namespace Monitor.Backend.Analysis;

public enum AccKind : byte
{
    Counter = 1,
    Ratio = 2,
    Sum = 3,
    Min = 4,
    Max = 5,
    Histogram = 6,
    RunLength = 7,
    Series = 8,
}

/// <summary>
/// 所有指标的中间状态只能是下面几种累加器之一。合并规则只写一次，
/// 存储、降采样、按时间范围聚合都通用处理。
/// </summary>
public abstract class Accumulator
{
    public abstract AccKind Kind { get; }

    /// <summary>加入一个观测值。含义由累加器决定：Counter 加 <paramref name="a"/>，Ratio 的 (a, b) 是 (分子, 分母)，Histogram 记录 a。</summary>
    public abstract void Add(double a, double b = 0);

    /// <summary>合并同类型的另一个累加器（精确合并，不是平均）。</summary>
    public abstract void Merge(Accumulator other);

    public abstract Accumulator Clone();

    /// <summary>样本数，用于置信说明。</summary>
    public abstract ulong SampleCount { get; }

    public static Accumulator Create(AccKind kind) => kind switch
    {
        AccKind.Counter => new CounterAcc(),
        AccKind.Ratio => new RatioAcc(),
        AccKind.Sum => new SumAcc(),
        AccKind.Min => new MinAcc(),
        AccKind.Max => new MaxAcc(),
        AccKind.Histogram => new HistogramAcc(),
        AccKind.RunLength => new RunLengthAcc(),
        AccKind.Series => new SeriesAcc(),
        _ => throw new ArgumentOutOfRangeException(nameof(kind)),
    };

    protected T Cast<T>(Accumulator other)
        where T : Accumulator =>
        other as T ?? throw new InvalidOperationException($"cannot merge {other.Kind} into {Kind}");
}

public sealed class CounterAcc : Accumulator
{
    public ulong Value { get; set; }

    public override AccKind Kind => AccKind.Counter;

    public override ulong SampleCount => Value;

    public override void Add(double a, double b = 0) => Value += (ulong)Math.Max(0, a);

    public override void Merge(Accumulator other) => Value += Cast<CounterAcc>(other).Value;

    public override Accumulator Clone() => new CounterAcc { Value = Value };
}

public sealed class RatioAcc : Accumulator
{
    public ulong Numerator { get; set; }

    public ulong Denominator { get; set; }

    public override AccKind Kind => AccKind.Ratio;

    public override ulong SampleCount => Denominator;

    /// <summary>百分比（0–100）；没有样本时为 null。</summary>
    public double? Percent => Denominator == 0 ? null : 100.0 * Numerator / Denominator;

    public override void Add(double a, double b = 0)
    {
        Numerator += (ulong)Math.Max(0, a);
        Denominator += (ulong)Math.Max(0, b);
    }

    public override void Merge(Accumulator other)
    {
        var o = Cast<RatioAcc>(other);
        Numerator += o.Numerator;
        Denominator += o.Denominator;
    }

    public override Accumulator Clone() => new RatioAcc { Numerator = Numerator, Denominator = Denominator };
}

public sealed class SumAcc : Accumulator
{
    public double Sum { get; set; }

    public ulong Count { get; set; }

    public override AccKind Kind => AccKind.Sum;

    public override ulong SampleCount => Count;

    public double? Mean => Count == 0 ? null : Sum / Count;

    public override void Add(double a, double b = 0)
    {
        Sum += a;
        Count++;
    }

    public override void Merge(Accumulator other)
    {
        var o = Cast<SumAcc>(other);
        Sum += o.Sum;
        Count += o.Count;
    }

    public override Accumulator Clone() => new SumAcc { Sum = Sum, Count = Count };
}

public sealed class MinAcc : Accumulator
{
    public double Value { get; set; } = double.PositiveInfinity;

    public ulong Count { get; set; }

    public override AccKind Kind => AccKind.Min;

    public override ulong SampleCount => Count;

    public double? Result => Count == 0 ? null : Value;

    public override void Add(double a, double b = 0)
    {
        Value = Math.Min(Value, a);
        Count++;
    }

    public override void Merge(Accumulator other)
    {
        var o = Cast<MinAcc>(other);
        Value = Math.Min(Value, o.Value);
        Count += o.Count;
    }

    public override Accumulator Clone() => new MinAcc { Value = Value, Count = Count };
}

public sealed class MaxAcc : Accumulator
{
    public double Value { get; set; } = double.NegativeInfinity;

    public ulong Count { get; set; }

    public override AccKind Kind => AccKind.Max;

    public override ulong SampleCount => Count;

    public double? Result => Count == 0 ? null : Value;

    public override void Add(double a, double b = 0)
    {
        Value = Math.Max(Value, a);
        Count++;
    }

    public override void Merge(Accumulator other)
    {
        var o = Cast<MaxAcc>(other);
        Value = Math.Max(Value, o.Value);
        Count += o.Count;
    }

    public override Accumulator Clone() => new MaxAcc { Value = Value, Count = Count };
}

/// <summary>连续段长度（例如连续丢包）：只保留次数、总和和最大值，合并是精确的。</summary>
public sealed class RunLengthAcc : Accumulator
{
    public ulong Runs { get; set; }

    public double Total { get; set; }

    public double Max { get; set; }

    public override AccKind Kind => AccKind.RunLength;

    public override ulong SampleCount => Runs;

    public double? Mean => Runs == 0 ? null : Total / Runs;

    public override void Add(double a, double b = 0)
    {
        Runs++;
        Total += a;
        Max = Math.Max(Max, a);
    }

    public override void Merge(Accumulator other)
    {
        var o = Cast<RunLengthAcc>(other);
        Runs += o.Runs;
        Total += o.Total;
        Max = Math.Max(Max, o.Max);
    }

    public override Accumulator Clone() => new RunLengthAcc { Runs = Runs, Total = Total, Max = Max };
}

/// <summary>固定分辨率的时间序列（仅用于测试结果）：Add(a = 序号, b = 值)，合并为拼接。</summary>
public sealed class SeriesAcc : Accumulator
{
    public List<double> Points { get; } = [];

    public override AccKind Kind => AccKind.Series;

    public override ulong SampleCount => (ulong)Points.Count;

    public override void Add(double a, double b = 0)
    {
        int index = (int)a;
        while (Points.Count <= index)
        {
            Points.Add(double.NaN);
        }

        Points[index] = b;
    }

    public override void Merge(Accumulator other) => Points.AddRange(Cast<SeriesAcc>(other).Points);

    public override Accumulator Clone()
    {
        var copy = new SeriesAcc();
        copy.Points.AddRange(Points);
        return copy;
    }
}