonly object _gate = new();
    private readonly long[] _second = new long[Buckets];
    private readonly int[] _sent = new int[Buckets];
    private readonly int[] _replied = new int[Buckets];

    private long _startUs = -1;
    private long _lastLossUs = long.MinValue;
    private long _activeUntilUs = long.MinValue;
    private long _lastReplyUs = long.MinValue;
    private long _deadSinceUs = long.MinValue;
    private int _deadProbes;
    private readonly Queue<(long Second, double Ms)> _rtts = new();
    private long _lastGuardSecond = long.MinValue;
    private double _guardHz = double.NaN;
    private TimeSpan _lastDelay = TimeSpan.Zero;
    private double _rttFast = double.NaN;
    private double _rttSlow = double.NaN;
    private int _rttHighSeconds;
    private long _lastRttSecond = long.MinValue;

    public AdaptiveRate(AdaptiveRateOptions options, Random? random = null)
    {
        _options = options;
        _random = random ?? Random.Shared;
        State = ProbeState.Watch;
        Hz = _options.MaxHz;
    }

    public ProbeState State { get; private set; }

    /// <summary>当前的探测频率（Hz）。Dead 时是退避后的近似频率。</summary>
    public double Hz { get; private set; }

    /// <summary>发出一个探测包。</summary>
    public void OnSend(long nowUs)
    {
        lock (_gate)
        {
            if (_startUs < 0)
            {
                _startUs = nowUs;
            }

            long second = nowUs / UsPerSecond;
            int slot = Slot(second);
            if (_second[slot] != second)
            {
                _second[slot] = second;
                _sent[slot] = 0;
                _replied[slot] = 0;
            }

            _sent[slot]++;
            if (State == ProbeState.Dead)
            {
                _deadProbes++;
            }
        }
    }

    /// <summary>收到回包：<paramref name="sentUs"/> 是它发出的时间，<paramref name="rttMs"/> 是往返时延。</summary>
    public void OnReply(long sentUs, long nowUs, double rttMs)
    {
        lock (_gate)
        {
            long second = sentUs / UsPerSecond;
            int slot = Slot(second);
            if (_second[slot] == second && _replied[slot] < _sent[slot])
            {
                _replied[slot]++;
            }

            if (double.IsNaN(_rttSlow))
            {
                _rttSlow = _rttFast = rttMs;
            }
            else
            {
                // 时间常数：快的 5 秒，慢的 15 分钟（按时间而不是按包数，频率变了也一样）
                double dt = Math.Max(0, (nowUs - Math.Max(_lastReplyUs, 0)) / (double)UsPerSecond);
                _rttFast += (1 - Math.Exp(-dt / 5)) * (rttMs - _rttFast);
                _rttSlow += (1 - Math.Exp(-dt / 900)) * (rttMs - _rttSlow);
            }

            _lastReplyUs = nowUs;
            _rtts.Enqueue((nowUs / UsPerSecond, rttMs));
            while (_rtts.Count > 6000)
            {
                _rtts.Dequeue();
            }
        }
    }

    /// <summary>连接（重新）建立了：之前线路不通退避着的话，马上恢复探测，不用等退避间隔。</summary>
    public void Reconnected(long nowUs)
    {
        lock (_gate)
        {
            if (State == ProbeState.Dead)
            {
                _lastReplyUs = nowUs;
            }
        }
    }

    /// <summary>距离下一个探测包还有多久（带抖动，平均值等于频率的倒数）。</summary>
    public TimeSpan NextDelay()
    {
        lock (_gate)
        {
            if (State == ProbeState.Dead)
            {
                double seconds = Math.Min(_options.DeadMaxIntervalSeconds, Math.Pow(2, Math.Min(_deadProbes, 10)));
                return _lastDelay = TimeSpan.FromSeconds(seconds * (0.8 + 0.4 * _random.NextDouble()));
            }

            // 平稳时间隔精确（不加抖动）：发包时刻规整，列块里的时间戳差分更小；抖动只用在退避里避免多条线路同步重试
            return _lastDelay = TimeSpan.FromSeconds(1.0 / Hz);
        }
    }

    /// <summary>刚刚等过的那个间隔所代表的权重（以 1/MaxHz 为单位）：下一个发出的包用它。平稳时是 MaxHz/Hz，退避时是几十到几百。</summary>
    public int Weight
    {
        get
        {
            lock (_gate)
            {
                return (int)Math.Clamp(Math.Round(_lastDelay.TotalSeconds * _options.MaxHz), 1, 65535);
            }
        }
    }

    /// <summary>重新评估状态，大约每秒调用一次；状态或频率变了就返回变化。</summary>
    public ProbeTransition? Evaluate(long nowUs)
    {
        lock (_gate)
        {
            if (_startUs < 0)
            {
                return null;
            }

            long finalSecond = nowUs / UsPerSecond - (long)Math.Ceiling(_options.GraceSeconds);
            (int sent, int replied, int _) shortWindow = Window(finalSecond, (int)_options.DeadWindowSeconds);
            (int sent, int replied, int lossSeconds) certify = Window(finalSecond, (int)_options.CertifySeconds);
            int lostCertify = certify.sent - certify.replied;
            double lossPct = certify.sent == 0 ? 0 : 100.0 * lostCertify / certify.sent;

            // 最近一次有丢包的时间：用最近 CertifySeconds 里最晚一个有丢包的整秒
            for (long second = finalSecond; second > finalSecond - (long)_options.CertifySeconds; second--)
            {
                int slot = Slot(second);
                if (_second[slot] == second && _sent[slot] > _replied[slot])
                {
                    _lastLossUs = Math.Max(_lastLossUs, second * UsPerSecond);
                    break;
                }
            }

            UpdateRttTrend(nowUs);

            ProbeState next;
            string reason;
            if (State == ProbeState.Dead)
            {
                if (_lastReplyUs > _deadSinceUs)
                {
                    next = ProbeState.Watch;
                    reason = "reply received";
                    _deadProbes = 0;
                    _lastLossUs = nowUs;
                }
                else
                {
                    return null;
                }
            }
            else if (shortWindow.sent >= _options.DeadMinSent && shortWindow.replied <= shortWindow.sent * (1 - _options.DeadLossRatio))
            {
                next = ProbeState.Dead;
                reason = "line unreachable";
                _deadSinceUs = nowUs;
                _deadProbes = 0;
            }
            else
            {
                (next, reason) = Decide(nowUs, certify.lossSeconds, lostCertify, lossPct);
            }

            UpdateGuard(nowUs);
            double hz = next switch
            {
                ProbeState.Quiet => Quantize(Math.Max(QuietHz(QuietWindowSeconds(nowUs)), !_options.RttGuard ? _options.MinHz : double.IsNaN(_guardHz) ? _options.MaxHz : _guardHz)),
                ProbeState.Watch or ProbeState.Active => _options.MaxHz,
                _ => Math.Max(_options.MinHz / 4, 1.0 / _options.DeadMaxIntervalSeconds),
            };
            hz = Math.Min(_options.MaxHz, hz);

            if (next == State && Math.Abs(hz - Hz) / Hz < 0.05)
            {
                return null;
            }

            ProbeTransition result = new(State, next, hz, reason, lossPct, certify.sent, lostCertify);
            State = next;
            Hz = hz;
            return result;
        }
    }

    private (ProbeState State, string Reason) Decide(long nowUs, int lossSeconds, int lostCertify, double lossPct)
    {
        ProbeState state = ProbeState.Quiet;
        string reason = "quiet";

        double sinceLoss = _lastLossUs == long.MinValue ? double.PositiveInfinity : (nowUs - _lastLossUs) / (double)UsPerSecond;
        if (lossSeconds >= _options.ActiveLossCount || (lossPct >= _options.TauPct * 2 && lossSeconds >= 2))
        {
            _activeUntilUs = nowUs + (long)(_options.ActiveHoldSeconds * UsPerSecond);
        }

        if (nowUs < _activeUntilUs)
        {
            return (ProbeState.Active, "sustained loss");
        }

        if (sinceLoss <= _options.WatchHoldSeconds)
        {
            state = ProbeState.Watch;
            reason = "recent loss";
        }

        if (_rttHighSeconds >= 3)
        {
            state = ProbeState.Watch;
            reason = "rtt above baseline";
        }

        double? prior = _options.Prior?.Invoke(nowUs);
        double? ahead = _options.Prior?.Invoke(nowUs + (long)(_options.PriorLeadSeconds * UsPerSecond));
        double expected = Math.Max(prior ?? 0, ahead ?? 0);
        if (expected >= _options.PriorActiveAt)
        {
            return (ProbeState.Active, "expected peak");
        }

        if (expected >= _options.PriorWatchAt && state == ProbeState.Quiet)
        {
            state = ProbeState.Watch;
            reason = "expected loss window";
        }

        if (state == ProbeState.Quiet && _options.Prior is null && nowUs - _startUs < (long)(_options.WarmupSeconds * UsPerSecond))
        {
            state = ProbeState.Watch;
            reason = "warm-up";
        }

        return (state, reason);
    }

    /// <summary>历史上这个时段（现在和将要到来的一段时间）都平稳时，证明窗口可以更长，频率更低。</summary>
    private double QuietWindowSeconds(long nowUs)
    {
        if (_options.Prior is null)
        {
            return _options.CertifySeconds;
        }

        double now = _options.Prior(nowUs) ?? 1;
        double ahead = _options.Prior(nowUs + (long)(_options.PriorLeadSeconds * UsPerSecond)) ?? 1;
        return Math.Max(now, ahead) <= 0.02 ? _options.QuietCertifySeconds : _options.CertifySeconds;
    }

    /// <summary>频率取 MaxHz/k（k 为整数）：每个包的权重正好是整数 k，分析时没有舍入误差。</summary>
    private double Quantize(double hz)
    {
        double k = Math.Max(1, Math.Floor(_options.MaxHz / Math.Max(hz, 1e-9)));
        return _options.MaxHz / k;
    }

    /// <summary>
    /// 分位数精度保护：用最近一个证明窗口里的 RTT 样本估计 p99 的 95% 置信区间（次序统计量），
    /// 半宽超过容差就升频，远小于容差才降频（单步最多减半），使平稳线路的 RTT 分位数始终保持在设定的精度内。
    /// </summary>
    private void UpdateGuard(long nowUs)
    {
        long second = nowUs / UsPerSecond;
        if (_lastGuardSecond != long.MinValue && second - _lastGuardSecond < 10)
        {
            return;
        }

        _lastGuardSecond = second;
        long from = second - (long)_options.CertifySeconds;
        double[] window = [.. _rtts.Where(r => r.Second >= from).Select(r => r.Ms).Order()];
        if (window.Length < 200)
        {
            _guardHz = double.NaN; // 样本不够估计精度：按最高频率
            return;
        }

        int n = window.Length;
        double rank = n * 0.99;
        double half = 1.96 * Math.Sqrt(n * 0.99 * 0.01);
        int lo = Math.Clamp((int)Math.Floor(rank - half), 0, n - 1);
        int hi = Math.Clamp((int)Math.Ceiling(rank + half), 0, n - 1);
        double halfWidth = (window[hi] - window[lo]) / 2;
        double tolerance = Math.Max(_options.RttTolFloorMs, _options.RttTolPct / 100.0 * window[Math.Min(n - 1, (int)rank)]);
        double ratio = Math.Clamp(halfWidth / tolerance, 0.7, 2.0); // 单步变化有限，避免来回跳
        double current = Hz;
        _guardHz = Math.Clamp(current * ratio * ratio, _options.MinHz, _options.MaxHz);
    }

    private double QuietHz(double windowSeconds) => Math.Min(_options.MaxHz, Math.Max(_options.MinHz, 3.0 / (_options.TauPct / 100.0 * windowSeconds)));

    /// <summary>窗口内发出、回来的包数，以及有丢包的整秒数（一次连续丢包只算一个"事件"，不会因为一次短暂中断就判成持续丢包）。</summary>
    private (int Sent, int Replied, int LossSeconds) Window(long finalSecond, int seconds)
    {
        int sent = 0;
        int replied = 0;
        int lossSeconds = 0;
        for (long second = finalSecond; second > finalSecond - seconds; second--)
        {
            int slot = Slot(second);
            if (_second[slot] == second)
            {
                sent += _sent[slot];
                replied += _replied[slot];
                if (_sent[slot] > _replied[slot])
                {
                    lossSeconds++;
                }
            }
        }

        return (sent, replied, lossSeconds);
    }

    private void UpdateRttTrend(long nowUs)
    {
        long second = nowUs / UsPerSecond;
        if (second == _lastRttSecond || double.IsNaN(_rttFast) || double.IsNaN(_rttSlow))
        {
            return;
        }

        _lastRttSecond = second;
        bool high = _rttFast > _rttSlow * _options.RttFactor + _options.RttFloorMs;
        _rttHighSeconds = high ? _rttHighSeconds + 1 : 0;
    }

    private static int Slot(long second) => (int)(((second % Buckets) + Buckets) % Buckets);
}