using System.Security.Cryptography;
using System.Text;
using MoShiro.Protocol;

namespace MoShiro.Backend.Analysis;

public enum Unit
{
    None,
    Ms,
    Pct,
    Count,
    PerMin,
    PerHour,
    Mbps,
    Packets,
    Score,
}

public enum Better
{
    None,
    Lower,
    Higher,
}

public enum Stat
{
    P50,
    P90,
    P95,
    P99,
    Mean,
    Max,
    Min,
    Pct,
    Count,
    PerMin,
    PerHour,
    P999,

    /// <summary>长尾比：p99 / p50（从同一个直方图导出，所以窗口合并之后仍然精确）。</summary>
    Tail,
}

/// <summary>算子：把一个流里的原始行变成观测值。算子都是纯函数，每个都有边界用例测试。</summary>
public abstract class MetricOp
{
    /// <summary>算子的名称和参数，参与指标指纹；改变语义时必须同时改变它。</summary>
    public abstract string Signature { get; }

    public abstract void Evaluate(ChunkView view, RawStream stream, IObservationSink sink);
}

/// <summary>导出值：<c>指标id.统计量</c>，例如 <c>rtt.p95</c>。已发布的 id 不能改名，只能新增。</summary>
public sealed record MetricExport(string Id, Stat Stat, Unit Unit, double? Good = null, double? Poor = null);

/// <summary>一行声明 = 一个指标：输入（kind.stream）+ 算子 + 累加器 + 导出。</summary>
public sealed class MetricDef
{
    public MetricDef(string id, AccKind kind, IReadOnlyList<string> from, MetricOp op, string label, string description, Better better, IReadOnlyList<MetricExport> exports, int version = 1)
    {
        Id = id;
        Kind = kind;
        From = from;
        Op = op;
        Label = label;
        Description = description;
        Better = better;
        Exports = exports;
        Version = version;
        Fingerprint = ComputeFingerprint();
    }

    public string Id { get; }

    public AccKind Kind { get; }

    /// <summary><c>kind.stream</c>；<c>*</c> 表示所有具有该 stream 的 Kind。</summary>
    public IReadOnlyList<string> From { get; }

    public MetricOp Op { get; }

    public string Label { get; }

    public string Description { get; }

    public Better Better { get; }

    public IReadOnlyList<MetricExport> Exports { get; }

    public int Version { get; }

    /// <summary>由声明内容和算子签名计算得出，变化时触发回填。</summary>
    public string Fingerprint { get; }

    public bool Matches(string kind, string stream)
    {
        foreach (string source in From)
        {
            int dot = source.IndexOf('.', StringComparison.Ordinal);
            if (source.AsSpan(dot + 1).SequenceEqual(stream) && (source.AsSpan(0, dot).SequenceEqual("*") || source.AsSpan(0, dot).SequenceEqual(kind)))
            {
                return true;
            }
        }

        return false;
    }

    /// <summary>从累加器导出一个值；没有样本时返回 null。<paramref name="durationSeconds"/> 用于速率类导出。</summary>
    public static double? Export(Accumulator acc, MetricExport export, double durationSeconds)
    {
        switch (acc)
        {
            case HistogramAcc h:
                return export.Stat switch
                {
                    Stat.P50 => h.Quantile(0.5),
                    Stat.P90 => h.Quantile(0.9),
                    Stat.P95 => h.Quantile(0.95),
                    Stat.P99 => h.Quantile(0.99),
                    Stat.P999 => h.Quantile(0.999),
                    Stat.Tail => h.Quantile(0.5) is > 0 and double p50 && h.Quantile(0.99) is { } p99 ? p99 / p50 : null,
                    Stat.Mean => h.Mean,
                    Stat.Max => h.Count == 0 ? null : h.Max,
                    Stat.Min => h.Count == 0 ? null : h.Min,
                    _ => null,
                };
            case RatioAcc r:
                return export.Stat == Stat.Pct ? r.Percent : null;
            case CounterAcc c:
                return export.Stat switch
                {
                    Stat.Count => c.Value,
                    Stat.PerMin => durationSeconds > 0 ? c.Value / (durationSeconds / 60.0) : null,
                    Stat.PerHour => durationSeconds > 0 ? c.Value / (durationSeconds / 3600.0) : null,
                    _ => null,
                };
            case MaxAcc m:
                return m.Result;
            case MinAcc n:
                return n.Result;
            case SumAcc s:
                return export.Stat == Stat.Mean ? s.Mean : s.Count == 0 ? null : s.Sum;
            case RunLengthAcc rl:
                return export.Stat switch
                {
                    Stat.Max => rl.Runs == 0 ? null : rl.Max,
                    Stat.Mean => rl.Mean,
                    _ => null,
                };
            default:
                return null;
        }
    }

    private string ComputeFingerprint()
    {
        var text = new StringBuilder()
            .Append('v').Append(Version).Append('|').Append(Id).Append('|').Append(Kind).Append('|')
            .AppendJoin(',', From).Append('|').Append(Op.Signature).Append('|');
        foreach (MetricExport e in Exports)
        {
            text.Append(e.Id).Append(':').Append(e.Stat).Append(';');
        }

        return Convert.ToHexStringLower(SHA256.HashData(Encoding.UTF8.GetBytes(text.ToString())))[..16];
    }
}

/// <summary>声明指标的简写。</summary>
public static class Metric
{
    public static MetricDef Histogram(string id, string[] from, MetricOp op, string label, string description, Unit unit, Stat[] exports, Better better = Better.Lower, double? good = null, double? poor = null) =>
        new(id, AccKind.Histogram, from, op, label, description, better, [.. exports.Select(s => s == Stat.Tail ? new MetricExport(id + "." + StatName(s), s, Unit.None) : new MetricExport(id + "." + StatName(s), s, unit, good, poor))]);

    public static MetricDef Ratio(string id, string[] from, MetricOp op, string label, string description, Better better = Better.Lower, double? good = null, double? poor = null, string? exportId = null) =>
        new(id, AccKind.Ratio, from, op, label, description, better, [new MetricExport(exportId ?? id, Stat.Pct, Unit.Pct, good, poor)]);

    public static MetricDef Counter(string id, string[] from, MetricOp op, string label, string description, MetricExport export, Better better = Better.Lower) =>
        new(id, AccKind.Counter, from, op, label, description, better, [export]);

    public static MetricDef Max(string id, string[] from, MetricOp op, string label, string description, Unit unit, string? exportId = null, Better better = Better.Lower, double? good = null, double? poor = null) =>
        new(id, AccKind.Max, from, op, label, description, better, [new MetricExport(exportId ?? id, Stat.Max, unit, good, poor)]);

    public static MetricDef RunLength(string id, string[] from, MetricOp op, string label, string description, params MetricExport[] exports) =>
        new(id, AccKind.RunLength, from, op, label, description, Better.Lower, exports);

    public static string StatName(Stat stat) => stat switch
    {
        Stat.P50 => "p50",
        Stat.P90 => "p90",
        Stat.P95 => "p95",
        Stat.P99 => "p99",
        Stat.Mean => "mean",
        Stat.Max => "max",
        Stat.Min => "min",
        Stat.Pct => "pct",
        Stat.Count => "count",
        Stat.PerMin => "per_min",
        Stat.P999 => "p999",
        Stat.Tail => "tail",
        _ => "per_h",
    };
}